Day 16 |
The advantage of having the input and output kept apart from the language and handled in libraries is that it is easier to make the language "platform-independent." That is, you can write C++ programs on a PC and then recompile them and run them on a Sun Workstation. The compiler manufacturer just supplies the right library, and everything works. At least that's the theory.
NOTE: A library is a collection of OBJ files that can be linked to your program to provide additional functionality. This is the most basic form of code reuse, and has been around since ancient programmers chiseled 1s and 0s into the walls of caves.
One principal goal of streams is to encapsulate the problems of getting the data to and from the disk or the screen. Once a stream is created, your program works with the stream and the stream sweats the details. Figure 16.1 illustrates this fundamental idea.
Figure 16.1. Encapsulation through streams.
Picture water trickling into the top of a tank, and the tank filling and filling, but no water running out of the bottom. Figure 16.2 illustrates this idea.
When the water (data) reaches the top, the valve opens and all the water flows out in a rush. Figure 16.3 illustrates this.
Once the buffer is empty, the bottom valve closes, the top valve opens, and more water flows into the buffer tank. Figure 16.4 illustrates this.
Every once in a while you need to get the water out of the tank even before it is full. This is called "flushing the buffer." Figure 16.5 illustrates this idea.
Figure 16.2. Filling the buffer.
Figure 16.3. Emptying the buffer.
Figure 16.4. Refilling the buffer.
Figure 16.5. Flushing the buffer.
NOTE: The iostream class library is added automatically to your program by the compiler. All you need to do to use these functions is to put the appropriate include statement at the top of your program listing.
DOS provides rudimentary redirection commands, such as redirect output (>) and (>)redirect input (<). UNIX provides more advanced redirection capabilities, but the general idea is the same: Take the output intended for the screen and write it to a file, or pipe it into another program. Alternatively, the input for a program can be extracted from a file rather than from the keyboard.
Redirection is more a function of the operating system than of the iostream libraries. C++ just provides access to the four standard devices; it is up to the user to redirect the devices to whatever alternatives are needed.
int someVariable; cout << "Enter a number: "; cin >> someVariable;The global object cout is discussed later today; for now, focus on the third line, cin >> someVariable;. What can you guess about cin?
Clearly it must be a global object, because you didn't define it in your own code. You know from previous operator experience that cin has overloaded the extraction operator (>>) and that the effect is to write whatever data cin has in its buffer into your local variable, someVariable.
What may not be immediately obvious is that cin has overloaded the extraction operator for a great variety of parameters, among them int&, short&, long&, double&, float&, char&, char*, and so forth. When you write cin >> someVariable;, the type of someVariable is assessed. In the example above, someVariable is an integer, so the following function is called:
istream & operator>> (int &)Note that because the parameter is passed by reference, the extraction operator is able to act on the original variable. Listing 16.1 illustrates the use of cin.
Listing 16.1. cin handles different data types.
1: //Listing 16.1 -- character strings and cin
2:
3: #include <iostream.h>
4:
5: int main()
6: {
7: int myInt;
8: long myLong;
9: double myDouble;
10: float myFloat;
11: unsigned int myUnsigned;
12:
13: cout << "int: ";
14: cin >> myInt;
15: cout << "Long: ";
16: cin >> myLong;
17: cout << "Double: ";
18: cin >> myDouble;
19: cout << "Float: ";
20: cin >> myFloat;
21: cout << "Unsigned: ";
22: cin >> myUnsigned;
23:
24: cout << "\n\nInt:\t" << myInt << endl;
25: cout << "Long:\t" << myLong << endl;
26: cout << "Double:\t" << myDouble << endl;
27: cout << "Float:\t" << myFloat << endl;
28: cout << "Unsigned:\t" << myUnsigned << endl;
29: return 0;
30: }
Output: int: 2
Long: 70000
Double: 987654321
Float: 3.33
Unsigned: 25
Int: 2
Long: 70000
Double: 9.87654e+08
Float: 3.33
Unsigned: 25
Analysis: On
lines 7-11, variables of various types are declared. On lines 13-22, the
user is prompted to enter values for these variables, and the results are
printed (using
cout) on lines 24-28.
The output reflects that the variables were put into the right "kinds" of variables, and the program works as you might expect.
char YourName[50] cout << "Enter your name: "; cin >> YourName;If you enter Jesse, the variable YourName will be filled with the characters J, e, s, s, e, \0. The last character is a null; cin automatically ends the string with a null character, and you must have enough room in the buffer to allow for the entire string plus the null. The null signals "end of string" to the standard library functions discussed on Day 21, "What's Next."
Listing 16.2. Trying to write more than one word to cin.
1: //Listing 16.2 -- character strings and cin
2:
3: #include <iostream.h>
4:
5: int main()
6: {
7: char YourName[50];
8: cout << "Your first name: ";
9: cin >> YourName;
10: cout << "Here it is: " << YourName << endl;
11: cout << "Your entire name: ";
12: cin >> YourName;
13: cout << "Here it is: " << YourName << endl;
14: return 0;
15: }
Output: Your first name: Jesse
Here it is: Jesse
Your entire name: Jesse Liberty
Here it is: Jesse
Analysis: On line 7, a character array
is created to hold the user's input. On line 8, the user is prompted to
enter one name, and that name is stored properly, as shown in the output.
On line 11, the user is again prompted, this time for a full name. cin reads the input, and when it sees the space between the names, it puts a null character after the first word and terminates input. This is not exactly what was intended.
