Pass object to a function without wrapping it into std::ref, while argument is specified as a const reference

Question

I have the following c++ code(just a simple example for a question)

#include <iostream>
#include <string>
#include <vector>

double get_first(const std::vector<double>& vec) {
    return vec[0];
}

int main()
{
  std::vector<double> some_vec = {1, 2};
  std::cout << get_first(some_vec);
}

So here parameter of the function get_first is const reference,

while I pass whole vector some_vec instead of wrapping it into std::ref. Does c++ copies full object here?

Answer 1

std::ref is not for that. It is used to convert an existing reference to an object, used where a reference cannot be accepted, like within a std::vector<>. The idea is that references cannot be reassigned or set to null, so stuff in STL containers like moving etc wouldn't work with a raw reference. Most likely it internally converts the wrapped reference into a pointer.

In your example, the paramter is automatically passed as a reference.