CODEWITHSUNDEEP
c++templates
Paster Keller
Paster Keller

Reputation: 33

Template function parameter and template member function parameter

I have this problem:

template <void (*F)(int)> struct FunctionWrapper // This compiles and works
{
    static void call_it() 
    { 
        F(0); 
    }
};

class MyClass
{
public:
    static void callMe(int k)
    {
    }
};

template <void (MyClass::*F)(int)> struct FunctionWrapper // Error - F incompatible with declaration
{
    static void call_it() 
    { 
        MyClass::F(0); 
    }
};

Why I can use a function pointer (compile time constant) but not a class-member (even static) one?

Upvotes: 2

Views: 94

Answers (2)

Andy Prowl
Andy Prowl

Reputation: 126432

Why I can use a function pointer (compile time constant) but not a class-member (even static) one?

You can use a pointer to a static function as a regular function pointer. For instance, the following works:

template <void (*F)(int)> struct FunctionWrapper
{
    static void call_it()
    {
        F(0);
    }
};

class MyClass
{
public:
    static void callMe(int k)
    {
    }
};

int main()
{
    FunctionWrapper<&MyClass::callMe> obj;
    obj.call_it();
}

Concerning your attempt, this is what Paragraph 14.1/4 of the C++11 Standard mandates:

A non-type template-parameter shall have one of the following (optionally cv-qualified) types:

— integral or enumeration type,

— pointer to object or pointer to function,

— lvalue reference to object or lvalue reference to function,

— pointer to member,

— std::nullptr_t.

A pointer to member function (even if static) is not an option for non-type template arguments.

Upvotes: 0

Alex Chamberlain
Alex Chamberlain

Reputation: 4207

A static function has the same type signature as a normal function; it is not really a member function.

Upvotes: 2

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