To understand why this works this way, examine Listing 16.3, which shows input for a number of fields.
1: //Listing 16.3 - character strings and cin
2:
3: #include <iostream.h>
4:
5: int main()
6: {
7: int myInt;
8: long myLong;
9: double myDouble;
10: float myFloat;
11: unsigned int myUnsigned;
12: char myWord[50];
13:
14: cout << "int: ";
15: cin >> myInt;
16: cout << "Long: ";
17: cin >> myLong;
18: cout << "Double: ";
19: cin >> myDouble;
20: cout << "Float: ";
21: cin >> myFloat;
22: cout << "Word: ";
23: cin >> myWord;
24: cout << "Unsigned: ";
25: cin >> myUnsigned;
26:
27: cout << "\n\nInt:\t" << myInt << endl;
28: cout << "Long:\t" << myLong << endl;
29: cout << "Double:\t" << myDouble << endl;
30: cout << "Float:\t" << myFloat << endl;
31: cout << "Word: \t" << myWord << endl;
32: cout << "Unsigned:\t" << myUnsigned << endl;
33:
34: cout << "\n\nInt, Long, Double, Float, Word, Unsigned: ";
35: cin >> myInt >> myLong >> myDouble;
36: cin >> myFloat >> myWord >> myUnsigned;
37: cout << "\n\nInt:\t" << myInt << endl;
38: cout << "Long:\t" << myLong << endl;
39: cout << "Double:\t" << myDouble << endl;
40: cout << "Float:\t" << myFloat << endl;
41: cout << "Word: \t" << myWord << endl;
42: cout << "Unsigned:\t" << myUnsigned << endl;
43:
44:
45: return 0;
46: }
Output: Int: 2
Long: 30303
Double: 393939397834
Float: 3.33
Word: Hello
Unsigned: 85
Int: 2
Long: 30303
Double: 3.93939e+11
Float: 3.33
Word: Hello
Unsigned: 85
Int, Long, Double, Float, Word, Unsigned: 3 304938 393847473 6.66 bye -2
Int: 3
Long: 304938
Double: 3.93847e+08
Float: 6.66
Word: bye
Unsigned: 65534Analysis:Once again, a number of variables are created, this time including a char array. The user is prompted for input and the output is faithfully printed.
On line 34, the user is prompted for all the input at once, and then each "word" of input is assigned to the appropriate variable. It is in order to facilitate this kind of multiple assignment that cin must consider each word in the input to be the full input for each variable. If cin was to consider the entire input to be part of one variable's input, this kind of concatenated input would be impossible.
Note that on line 35 the last object requested was an unsigned integer, but the user entered -2. Because cin believes it is writing to an unsigned integer, the bit pattern of -2 was evaluated as an unsigned integer, and when written out by cout, the value 65534 was displayed. The unsigned value 65534 has the exact bit pattern of the signed value -2.
Later in this chapter you will see how to enter an entire string into a buffer, including multiple words. For now, the question arises, "How does the extraction operator manage this trick of concatenation?"
int VarOne, varTwo, varThree; cout << "Enter three numbers: " cin >> VarOne >> varTwo >> varThree;When you write cin >> VarOne >> varTwo >> varThree;, the first extraction is evaluated (cin >> VarOne). The return value from this is another istream object, and that object's extraction operator gets the variable varTwo. It is as if you had written this:
((cin >> varOne) >> varTwo) >> varThree;You'll see this technique repeated later when cout is discussed.
cin.get() >>myVarOne >> myVarTwo; // illegalThe return value of (cin.get() >> myVarOne) is an integer, not an iostream object.
A common use of get() with no parameters is illustrated in Listing 16.4.
Listing 16.4. Using get() with no parameters.
1: // Listing 16.4 - Using get() with no parameters
2: #include <iostream.h>
3:
4: int main()
5: {
6: char ch;
7: while ( (ch = cin.get()) != EOF)
8: {
9: cout << "ch: " << ch << endl;
10: }
11: cout << "\nDone!\n";
12: return 0;
13: }
NOTE: To exit this program, you must send end of file from the keyboard. On DOS computers use Ctrl+Z; on UNIX units use Ctrl+D.
Output: Hello ch: H ch: e ch: l ch: l ch: o ch: World ch: W ch: o ch: r ch: l ch: d ch: (ctrl-z) Done!Analysis: On line 6, a local character variable is declared. The while loop assigns the input received from cin.get() to ch, and if it is not EOF the string is printed out. This output is buffered until an end of line is read, however. Once EOF is encountered (by pressing Ctrl+Z on a DOS machine, or Ctrl+D on a UNIX machine), the loop exits.
Note that not every implementation of istream supports this version of get(). Using get() with a Character Reference Parameter When a character is passed as input to get(), that character is filled with the next character in the input stream. The return value is an iostream object, and so this form of get() can be concatenated, as illustrated in Listing 16.5.
Listing 16.5 Using get() with parameters.
1: // Listing 16.5 - Using get() with parameters
2: #include <iostream.h>
3:
4: int main()
5: {
6: char a, b, c;
7:
8: cout << "Enter three letters: ";
9:
10: cin.get(a).get(b).get(c);
11:
12: cout << "a: " << a << "\nb: " << b << "\nc: " << c << endl;
13: return 0;
14: }
Output: Enter three letters: one
a: o
b: n
c: e
Analysis:On
line 6, three character variables are created. On line 10, cin.get()
is called three times, concatenated. First cin.get(a) is called.
