CODEWITHSUNDEEP
c++templatesmetaprogramming
Klaus
Klaus

Reputation: 25603

template to check for existence of overloaded member function

I try to specialize a template if a class has a special member function like this (found here in another example):

template <typename T>
class has_begin
{
    typedef char one;
    typedef long two;

    template <typename C> static one test( decltype( &C::AnyFunc) ) ;
    template <typename C> static two test(...);

public:
    enum { value = sizeof(test<T>(0)) == sizeof(char) };
    enum { Yes = sizeof(has_begin<T>::test<T>(0)) == 1 };
    enum { No = !Yes };
};

This works well until AnyFunc is overloaded:

class B : public vector<int>
{
public:
    void AnyFunc() const;
    void AnyFunc();
};

How can I rewrite my test code to get a "Yes" from my template?

Upvotes: 5

Views: 177

Answers (2)

ecatmur
ecatmur

Reputation: 157334

The use of an overloaded function name without arguments (13.4p1) must be resolved to a single overload (13.4p4), otherwise substitution failure will occur.

If you are testing for the existence of a member function then you should know the arguments you plan to call it with:

    template <typename C> static one test(
        typename std::add_pointer<decltype(std::declval<C>().AnyFunc())>::type);

In general, you can use a variadic template and a pattern similar to result_of:

    template <typename C, typename... Args> static one test(
        typename std::add_pointer<decltype(
            std::declval<C>(std::declval<Args>()...).AnyFunc())>::type);

Using add_pointer allows this to work with function return types that are not allowable as function argument types (e.g. void).

Upvotes: 2

PiotrNycz
PiotrNycz

Reputation: 24392

Found the version which works:

    template <typename C> static one test( decltype(((C*)0)->AnyFunc())* ) ;

If you want to verify that object has const function, use this:

    template <typename C> static one test( decltype(((const C*)0)->AnyFunc())* ) ;

this version will not detect function with arguments:

class B : public std::vector<int>
{
public:
    //void AnyFunc() const;
    //void AnyFunc();
    int AnyFunc(int);
};

Upvotes: 1

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