Question

I’m tracking down a C++ compiler error which I can’t figure out. I’ve reduced it to this code:

#include <boost/config.hpp>
#include <boost/type_traits/remove_reference.hpp>

template<typename T> inline const T bar( T in )
{
    typedef BOOST_DEDUCED_TYPENAME boost::remove_reference<T>::type nonref;
    const nonref* inPtr = &in;
    return *inPtr;
}

class Foo
{
};

int main()
{
    Foo foo;
    const Foo& ref = bar< Foo& >( foo );
}

which results in:

tt.cpp: In function ‘const T bar(T) [with T = Foo&]’:
tt.cpp:19:39:   instantiated from here
tt.cpp:9:13: error: invalid initialization of reference of type ‘Foo&’ from expression of type ‘const nonref {aka const Foo}’

What’s the actual issue here? Why is the const missing in the return value? I need the remove_reference since the actual code requires it.

Answer 1

Applying const to a reference type does nothing. You need to make the template argument const foo &, or else remove the reference and then add back both const and the reference in the function signature itself.

See also When should I use remove_reference and add_reference? particularly the second paragraph.