This puts the first letter into a and returns cin so
that when it is done,
cin.get(b) is called, putting the next letter
into b. The end result of this is that cin.get(c) is
called and the third letter is put in c.
Because cin.get(a) evaluates to cin, you could have written this:
cin.get(a) >> b;In this form, cin.get(a) evaluates to cin, so the second phrase is cin >> b;.
DO use the extraction operator (>>) when you need to skip over white space. DO use get() with a character parameter when you need to examine every character, including white space. DON'T use get() with no parameters at all; it is more or less obsolete.
The final form of get() takes three parameters. The first parameter is a pointer to a character array, the second parameter is the maximum number of characters to read plus one, and the third parameter is the termination character.
If you enter 20 as the second parameter, get() will read 19 characters and then will null-terminate the string, which it will store in the first parameter. The third parameter, the termination character, defaults to newline (`\n'). If a termination character is reached before the maximum number of characters is read, a null is written and the termination character is left in the buffer.
Listing 16.6 illustrates the use of this form of get().
Listing 16.6. Using get() with a character array.
1: // Listing 16.6 - Using get() with a character array
2: #include <iostream.h>
3:
4: int main()
5: {
6: char stringOne[256];
7: char stringTwo[256];
8:
9: cout << "Enter string one: ";
10: cin.get(stringOne,256);
11: cout << "stringOne: " << stringOne << endl;
12:
13: cout << "Enter string two: ";
14: cin >> stringTwo;
15: cout << "StringTwo: " << stringTwo << endl;
16: return 0;
17: }
Output: Enter string one: Now is the time
stringOne: Now is the time
Enter string two: For all good
StringTwo: For
Analysis:On
lines 6 and 7, two character arrays are created. On line 9, the user is
prompted to enter a string, and cin.get() is called on line 10.
The first parameter is the buffer to fill, and the second is one more than
the maximum number for get() to accept (the extra position being
given to the null character, (`\0')). The defaulted third parameter
is a newline.
The user enters Now is the time. Because the user ends the phrase with a newline, that phrase is put into stringOne, followed by a terminating null.
The user is prompted for another string on line 13, and this time the extraction operator is used. Because the extraction operator takes everything up to the first white space, the string For, with a terminating null character, is stored in the second string, which of course is not what was intended.
Another way to solve this problem is to use getline(), as illustrated in Listing 16.7.
Listing 16.7. Using getline().
1: // Listing 16.7 - Using getline()
2: #include <iostream.h>
3:
4: int main()
5: {
6: char stringOne[256];
7: char stringTwo[256];
8: char stringThree[256];
9:
10: cout << "Enter string one: ";
11: cin.getline(stringOne,256);
12: cout << "stringOne: " << stringOne << endl;
13:
14: cout << "Enter string two: ";
15: cin >> stringTwo;
16: cout << "stringTwo: " << stringTwo << endl;
17:
18: cout << "Enter string three: ";
19: cin.getline(stringThree,256);
20: cout << "stringThree: " << stringThree << endl;
21: return 0;
22: }
Output: Enter string one: one two three
stringOne: one two three
Enter string two: four five six
stringTwo: four
Enter string three: stringThree: five six
Analysis:This
example warrants careful examination; there are some potential surprises.
On lines 6-8, three character arrays are declared.
On line 10, the user is prompted to enter a string, and that string is read by getline(). Like get(), getline() takes a buffer and a maximum number of characters. Unlike get(), however, the terminating newline is read and thrown away. With get() the terminating newline is not thrown away. It is left in the input buffer.
On line 14, the user is prompted again, and this time the extraction operator is used. The user enters four five six, and the first word, four, is put in stringTwo. The string Enter string three is then displayed, and getline() is called again. Because five six is still in the input buffer, it is immediately read up to the newline; getline() terminates and the string in stringThree is printed on line 20.
The user has no chance to enter string three, because the second getline() call is fulfilled by the string remaining in the input buffer after the call to the extraction operator on line 15.
The extraction operator (>>) reads up to the first white space and puts the word into the character array.
The member function get() is overloaded. In one version, it takes no parameters and returns the value of the character it receives. In the second version, it takes a single character reference and returns the istream object by reference.
In the third and final version, get() takes a character array, a number of characters to get, and a termination character (which defaults to newline). This version of get() reads characters into the array until it gets to one fewer than its maximum number of characters or it encounters the termination character, whichever comes first. If get() encounters the termination character, it leaves that character in the input buffer and stops reading characters.
The member function getline() also takes three parameters: the buffer to fill, one more than the maximum number of characters to get, and the termination character. getline()functions exactly like get() does with these parameters, except getline() throws away the terminating character.
1: // Listing 16.8 - Using ignore()
2: #include <iostream.h>
3:
4: int main()
5: {
6: char stringOne[255];
7: char stringTwo[255];
8:
9: cout << "Enter string one:";
10: cin.get(stringOne,255);
11: cout << "String one" << stringOne << endl;
12:
13: cout << "Enter string two: ";
14: cin.getline(stringTwo,255);
15: cout << "String two: " << stringTwo << endl;
16:
17: cout << "\n\nNow try again...\n";
18:
19: cout << "Enter string one: ";
20: cin.get(stringOne,255);
21: cout << "String one: " << stringOne<< endl;
22:
23: cin.ignore(255,'\n');
24:
25: cout << "Enter string two: ";
26: cin.getline(stringTwo,255);
27: cout << "String Two: " << stringTwo<< endl;
28: return 0;
29: }
Output: Enter string one:once upon a time
String oneonce upon a time
Enter string two: String two:
Now try again...
