ralzaul ralzaul - 9 days ago 5
C++ Question

Why post-increment needs to make a copy while pre-increment does not

I know this issue has been discussed several times , but I could not find a post which explains why a copy needs to be made in case of a post-increment operation.

Quoting from a stackoverflow reply:

int j = i++; // j will contain i, i will be incremented.
int j = ++i; // i will be incremented, and j will contain i+1.


Which perfectly makes sense when the definition of post/pre increment is considered. Many times when comparing the performance of pre/post increment it is said that post increment needs to make a copy, increment it and return the copy while pre-increment just increases value and does not create a copy.

Although performance has been compared in tens of posts, I really could not find any explanation on why a copy has to be made in the case of post-increment. Why doesn't it return the old value and then increments the value of the variable by one(or however way operator is overloaded), rather than creating a new object and return that one.

Answer

The difference is someval++ returns what someval is before the increment and to do that you need remember what someval is with a copy. How else would you return the original value while updating it if the original value wasn't stored somewhere?

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