Prefix and Postfix Operators

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Postfix Increment and Decrement Operators

The postfix increment or decrement operators act on a single operand, and appear to the right side. Operators that act on only one operand are called unary operators.

The postfix increment operator is used to increment the value of its operand by 1 unit, and the postfix decrement operator is the decrement counterpart that decreases the value by 1 unit. You might have noticed, that I have written "1 unit" and not "1". There is a slight difference of behaviour when the operator operates on a value, and when it operates on an address. The primitive types like an int, char are value types, and in their case the value increases or decreases by 1. Here's the code-

int main()
{
    int x = 0;
 
    char c = 'A';
 
    x++;
 
    c++;
 
    // displays 1
    cout << x << endl;
 
    // displays B
    cout << c << endl;
 
    return 0;
}

If the same operator is applied to an address, the increase or decrease is equal to the number of bytes, and not 1. For example, if the operand is an address of a "short" type, then the increment is of 2 because a short occupies two bytes. If a type such as an int takes 4 bytes, then the increment will cause an increment of 4 units. Have a look at this code.

int main()
{
    int x = 0;
 
    int *p = &x;
 
    // displays the address
    cout << (int)p << endl;
 
    p++;
 
    // shows an increment of
    // 4 on my machine
    cout << (int)p << endl;
 
    return 0;
}

If the postfix increment is applied to a boolean type, the result is true. The postfix decrement cannot be applied to a boolean type; it is a compilation error.

int main()
{
    bool b1 = false;
 
    b1++;
 
    // true or 1
    cout << b1 << endl;
 
    // ERROR,
    // b1--;
 
    return 0;
}

It is not possible to apply postfix or prefix operators to an enum type.

int main()
{
    enum eWeekDays { MON, TUE, WED, THU, FRI };
 
    // ERROR
    eWeekDays++;
 
    return 0;
}

Prefix Increment and Decrement Operators

Prefix operators appear to the left side of their operand. They also increment or decrement the value by 1 unit.

Larger Expressions

When an expression is large, the postfix and prefix operators behave differently. In the code below, the postfix operator appears in a larger expression. In such cases the value is evaluated before the increment occurs. So y gets 8 + 0 = 8. x++ gives 0, and the increment of x in x++ occurs after.

int main()
{
    int x = 0;
 
    int y = (8 + x++);
 
    // displays 8
    cout << y << endl;
 
    return 0;
}

The behaviour of prefix increment in a larger expression can be seen in the code below. Here the value of x is increment before returning the value of x. So y gets 8 + 1 = 9.

int main()
{
    int x = 0;
 
    int y = (8 + ++x);
 
    // displays 9
    cout << y << endl;
 
    return 0;
}

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