Why are cv-qualifiers in template parameters ignored?

Question

I had some code that was failing to compile, which amounts to something like what's shown below. After some digging around, I came across paragraph 14.1 note 5, which states:

The top-level cv-qualifiers on the template-parameter are ignored when determining its type.

My code looks like this:

#include <iostream>
#include <typeinfo>

class Bar {};

template<class T>
void Func(T t)
{
   std::cout << typeid(T).name() << "\n";
}

template<class T>
void Func(const T& t)
  {
     std::cout << "const ref : " << typeid(T).name() << "\n";
   }


 int main()
  {
    Bar bar;
    const Bar& constBar = bar;

    Func(constBar);

    return 0;
 }

It gives this compilation error:

In function 'int main()'  
error: call of overloaded 'Func(const Bar&)' is ambiguous

Can someone comment on the reasoning behind the this rule in the standard?

Answer 1

The problem with your code is that the function call is ambiguous. The const Bar & can match either the value or the const reference. G++ says:

xx.cpp:24: error: call of overloaded 'Func(const Bar&)' is ambiguous

This has nothing specifically to do with templates - you would get the same error if you overloaded a non-template function.

And as people have told you here time after time, you will not learn C++ by reading the Standard.

Answer 2

The call is ambiguous because anything can match T or const T &.

Just try Func(0);: it will give the same error message.

Answer 3

As you could have found easily yourself, this has nothing to do with templates. This

class Bar {};

void Func(Bar) {}
void Func(const Bar&) {}

int main()
{
    Bar bar;
    const Bar& constBar = bar;

    Func(bar);
    Func(constBar);

    return 0;
}

gives the same errors.