Enter string one: once upon a time
String one: once upon a time
Enter string two: there was a
String Two: there was a
Analysis: On
lines 6 and 7, two character arrays are created. On line 9, the user is
prompted for input and types once upon a time, followed by Enter.
On line 10, get() is used to read this string. get()
fills stringOne and terminates on the newline, but leaves the
newline character in the input buffer.
On line 13, the user is prompted again, but the getline() on line 14 reads the newline that is already in the buffer and terminates immediately, before the user can enter any input.
On line 19, the user is prompted again and puts in the same first line of input. This time, however, on line 23, ignore() is used to "eat" the newline character. Thus, when the getline() call on line 26 is reached, the input buffer is empty, and the user can input the next line of the story.
Listing 16.9. Using peek() and putback().
1: // Listing 16.9 - Using peek() and putback()
2: #include <iostream.h>
3:
4: int main()
5: {
6: char ch;
7: cout << "enter a phrase: ";
8: while ( cin.get(ch) )
9: {
10: if (ch == `!')
11: cin.putback(`$');
12: else
13: cout << ch;
14: while (cin.peek() == `#')
15: cin.ignore(1,'#');
16: }
17: return 0;
18: }
Output: enter a phrase: Now!is#the!time#for!fun#!
Now$isthe$timefor$fun$
Analysis: On
line 6, a character variable, ch, is declared, and on line 7,
the user is prompted to enter a phrase. The purpose of this program is
to turn any exclamation marks (!) into dollar signs ($)
and to remove any pound symbols (#).
The program loops as long as it is getting characters other than the end of file (remember that cin.get() returns 0 for end of file). If the current character is an exclamation point, it is thrown away and the $ symbol is put back into the input buffer; it will be read the next time through. If the current item is not an exclamation point, it is printed. The next character is "peeked" at, and when pound symbols are found, they are removed.
This is not the most efficient way to do either of these things (and it won't find a pound symbol if it is the first character), but it does illustrate how these methods work. They are relatively obscure, so don't spend a lot of time worrying about when you might really use them. Put them into your bag of tricks; they'll come in handy sooner or later.
TIP: peek() and putback() are typically used for parsing strings and other data, such as when writing a compiler.
cout << flushThis can be convenient when you need to ensure that the output buffer is emptied and that the contents are written to the screen.
The function put() is used to write a single character to the output device. Because put() returns an ostream reference, and because cout is an ostream object, you can concatenate put() just as you do the insertion operator. Listing 16.10 illustrates this idea.
1: // Listing 16.10 - Using put()
2: #include <iostream.h>
3:
4: int main()
5: {
6: cout.put(`H').put(`e').put(`l').put(`l').put(`o').put(`\n');
7: return 0;
8: }
Output: Hello
Analysis:Line
6 is evaluated like this: cout.put(`H') writes the letter H
to the screen and returns the cout object. This leaves the following:
cout.put(`e').put(`l').put(`l').put(`o').put(`\n');The letter e is written, leaving cout.put(`l'). This process repeats, each letter being written and the cout object returned until the final character (`\n') is written and the function returns.
The function write() works just like the insertion operator (<<), except that it takes a parameter that tells the function the maximum number of characters to write. Listing 16.11 illustrates its use.
1: // Listing 16.11 - Using write()
2: #include <iostream.h>
3: #include <string.h>
4:
5: int main()
6: {
7: char One[] = "One if by land";
8:
9:
10:
11: int fullLength = strlen(One);
12: int tooShort = fullLength -4;
13: int tooLong = fullLength + 6;
14:
15: cout.write(One,fullLength) << "\n";
16: cout.write(One,tooShort) << "\n";
17: cout.write(One,tooLong) << "\n";
18: return 0;
19: }
Output: One if by land
One if by
One if by land i?!
Analysis: On line 7, one phrase is created. On line 11, the integer fullLength is set to the length of the phrase and tooShort is set to that length minus four, while tooLong is set to fullLength plus six.
NOTE: The last line of output may look different on your computer.
On line 15, the complete phrase is printed using write(). The length is set to the actual length of the phrase, and the correct phrase is printed.
On line 16, the phrase is printed again, but is four characters shorter than the full phrase, and that is reflected in the output.
On line 17, the phrase is printed again, but this time write() is instructed to write an extra six characters. Once the phrase is written, the next six bytes of contiguous memory are written.
Listing 16.12. Adjusting the width of output.
1: // Listing 16.12 - Adjusting the width of output
2: #include <iostream.h>
3:
4: int main()
5: {
6: cout << "Start >";
7: cout.width(25);
8: cout << 123 << "< End\n";
9:
10: cout << "Start >";
11: cout.width(25);
12: cout << 123<< "< Next >";
13: cout << 456 << "< End\n";
14:
15: cout << "Start >";
16: cout.width(4);
17: cout << 123456 << "< End\n";
18:
19: return 0;
20: }
Output: Start > 123< End
Start > 123< Next >456< End
Start >123456< End
Analysis:The
first output, on lines 6-8, prints the number 123 within a field whose
width is set to 25 on line 7. This is reflected in the first line of output.
