Consider the following code snippet.
 
    int x = 10;
    std::reference_wrapper<int> rx( x );
    std::swap( rx, x );
 
The compiler will issue an error because either rx can be implicitly converted to the type int or x can be implicitly converted to the type std::reference_wrapper<int>.
 
So a pair of questions are arising.
 
Should the constructor of the class template std::reference_wrapper be explicit?
What is the reason that the constructor is implicit? Is that so necessary?
 
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