Expressions in C++

Anything that evaluates to a value is an expression in C++. An expression is said to return a value. Thus, 3+2; returns the value 5 and so is an expression. All expressions are statements.

The myriad pieces of code that qualify as expressions might surprise you. Here are three examples:

3.2                       // returns the value 3.2
PI                        // float const that returns the value 3.14
SecondsPerMinute          // int const that returns 60

Assuming that PI is a constant equal to 3.14 and SecondsPerMinute is a constant equal to 60, all three of these statements are expressions.

The complicated expression

x = a + b;

not only adds a and b and assigns the result to x, but returns the value of that assignment (the value of x) as well. Thus, this statement is also an expression. Because it is an expression, it can be on the right side of an assignment operator:

y = x = a + b;

This line is evaluated in the following order: Add a to b.

Assign the result of the expression a + b to x.

Assign the result of the assignment expression x = a + b to y.

If a, b, x, and y are all integers, and if a has the value 2 and b has the value 5, both x and y will be assigned the value 7.

Listing 4.1. Evaluating complex expressions.

1: #include <iostream.h>
2: int main()
3: {
4: int a=0, b=0, x=0, y=35;
5: cout << "a: " << a << " b: " << b;
6: cout << " x: " << x << " y: " << y << endl;
7: a = 9;
8: b = 7;
9: y = x = a+b;
10: cout << "a: " << a << " b: " << b;
11: cout << " x: " << x << " y: " << y << endl;
12: return 0;
13: }
Output: a: 0 b: 0 x: 0 y: 35
a: 9 b: 7 x: 16 y: 16

Analysis: On line 4, the four variables are declared and initialized. Their values are printed on lines 5 and 6. On line 7, a is assigned the value 9. One line 8, b is assigned the value 7. On line 9, the values of a and b are summed and the result is assigned to x. This expression (x = a+b) evaluates to a value (the sum of a + b), and that value is in turn assigned to y.