The second line of output first prints the value 123 in the same field whose width is set to 25, and then prints the value 456. Note that 456 is printed in a field whose width is reset to just large enough; as stated, the effect of width() lasts only as long as the very next output.
The final output reflects that setting a width that is smaller than the output is exactly like setting a width that is just large enough.
1: // Listing 16.3 - fill()
2:
3: #include <iostream.h>
4:
5: int main()
6: {
7: cout << "Start >";
8: cout.width(25);
9: cout << 123 << "< End\n";
10:
11:
12: cout << "Start >";
13: cout.width(25);
14: cout.fill(`*');
15: cout << 123 << "< End\n";
16: return 0;
17: }
Output: Start > 123< End
Start >******************123< End
Analysis:Lines
7-9 repeat the functionality from the previous example. Lines 12-15 repeat
this again, but this time, on line 14, the fill character is set to asterisks,
as reflected in the output.
The enumerated constants are scoped to the iostream class (ios) and thus are called with the full qualification ios::flagname, such as ios::showpoint.
You can turn on the plus sign (+) before positive numbers by using ios::showpos. You can change the alignment of the output by using ios::left, ios::right, or ios::internal.
Finally, you can set the base of the numbers for display by using ios::dec (decimal), ios::oct (octal--base eight), or ios::hex (hexadecimal--base sixteen). These flags can also be concatenated into the insertion operator. Listing 16.14 illustrates these settings. As a bonus, Listing 16.14 also introduces the setw manipulator, which sets the width but can also be concatenated with the insertion operator.
1: // Listing 16.14 - Using setf
2: #include <iostream.h>
3: #include <iomanip.h>
4:
5: int main()
6: {
7: const int number = 185;
8: cout << "The number is " << number << endl;
9:
10: cout << "The number is " << hex << number << endl;
11:
12: cout.setf(ios::showbase);
13: cout << "The number is " << hex << number << endl;
14:
15: cout << "The number is " ;
16: cout.width(10);
17: cout << hex << number << endl;
18:
19: cout << "The number is " ;
20: cout.width(10);
21: cout.setf(ios::left);
22: cout << hex << number << endl;
23:
24: cout << "The number is " ;
25: cout.width(10);
26: cout.setf(ios::internal);
27: cout << hex << number << endl;
28:
29: cout << "The number is:" << setw(10) << hex << number << endl;
30: return 0;
31: }
Output: The number is 185
The number is b9
The number is 0xb9
The number is 0xb9
The number is 0xb9
The number is 0x b9
The number is:0x b9
Analysis:On
line 7, the constant int number is initialized to the
value
185. This is displayed on line 8.
The value is displayed again on line 10, but this time the manipulator hex is concatenated, causing the value to be displayed in hexadecimal as b9. (b=11; 11*16=176+9=185).
On line 12, the flag showbase is set. This causes the prefix 0x to be added to all hexadecimal numbers, as reflected in the output.
On line 16, the width is set to 10, and the value is pushed to the extreme right. On line 20, the width is again set to 10, but this time the alignment is set to the left, and the number is again printed flush left.
On line 25, once again the width is set to 10, but this time the alignment is internal. Thus the 0x is printed flush left, but the value, b9, is printed flush right.
Finally, on line 29, the concatenation operator setw() is used to set the width to 10, and the value is printed again.
printf() does not provide type safety, so it is easy to inadvertently tell it to display an integer as if it was a character and vice versa. printf() also does not support classes, and so it is not possible to teach it how to print your class data; you must feed each class member to printf() one by one.
On the other hand, printf() does make formatting much easier, because you can put the formatting characters directly into the printf() statement. Because printf() has its uses and many programmers still make extensive use of it, this section will briefly review its use.
To use printf(), be sure to include the STDIO.H header file. In its simplest form, printf() takes a formatting string as its first parameter and then a series of values as its remaining parameters.
The formatting string is a quoted string of text and conversion specifiers. All conversion specifiers must begin with the percent symbol (%). The common conversion specifiers are presented in Table 16.1.
Table 16.1. The Common Conversion Specifiers.
| Specifier | Used For |
| %s | strings |
| %d | integers |
| %l | long integer |
| %ld | long integers |
| %f | float |
Each of the conversion specifiers can also provide a width statement and a precision statement, expressed as a float, where the digits to the left of the decimal are used for the total width, and the digits to the right of the decimal provide the precision for floats. Thus, %5d is the specifier for a five-digit-wide integer, and %15.5f is the specifier for a 15-digit-wide float, of which the final five digits are dedicated to the decimal portion. Listing 16.15 illustrates various uses of printf().
Listing 16.15. Printing with printf().
1: #include <stdio.h>
2: int main()
3: {
4: printf("%s","hello world\n");
5:
6: char *phrase = "Hello again!\n";
7: printf("%s",phrase);
8:
9: int x = 5;
10: printf("%d\n",x);
11:
12: char *phraseTwo = "Here's some values: ";
13: char *phraseThree = " and also these: ";
14: int y = 7, z = 35;
15: long longVar = 98456;
16: float floatVar = 8.8;
17:
18: printf("%s %d %d %s %ld %f\n",phraseTwo,y,z,phraseThree,longVar,floatVar);
19:
20: char *phraseFour = "Formatted: ";
21: printf("%s %5d %10d %10.5f\n",phraseFour,y,z,floatVar);
22: return 0;
23: }
Output: hello world
Hello again!
