Question:
Can you tell me which of these implementations is more efficient and why, assumming this function cannot be inlined and is called often?

First option:

int even( int &val){if (val%2==0)return val*2;else return val;}

Second option:

int even(const int &val){if (val%2==0)return val*2;else return val;}

Answer:
I see no reason why there’d be any performance difference between the two. In fact, I compiled them under VC++ 6.0 and the generated code was identical in both versions.

An argument with the const attribute specifies that a given routine will not modify that argument. The compiler then allows you to pass arguments that cannot be modified (declared as const). This is enforced by the compiler at compile time and does not affect the run-time code.