How to correctly use std::reference_wrappers

Question

I am trying to understand std::reference_wrapper.

The following code shows that the reference wrapper does not behave exactly like a reference.

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Answer 1

Class std::reference_wrapper implements an implicit converting operator to T&:

operator T& () const noexcept;

and a more explicit getter:

T& get() const noexcept;

The implicit operator is called when a T (or T&) is required. For instance

void f(some_type x);
// ...
std::reference_wrapper x;
some_type y = x; // the implicit operator is called
f(x);            // the implicit operator is called and the result goes to f.

However, sometimes a T is not necessarily expected and, in this case, you must use get. This happens, mostly, in automatic type deduction contexts. For instance,

template 
g(U x);
// ...
std::reference_wrapper x;
auto y = x; // the type of y is std::reference_wrapper
g(x);       // U = std::reference_wrapper

To get some_type instead of std::reference_wrapper above you should do

auto y = x.get(); // the type of y is some_type
g(x.get());       // U = some_type

Alternativelly the last line above could be replaced by g(x);. However, for templatized operators (e.g. ostream::operator <<()) I believe you can't explicit the type.