5
Here's some values: 7 35 and also these: 98456 8.800000
Formatted: 7 35 8.800000
Analysis: The first printf()
statement, on line 4, uses the standard form: the term printf,
followed by a quoted string with a conversion specifier (in this case %s),
followed by a value to insert into the conversion specifier.
The %s indicates that this is a string, and the value for the string is, in this case, the quoted string "hello world".
The second printf() statement is just like the first, but this time a char pointer is used, rather than quoting the string right in place in the printf() statement.
The third printf(), on line 10, uses the integer conversion specifier, and for its value the integer variable x. The fourth printf() statement, on line 18, is more complex. Here six values are concatenated. Each conversion specifier is supplied, and then the values are provided, separated by commas.
Finally, on line 21, format specifications are used to specify width and precision. As you can see, all of this is somewhat easier than using manipulators.
As stated previously, however, the limitation here is that there is no type checking and printf() cannot be declared a friend or member function of a class. So if you want to print the various member data of a class, you must feed each accessor method to the printf() statement explicitly.
To get started with writing to a file, you must first create an ofstream object, and then associate that object with a particular file on your disk. To use ofstream objects, you must be sure to include fstream.h in your program.
NOTE: Because fstream.h includes iostream.h, there is no need for you to include iostream explicitly.
ofstream fout("myfile.cpp");
Opening this file for input works exactly the same way, except it uses
an ifstream object:
ifstream fin("myfile.cpp");
Note that fout and fin are names you assign; here fout
has been used to reflect its similarity to cout, and fin
has been used to reflect its similarity to cin.
One important file stream function that you will need right away is close(). Every file stream object you create opens a file for either reading or writing (or both). It is important to close() the file after you finish reading or writing; this ensures that the file won't be corrupted and that the data you've written is flushed to the disk.
Once the stream objects are associated with files, they can be used like any other stream objects. Listing 16.16 illustrates this.
Listing 16.16. Opening files for read and write.
1: #include <fstream.h>
2: int main()
3: {
4: char fileName[80];
5: char buffer[255]; // for user input
6: cout << "File name: ";
7: cin >> fileName;
8:
9: ofstream fout(fileName); // open for writing
10: fout << "This line written directly to the file...\n";
11: cout << "Enter text for the file: ";
12: cin.ignore(1,'\n'); // eat the newline after the file name
13: cin.getline(buffer,255); // get the user's input
14: fout << buffer << "\n"; // and write it to the file
15: fout.close(); // close the file, ready for reopen
16:
17: ifstream fin(fileName); // reopen for reading
18: cout << "Here's the contents of the file:\n";
19: char ch;
20: while (fin.get(ch))
21: cout << ch;
22:
23: cout << "\n***End of file contents.***\n";
24:
25: fin.close(); // always pays to be tidy
26: return 0;
27: }
Output: File name: test1
Enter text for the file: This text is written to the file!
Here's the contents of the file:
This line written directly to the file...
This text is written to the file!
***End of file contents.***
Analysis: On line 4, a buffer is set
aside for the filename, and on line 5 another buffer is set aside for user
input. The user is prompted to enter a filename on line 6, and this response
is written to the fileName buffer. On line 9, an ofstream
object is created,
fout, which is associated with the new filename.
This opens the file; if the file already exists, its contents are thrown
away.
On line 10, a string of text is written directly to the file. On line 11, the user is prompted for input. The newline character left over from the user's input of the filename is eaten on line 12, and the user's input is stored into buffer on line 13. That input is written to the file along with a newline character on line 14, and then the file is closed on line 15.
On line 17, the file is reopened, this time in input mode, and the contents are read, one character at a time, on lines 20 and 21.
Valid arguments include:
Listing 16.17. Appending to the end of a file.
1: #include <fstream.h>
2: int main() // returns 1 on error
3: {
4: char fileName[80];
5: char buffer[255];
6: cout << "Please re-enter the file name: ";
7: cin >> fileName;
8:
9: ifstream fin(fileName);
10: if (fin) // already exists?
11: {
12: cout << "Current file contents:\n";
13: char ch;
14: while (fin.get(ch))
15: cout << ch;
16: cout << "\n***End of file contents.***\n";
17: }
18: fin.close();
19:
20: cout << "\nOpening " << fileName << " in append mode...\n";
21:
22: ofstream fout(fileName,ios::app);
23: if (!fout)
24: {
25: cout << "Unable to open " << fileName << " for appending.\n";
26: return(1);
27: }
28:
29: cout << "\nEnter text for the file: ";
30: cin.ignore(1,'\n');
31: cin.getline(buffer,255);
32: fout << buffer << "\n";
33: fout.close();
34:
35: fin.open(fileName); // reassign existing fin object!
36: if (!fin)
37: {
38: cout << "Unable to open " << fileName << " for reading.\n";
39: return(1);
40: }
41: cout << "\nHere's the contents of the file:\n";
42: char ch;
43: while (fin.get(ch))
44: cout << ch;
45: cout << "\n***End of file contents.***\n";
46: fin.close();
47: return 0;
48: }
Output: Please re-enter the file name: test1
Current file contents:
This line written directly to the file...
This text is written to the file!
***End of file contents.***
Opening test1 in append mode...
Enter text for the file: More text for the file!
Here's the contents of the file:
This line written directly to the file...
This text is written to the file!
More text for the file!
