Pre-increment vs. Post-increment in C++

Summary

This article describes the similarities and differences between pre-increment and post-increment in C++ and the requirements that dictate which one you should use.

What’s the same

Whether we pre-increment (++x) or post-increment (x++), we are adding one to the value of the variable (x += 1).

int x = 5;
++x;
std::cout << x << std::endl;  // prints "6"
x++;
std::cout << x << std::endl;  // prints "7"

What’s different

The only difference between the two operations is the return type.

Function Signatures

As the function signature for pre-increment (++x) reveals, it adds one to the supplied object and returns a reference to this modified object.

// Function signature for pre-increment operator in custom class.
Example & operator++();

In contrast, the function signature for post-increment (x++) returns by value.  It copies the value before modification, adds one to the supplied object, and returns the original copied value as an unnamed temporary object.

// Function signature for post-increment operator in custom class.
//  The dummy int argument only denotes post-increment.
Example operator++(int);

Your intent decides which operation you should use

  • If you don’t care about the return type because you aren’t capturing it, you should stick to the prefix form (++x) because it doesn’t waste time creating a temporary object that you won’t use.
  • If you want to increment and immediately use the object you should use pre-increment (++x).
  • If you want to increment an object but retrieve a copy of its value before it was modified you should use post-increment (x++).

Conclusion

Stick to pre-increment (++x) in almost all situations with the rare exception being when you want a copy of the variable before it was incremented.

Examples

int a = 5;
int b = 5;

int c = a++;  // c == 5
int d = ++b;  // d == 6

int & e = ++a; // e now references a which has been incremented to 7
int & f = b++; // compile time error - non const reference to temporary

About Mark Turney

I am a software developer who designs, develops, and maintains cross-platform applications.