Overload resolution and universal reference parameters - c++

Question

The following code works and the overloads are found as expected:
struct HasBuzz
{
void buzz() const {}
};
struct NoBuzz {};
template <typename T>
void foo(T const& t)
{
t.buzz();
}
void foo(NoBuzz const&){}
int main()
{
foo(HasBuzz{});
foo(NoBuzz{});
}
However, if I replace the first overload with a "universal reference" version then it no longer works. The correct overload is not found for NoBuzz.
struct HasBuzz
{
void buzz() const {}
};
struct NoBuzz {};
template <typename T>
void foo(T&& t)
{
t.buzz();
}
void foo(NoBuzz const&){}
int main()
{
foo(HasBuzz{});
foo(NoBuzz{}); // error: NoBuzz has no member function buzz
}
What can I do to make it work?

Answer 1

Simple solution
Add an overload that is callable with an rvalue of type NoBuzz.
void foo(NoBuzz const&){ };
void foo(NoBuzz&&) { }; // overload for rvalues
Note: Depending on your actual use case this might not be enough, because if you pass a non-const lvalue type NoBuzz to foo you'd still instantiate the template, since the two NoBuzz overloads doesn't match. At the end of this post is a more complex, but certainly cleaner, solution.
Explanation
template<class T>
void foo (T&&); // (A)
void foo (NoBuzz const&); // (B)
The problem with your snippet is that your template (A) can be instantiated in such a way that it's a better match than your overload (B).
When the compiler sees that you are trying to call a function named foo with an argument which is an rvalue of type NoBuzz, it will look for all functions named foo taking one argument where a NoBuzz would fit.
Let's say it starts of with your template (A), here it sees that T&& is deducable to any reference type (both lvalue, and rvalue), since we are passing an rvalue T = NoBuzz.
With T = NoBuzz the instantiated template would be semantically equivalent to:
void foo (NoBuzz&&); // (C), instantiated overload of template (A)
It will then continue to your overload (B). This overload accepts a const lvalue reference, which can bind to both lvalues and rvalues; but our previous template instantiation (C) can only bind to rvalues.
Since (C) is a better match than (B), binding rvalues to T&& is prefered over U const&, that overload is selected and you get the behavior you describe in your post.
Advanced solution
We can use a technique called SFINAE to conditionally make it impossible to call the template if the type passed doesn't implement .buzz ().
template <typename T>
auto foo(T&& t) -> decltype (t.buzz ())
{
return t.buzz();
}
The above solution uses a lot of new features of C++11, detailed information is available here:
wikipedia.org - C++11 - auto
wikipedia.org - decltype
Conditional overloading with trailing-return-type possible?

Answer 2

refp's answer explains the behavior you're seeing, and offers a possible solution. Another option is to ensure the foo function template does not make it into the candidate set for overload resolution unless T has a member function named buzz().
template <typename T>
auto foo(T&& t)
-> decltype((void)(t.buzz()), void())
{
t.buzz();
}
After making this change the foo(NoBuzz const&) overload will be selected when you pass it an instance of NoBuzz. Live demo
A detailed explanation of what's going on in the decltype expression in the trailing return type can be found here. The only thing I've done differently here is instead of using three subexpressions, with the middle one being void() to prevent a user defined operator, from being selected, I've cast the result of the first expression to void; the intent and result are identical in both cases.