***End of file contents.***
Analysis:The
user is again prompted to enter the filename. This time an input file stream
object is created on line 9. That open is tested on line 10, and if the
file already exists, its contents are printed on lines 12 to 16. Note that
if(fin) is synonymous with if (fin.good()).
The input file is then closed, and the same file is reopened, this time in append mode, on line 22. After this open (and every open), the file is tested to ensure that the file was opened properly. Note that if(!fout) is the same as testing if (fout.fail()). The user is then prompted to enter text, and the file is closed again on line 33.
Finally, as in Listing 16.16, the file is reopened in read mode; however, this time fin does not need to be redeclared. It is just reassigned to the same filename. Again the open is tested, on line 36, and if all is well, the contents of the file are printed to the screen and the file is closed for the final time.
DO test each open of a file to ensure that it opened successfully. DO reuse existing ifstream and ofstream objects. DO close all fstream objects when you are done using them. DON'T try to close or reassign cin or cout.
To help the file system distinguish between text and binary files, C++ provides the ios::binary flag. On many systems, this flag is ignored because all data is stored in binary format. On some rather prudish systems, the ios::binary flag is illegal and won't compile!
Binary files can store not only integers and strings, but entire data structures. You can write all the data at one time by using the write() method of fstream.
If you use write(), you can recover the data using read(). Each of these functions expects a pointer to character, however, so you must cast the address of your class to be a pointer to character.
The second argument to these functions is the number of characters to write, which you can determine using sizeof(). Note that what is being written is just the data, not the methods. What is recovered is just data. Listing 16.18 illustrates writing the contents of a class to a file.
Listing 16.18. Writing a class to a file.
1: #include <fstream.h>
2:
3: class Animal
4: {
5: public:
6: Animal(int weight, long days):itsWeight(weight),itsNumberDaysAlive(days){}
7: ~Animal(){}
8:
9: int GetWeight()const { return itsWeight; }
10: void SetWeight(int weight) { itsWeight = weight; }
11:
12: long GetDaysAlive()const { return itsNumberDaysAlive; }
13: void SetDaysAlive(long days) { itsNumberDaysAlive = days; }
14:
15: private:
16: int itsWeight;
17: long itsNumberDaysAlive;
18: };
19:
20: int main() // returns 1 on error
21: {
22: char fileName[80];
23: char buffer[255];
24:
25: cout << "Please enter the file name: ";
26: cin >> fileName;
27: ofstream fout(fileName,ios::binary);
28: if (!fout)
29: {
30: cout << "Unable to open " << fileName << " for writing.\n";
31: return(1);
32: }
33:
34: Animal Bear(50,100);
35: fout.write((char*) &Bear,sizeof Bear);
36:
37: fout.close();
38:
39: ifstream fin(fileName,ios::binary);
40: if (!fin)
41: {
42: cout << "Unable to open " << fileName << " for reading.\n";
43: return(1);
44: }
45:
46: Animal BearTwo(1,1);
47:
48: cout << "BearTwo weight: " << BearTwo.GetWeight() << endl;
49: cout << "BearTwo days: " << BearTwo.GetDaysAlive() << endl;
50:
51: fin.read((char*) &BearTwo, sizeof BearTwo);
52:
53: cout << "BearTwo weight: " << BearTwo.GetWeight() << endl;
54: cout << "BearTwo days: " << BearTwo.GetDaysAlive() << endl;
55: fin.close();
56: return 0;
57: }
Output: Please enter the file name: Animals BearTwo weight: 1 BearTwo days: 1 BearTwo weight: 50 BearTwo days: 100Analysis: On lines 3-18, a stripped-down Animal class is declared. On lines 22-32, a file is created and opened for output in binary mode. An animal whose weight is 50 and who is 100 days old is created on line 34, and its data is written to the file on line 35.
The file is closed on line 37 and reopened for reading in binary mode on line 39. A second animal is created on line 46 whose weight is 1 and who is only one day old. The data from the file is read into the new animal object on line 51, wiping out the existing data and replacing it with the data from the file.
SomeProgram Param1 Param2 Param3These parameters are not passed to main() directly. Instead, every program's main() function is passed two parameters. The first is an integer count of the number of arguments on the command line. The program name itself is counted, so every program has at least one parameter. The example command line shown previously has four. (The name SomeProgram plus the three parameters make a total of four command-line arguments.)
The second parameter passed to main() is an array of pointers to character strings. Because an array name is a constant pointer to the first element of the array, you can declare this argument to be a pointer to a pointer to char, a pointer to an array of char, or an array of arrays of char.
Typically, the first argument is called argc (argument count), but you may call it anything you like. The second argument is often called argv (argument vector), but again this is just a convention.
It is common to test argc to ensure you've received the expected number of arguments, and to use argv to access the strings themselves. Note that argv[0] is the name of the program, and argv[1] is the first parameter to the program, represented as a string. If your program takes two numbers as arguments, you will need to translate these numbers to strings. On Day 21 you will see how to use the standard library conversions. Listing 16.19 illustrates how to use the command-line arguments.
Listing 16.19. Using command-line arguments.
