How to use enable_if to enable member functions based on template parameter of class

问题

In code:

template<class T>
struct is_builtin
{
    enum {value = 0};
};

template<>
struct is_builtin<char>
{
    enum {value = 1};
};

template<>
struct is_builtin<int>
{
    enum {value = 1};
};

template<>
struct is_builtin<double>
{
    enum {value = 1};
};

template<class T>
struct My
{
    typename enable_if<is_builtin<T>::value,void>::type f(T arg)
    {
        std::cout << "Built-in as a param.\n";
    }


    typename enable_if<!is_builtin<T>::value,void>::type f(T arg)
    {
        std::cout << "Non - built-in as a param.\n";
    }
};

struct A
{
};

int main()
{
    A a;
    My<int> m;
    My<A> ma;
    m.f(1);
    ma.f(a);
    return 0;
}

I'm getting an error:

error C2039: 'type' : is not a member of 'std::tr1::enable_if<_Test,_Type>'

Obviously I don't understand how to use enable_if. What I was thinking was that I can enable one or the second one member function from a set of member functions during compilation time but it does not work. Could anyone please explain to me how to do it correctly?
Edited
What I really can't understand is why isn't there typedef in one of those def. Compiler cannot find it and it wont compile it.

回答1:

You can't use class template parameters to get SFINAE for member functions.

You either need to

make the member function a member function template instead and use enable_if on the member function template's template parameters or
move the member function f into a policy class and specialize the class template using enable_if.

回答2:

You can fix your code by using modified enable_if

template < typename T >
struct __Conflict {};

template <bool B, class T = void>
struct __enable_if { typedef __Conflict<T> type; };

template <class T>
struct __enable_if<true, T> { typedef T type; };

Example of usage:

template <typename T>
class Lazy
{
public:
    void _ctor(bool b);
    void _ctor(typename __enable_if<!std::is_same<T, bool>::value, T>::type);
};

template <typename T>
void Lazy<T>::_ctor(bool b)
{
    std::cout << "bool " << b << std::endl;
};

template <typename T>
void Lazy<T>::_ctor(typename __enable_if<!std::is_same<T, bool>::value, T>::type t)
{
    std::cout << "T " << t << std::endl;
};

int main(int argc, char **argv)
{
    Lazy<int> i;
    i._ctor(10);
    i._ctor(true);

    Lazy<bool> b;
    b._ctor(true);

    return 0;
}

回答3:

Here's how it works (note that for convenience I replaced your is_builtin trait with std::is_arithmetic and used further C++11 stuff, but it works any way):

template<class T>
struct My
{
    template<typename T_ = T, std::enable_if_t<std::is_arithmetic<T_>::value>* = nullptr>
    void f(T_ arg)
    {
        std::cout << "Built-in as a param.\n";
    }

    template<typename T_ = T, std::enable_if_t<!std::is_arithmetic<T_>::value>* = nullptr>
    void f(T_ arg)
    {
        std::cout << "Non - built-in as a param.\n";
    }
};

DEMO

The crucial part is to bring the template parameter into the immediate context by using a default function template parameter T_ which equals the class template parameter T. For more details, see this question.

回答4:

enable_if expects a metafunction. To use a bool you need enable_if_c. I'm surprised you're not getting errors explaining THAT problem.

You can fix your metafunction by declaring a 'type' typedef inside that is simply itself. Then you can use boost::enable_if<is_builtin<T>>::type

来源：https://stackoverflow.com/questions/4158727/how-to-use-enable-if-to-enable-member-functions-based-on-template-parameter-of-c

标签

c++

templates

metaprogramming

sfinae