std::vector reserve() and push_back() is faster than resize() and array index, why?

Question

I was doing a quick performance test on a block of code

void ConvertToFloat( const std::vector< short >& audioBlock, 
                     std::vec         


        

Answer 1

First code writes to out[i] which boils down to begin() + i (ie. an addition). Second code uses push_back, which probably writes immediately to a known pointer equivalent to end() (ie. no addition). You could probably make the first run as fast as the second by using iterators rather than integer indexing.

Edit: also to clarify some other comments: the vector contains floats, and constructing a float is effectively a no-op (the same way declaring "float f;" does not emit code, only tells the compiler to save room for a float on the stack). So I think that any performance difference between resize() and reserve() for a vector of floats is not to do with construction.