C++ Difference between std::ref(T) and T&?

Question

I have some questions regarding this program:

#include 
#include 
#include 
using namespace std;
template &l         


        

Answer 1

A reference (T& or T&&) is a special element in C++ language. It allows to manipulate an object by reference and has special use cases in the language. For example, you cannot create a standard container to hold references: vector is ill formed and generates a compilation error.

A std::reference_wrapper on the other hand is a C++ object able to hold a reference. As such, you can use it in standard containers.

std::ref is a standard function that returns a std::reference_wrapper on its argument. In the same idea, std::cref returns std::reference_wrapper to a const reference.

One interesting property of a std::reference_wrapper, is that it has an operator T& () const noexcept;. That means that even if it is a true object, it can be automatically converted to the reference that it is holding. So:

• as it is a copy assignable object, it can be used in containers or in other cases where references are not allowed
• thanks to its operator T& () const noexcept;, it can be used anywhere you could use a reference, because it will be automatically converted to it.