1: #include <iostream.h>
2: int main(int argc, char **argv)
3: {
4: cout << "Received " << argc << " arguments...\n";
5: for (int i=0; i<argc; i++)
6: cout << "argument " << i << ": " << argv[i] << endl;
7: return 0;
8: }
Output: TestProgram Teach Yourself C++ In 21 Days
Received 7 arguments...
argumnet 0: TestProgram.exe
argument 1: Teach
argument 2: Yourself
argument 3: C++
argument 4: In
argument 5: 21
argument 6: Days
Analysis: The function main()
declares two arguments: argc is an integer that contains the count
of command-line arguments, and argv is a pointer to the array
of strings. Each string in the array pointed to by argv is a command-line
argument. Note that argv could just as easily have been declared
as char *argv[] or char argv[][]. It is a matter of programming
style how you declare argv; even though this program declared
it as a pointer to a pointer, array offsets were still used to access the
individual strings.
On line 4, argc is used to print the number of command-line arguments: seven in all, counting the program name itself.
On lines 5 and 6, each of the command-line arguments is printed, passing the null-terminated strings to cout by indexing into the array of strings.
A more common use of command-line arguments is illustrated by modifying Listing 16.18 to take the filename as a command-line argument. This listing does not include the class declaration, which is unchanged.
Listing 16.20. Using command-line arguments.
1: #include <fstream.h>
2: int main(int argc, char *argv[]) // returns 1 on error
3: {
4: if (argc != 2)
5: {
6: cout << "Usage: " << argv[0] << " <filename>" << endl;
7: return(1);
8: }
9:
10: ofstream fout(argv[1],ios::binary);
11: if (!fout)
12: {
13: cout << "Unable to open " << argv[1] << " for writing.\n";
14: return(1);
15: }
16:
17: Animal Bear(50,100);
18: fout.write((char*) &Bear,sizeof Bear);
19:
20: fout.close();
21:
22: ifstream fin(argv[1],ios::binary);
23: if (!fin)
24: {
25: cout << "Unable to open " << argv[1] << " for reading.\n";
26: return(1);
27: }
28:
29: Animal BearTwo(1,1);
30:
31: cout << "BearTwo weight: " << BearTwo.GetWeight() << endl;
32: cout << "BearTwo days: " << BearTwo.GetDaysAlive() << endl;
33:
34: fin.read((char*) &BearTwo, sizeof BearTwo);
35:
36: cout << "BearTwo weight: " << BearTwo.GetWeight() << endl;
37: cout << "BearTwo days: " << BearTwo.GetDaysAlive() << endl;
38: fin.close();
39: return 0;
40: }
Output: BearTwo weight: 1
BearTwo days: 1
BearTwo weight: 50
BearTwo days: 100
Analysis: The declaration of the Animal
class is the same as in Listing 16.18, and so is left out of this example.
This time, however, rather than prompting the user for the filename, command-line
arguments are used. On line 2, main() is declared to take two
parameters: the count of the command-line arguments and a pointer to the
array of command-line argument strings.
On lines 4-8, the program ensures that the expected number of arguments (exactly two) is received. If the user fails to supply a single filename, an error message is printed:
Usage TestProgram <filename>Then the program exits. Note that by using argv[0] rather than hard-coding a program name, you can compile this program to have any name, and this usage statement will work automatically.
On line 10, the program attempts to open the supplied filename for binary output. There is no reason to copy the filename into a local temporary buffer. It can be used directly by accessing argv[1].
This technique is repeated on line 22 when the same file is reopened for input, and is used in the error condition statements when the files cannot be opened, on lines 13 and 25.
There are four standard stream objects created in every program: cout, cin, cerr, and clog. Each of these can be "redirected" by many operating systems.
The istream object cin is used for input, and its most common use is with the overloaded extraction operator (>>). The ostream object cout is used for output, and its most common use is with the overloaded insertion operator (<<).
Each of these objects has a number of other member functions, such as get() and put(). Because the common forms of each of these methods returns a reference to a stream object, it is easy to concatenate each of these operators and functions.
The state of the stream objects can be changed by using manipulators. These can set the formatting and display characteristics and various other attributes of the stream objects.
File I/O can be accomplished by using the fstream classes, which derive from the stream classes. In addition to supporting the normal insertion and extraction operators, these objects also support read() and write() for storing and retrieving large binary objects.
A. In general, it is easier to use the insertion and extraction
operators, and they are preferred when their behavior is what is needed.
In those unusual circumstances when these operators don't do the job (such
as reading in a string of words), the other functions can be used.
Q. What is the difference between cerr and clog?
A. cerr is not buffered. Everything written to cerr is immediately written out. This is fine for errors to be written to the screen, but may have too high a performance cost for writing logs to disk. clog buffers its output, and thus can be more efficient.
Q. Why were streams created if printf() works well?
A. printf() does not support the strong type system of C++, and it does not support user-defined classes.
Q. When would you ever use putback()?
A. When one read operation is used to determine whether or not a character is valid, but a different read operation (perhaps by a different object) needs the character to be in the buffer. This is most often used when parsing a file; for example, the C++ compiler might use putback().
Q. When would you use ignore()?
A. A common use of this is after using get(). Because get() leaves the terminating character in the buffer, it is not uncommon to immediately follow a call to get() with a call to ignore(1,'\n');. Once again, this is often used in parsing.
Q. My friends use printf() in their C++ programs. Can I?
A. Sure. You'll gain some convenience, but you'll pay by sacrificing type safety.
2. What is the extraction operator, and what does it do?
3. What are the three forms of cin.get(), and what are their differences?
4. What is the difference between cin.read() and cin.getline()?
5. What is the default width for outputting a long integer using the insertion operator?
6. What is the return value of the insertion operator?
7. What parameter does the constructor to an ofstream object take?
8. What does the ios::ate argument do?