C++ Primer Plus

The interesting thing is that you type an M, not the corresponding character code, 77. Also the program prints an M, not 77. Yet if you peer into memory, you find that 77 is the value stored in the ch variable. The magic, such as it is, lies not in the char type but in cin and cout. These worthy facilities make conversions on your behalf. On input, cin converts the keystroke input M to the value 77. On output, cout converts the value 77 to the displayed character M; cin and cout are guided by the type of variable. If you place the same value 77 into an int variable, cout displays it as 77. (That is, cout displays two 7 characters.) Listing 3.6 illustrates this point. It also shows how to write a character literal in C++: Enclose the character within two single quotation marks, as in 'M'. (Note that the example doesn’t use double quotation marks. C++ uses single quotation marks for a character and double quotation marks for a string. The cout object can handle either, but, as Chapter 4 discusses, the two are quite different from one another.) Finally, the program introduces a cout feature, the cout.put() function, which displays a single character.

Listing 3.6. morechar.cpp

// morechar.cpp -- the char type and int type contrasted

#include

int main()

{

    using namespace std;

    char ch = 'M';       // assign ASCII code for M to ch

    int i = ch;          // store same code in an int

    cout << "The ASCII code for " << ch << " is " << i << endl;

    cout << "Add one to the character code:" << endl;

    ch = ch + 1;          // change character code in ch

    i = ch;               // save new character code in i

    cout << "The ASCII code for " << ch << " is " << i << endl;

    // using the cout.put() member function to display a char

    cout << "Displaying char ch using cout.put(ch): ";

    cout.put(ch);

    // using cout.put() to display a char constant

    cout.put('!');

    cout << endl << "Done" << endl;

    return 0;

}

Here is the output from the program in Listing 3.6:

The ASCII code for M is 77

Add one to the character code:

The ASCII code for N is 78

Displaying char ch using cout.put(ch): N!

Done

Program Notes

In the program in Listing 3.6, the notation 'M' represents the numeric code for the M character, so initializing the char variable ch to 'M' sets ch to the value 77. The program then assigns the identical value to the int variable i, so both ch and i have the value 77. Next, cout displays ch as M and i as 77. As previously stated, a value’s type guides cout as it chooses how to display that value—just another example of smart objects.

Because ch is really an integer, you can apply integer operations to it, such as adding 1. This changes the value of ch to 78. The program then resets i to the new value. (Equivalently, you can simply add 1 to i.) Again, cout displays the char version of that value as a character and the int version as a number.

The fact that C++ represents characters as integers is a genuine convenience that makes it easy to manipulate character values. You don’t have to use awkward conversion functions to convert characters to ASCII and back.

Even digits entered via the keyboard are read as characters. Consider the following sequence:

char ch;

cin >> ch;

If you type 5 and Enter, this code reads the 5 character and stores the character code for the 5 character (53 in ASCII) in ch. Now consider this code:

int n;

cin >> n;

The same input results in the program reading the 5 character and running a routine converting the character to the corresponding numeric value of 5, which gets stored in n.

Finally, the program uses the cout.put() function to display both c and a character constant.

A Member Function: cout.put()

Just what is cout.put(), and why does it have a period in its name? The cout.put() function is your first example of an important C++ OOP concept, the member function. Remember that a class defines how to represent data and how to manipulate it. A member function belongs to a class and describes a method for manipulating class data. The ostream class, for example, has a put() member function that is designed to output characters. You can use a member function only with a particular object of that class, such as the cout object, in this case. To use a class member function with an object such as cout, you use a period to combine the object name (cout) with the function name (put()). The period is called the membership operator. The notation cout.put() means to use the class member function put() with the class object cout. You’ll learn about this in greater detail when you reach classes in Chapter 10, “Objects and Classes.” Now the only classes you have are the istream and ostream classes, and you can experiment with their member functions to get more comfortable with the concept.