c++ function template specialisation

Question

Given this code:

class X
{
public:
    template< typename T >
    void func( const T & v );
};

template<>
void X::func< int >( const int & v )
{
}

template<>
void X::func< char * >( const char * & v )       // 16
{
}

When I compile it I get the following error.

test.cpp:16: error: template-id 'func<char*>' for 'void X::func(const char*&)' does not match any template declaration

Can anyone shed any light on this?

Answer 1

If you change the declaration:

template<> void X::func< char * >( const char * & v )

to:

template<> void X::func< char * >( char * const & v )

This would work perfectly fine. Why does it happen? Because while const sometype is perfectly acceptable, it's only an alternative notation for sometype const. So, your const modifier is not applied to the basic type (char), but to the pointer, making "constant pointer to non-constant char" a valid type. If that's not what you want, you'll have to correct your basic template.

Finally, here's some interesting reading for you about why overloading templates is generally better than specializing them.

Answer 2

The reason you face this error is because you write const before the type. Although this is common practise, it is not conducive to understanding how const/volatile-qualifiers (cv-qualifier) work.

In this case const T when T is char* doesn't mean const char*. It rather means char* const because T is char* and no matter which side of T you put const it behaves as if const is on the right of T, that is, the pointer itself that is going to be const not the type pointed too.

It is easy to avoid this type of confusion if you make it a rule to always put const or volatile on the right of the type. For example, it makes it straightforward to mentally expand T const when T is char* to char* const.

This is the reason in boost sources you see cv-qualifiers after the type, not before.

Answer 3

Move const before &

template<> 
void X::func< char * >( char * const & v ) 
{ 
}