基于编译时常量禁用/隐藏模板中的函数

Question

Is it possible to conditionally hide or disable functions in a template class using compile time constants?

Imagine the following class:

template<size_t M, size_t N>
class MyClassT
{
    // I only want this function available if M == N, otherwise it is illegal to call
    static MyClassT<M, N> SomeFunc()
    {
        ...
    }
}


MyClassT<2,2>::SomeFunc(); // Fine
MyClassT<3,2>::SomeFunc(); // Shouldn't even compile

Answer 1

Use partial specialization, and inheritance:

// Factor common code in a base class
template <size_t n, size_t m>
class MyClassTBase
{
    // Put here the methods which must appear
    // in MyClassT independantly of n, m
};

// General case: no extra methods
template <size_t n, size_t m>
class MyClassT : MyClassTBase<n, m>
{};

// Special case: one extra method (you can add more here)
template <size_t n>
class MyClassT<n, n> : MyClassTBase<n, n>
{
    static MyClassT<n, n> SomeFunc()
    {
        ...
    }
};

Another option is to use SFINAE: std::enable_if or a variant thereof:

template <size_t n, size_t m>
class MyClassT
{
    template <typename EnableIf = char>
    static MyClassT<n, m> SomeFunc(EnableIf (*)[n == m] = 0)
    {
        ...
    }
};

the more verbose alternative (but less surprising if you don't know about SFINAE and pointer to arrays) being

template <size_t n, size_t m>
class MyClassT
{
    template <typename Dummy = char>
    static MyClassT<n, m>
    SomeFunc(typename std::enable_if<n == m, Dummy>::type * = 0)
    {
        ...
    }
};

Generally, I prefer SFINAE approaches where there is one or two member functions to enable or disable. As soon as it gets more complex than this, I prefer the partial specialization technique.

EDIT: The SFINAE code was wrong, since there were not template functions. Corrected.

Answer 2

The ideal way to provide specializations is to use template specializations. You can move all the basic functionality to a base class:

template< size_t M, size_t N >
class basic_class{ ... };

template< size_t M, size_t N >
class my_class : basic_class< M, N > { ... };


template< size_t M >
class my_class< M, M > : basic_class< M, N > { ... };

Alternatively, you could add a dummy template parameter and use enable_if :

template< typename Dummy = int >
typename std::enable_if< M == N, my_class >::type some_func( Dummy* = 0 );