function composition in C++ / C++11

Question

I am currently coding some cryptographic algorithms in C++11 that require a lot of function compositions. There are 2 types of composition I have to deal with :

Answer 1

You haven't shown the body of F, but if you can modify it so that it mutates the input to form the output then change the signature to:

void F(std::vector& x);

Thereafter you can implement Fn as:

void Fn(std::vector& x, size_t n)
{
    for (size_t i = 0; i < n; i++)
        F(x);
}

The compiler will unroll the loop for you if it is more efficient, but even if it doesn't an increment/compare of a local variable will be orders of magnitude faster than calling F.

You can then explcitly copy-construct new vectors when you actually want to make a copy:

vector v1 = ...;
vector v2 = v1; // explicitly take copy
Fn(v2,10);