Pre-increment and Post-increment in C/C++

In C/C++, Increment operators are used to increase the value of a variable by 1. This operator is represented by the ++ symbol. The increment operator can either increase the value of the variable by 1 before assigning it to the variable or can increase the value of the variable by 1 after assigning the variable. Thus it can be classified into two types:

• Pre-Increment Operator
• Post-Increment Operator

1) Pre-increment operator: A pre-increment operator is used to increment the value of a variable before using it in an expression. In the Pre-Increment, value is first incremented and then used inside the expression.
Syntax:

 a = ++x;

Here, if the value of ‘x’ is 10 then the value of ‘a’ will be 11 because the value of ‘x’ gets modified before using it in the expression.

CPP

// CPP program to demonstrate pre increment
// operator
#include <iostream>
using namespace std;

// Driver Code
int main()
{
    int x = 10, a;

    a = ++x;

    cout << "Pre Increment Operation";

    // Value of a will  change
    cout << "\na = " << a;

    // Value of x change does not change after execution of a= ++x , because it is already changed;
    cout << "\nx = " << x;

    return 0;
}

Output

Pre Increment Operation
a = 11
x = 11

2) Post-increment operator: A post-increment operator is used to increment the value of the variable after executing the expression completely in which post-increment is used. In the Post-Increment, value is first used in an expression and then incremented. 
Syntax:

 a = x++;

Here, suppose the value of ‘x’ is 10 then the value of variable ‘a’ will be 10 because the old value of ‘x’ is used.

CPP

// CPP program to demonstrate post increment
// operator
#include <iostream>
using namespace std;

// Driver Code
int main()
{
    int x = 10, a;

    a = x++;

    cout << "Post Increment Operation";

    // Value of a will not change
    cout << "\na = " << a;

    // Value of x change after execution of a=x++;
    cout << "\nx = " << x;

    return 0;
}

Output

Post Increment Operation
a = 10
x = 11

Special Case for Post-increment operator: If we assign the post-incremented value to the same variable then the value of that variable will not get incremented i.e. it will remain the same like it was before.

Syntax:

a = a++;

Here, if the value of ‘x’ is 10 then the value of ‘a’ will be 10 because the value of ‘x’ gets assigned to the post-incremented value of ‘x’.

C++

// CPP program to demonstrate special
// case of post increment operator
#include <iostream>
using namespace std;

int main()
{

    int x = 10;

    cout << "Value of x before post-incrementing";

    cout << "\nx = " << x;

    x = x++;

    cout << "\nValue of x after post-incrementing";

    // Value of a will not change
    cout << "\nx = " << x;

    return 0;
}

Output:

Value of x before post-incrementing
x = 10
Value of x after post-incrementing
x = 10

Note: This special case is only with post-increment and post-decrement operators, while the pre-increment and pre-decrement operators works normal in this case.

Evaluating Post and Pre-Increment Together
The precedence of postfix ++ is more than prefix ++ and their associativity is also different. Associativity of prefix ++ is right to left and associativity of postfix ++ is left to right.  

• Associativity of postfix ++ is left to right
• Associativity of prefix ++ is right to left