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

out.resize( audioBlock.size() );

Since out's size (= 0) is lesser than audioBlock.size() , additional elements are created and appended to the end of the out. This creates the new elements by calling their default constructor.

Reserve only allocates the memory.