CODEWITHSUNDEEP
c++c++11visual-c++
Adrian
Adrian

Reputation: 10911

Is it possible to get a template to use a class and a member function of that class?

I'm trying to make a for_each function for a generic object that uses an size function and an item index function. But I'm having some difficulty with the syntax.

This is what I have so far (starting at line 128):

class base1
{
protected:
    std::vector<int> items;
public:
    base1()
        : items({1,2,3})
    {
    }

    int GetCount() const
    {
    }
};

class base2 : public base1
{
public:
    base2()
        : base1()
    {
    }

    int GetItem(int i) const
    {
        return items[i];
    }
};

class derived : public base2
{
public:
    derived()
        : base2()
    {
    }
};

template <typename CONTAINER, typename CONTAINER_BASE1, typename CONTAINER_BASE2, typename SIZE, typename CONTAINED, typename FUNC>
void for_each(CONTAINER* container, SIZE (CONTAINER_BASE1::*GetSize)() const, CONTAINED (CONTAINER_BASE2::*GetItem)(SIZE) const, FUNC& body)
{
    for (SIZE i = 0; i < container->*GetSize(); ++i)
    {
        body(container->*GetItem(i));
    }
}

void fn()
{
    derived x;
    for_each(&x, &derived::GetCount, &derived::GetItem, [](int i){
        ++i;
    });
}

Right now, I get an error from VC++ 2013 stating:

1>d:\projects\test\test.cpp(169): error C2064: term does not evaluate to a function taking 0 arguments
1>          d:\projects\test\test.cpp(180) : see reference to function template instantiation 'void for_each<derived,base1,base2,int,int,fn::<lambda_862ea397905775f7e094cde6fe9b462c>>(CONTAINER *,SIZE (__thiscall base1::* )(void) const,CONTAINED (__thiscall base2::* )(SIZE) const,FUNC &)' being compiled
1>          with
1>          [
1>              CONTAINER=derived
1>  ,            SIZE=int
1>  ,            CONTAINED=int
1>  ,            FUNC=fn::<lambda_862ea397905775f7e094cde6fe9b462c>
1>          ]
1>d:\projects\test\test.cpp(171): error C2064: term does not evaluate to a function taking 1 arguments

Any ideas as to what the problem is?

Upvotes: 2

Views: 136

Answers (1)

T.C.
T.C.

Reputation: 137315

You have two bugs. You take the functor by non-const lvalue reference - FUNC& body - which doesn't bind to a temporary like a lambda; this was hidden by a terrible MSVC extension that allows such bindings. You should accept the function object by value (the way it is usually done by the standard library), by const lvalue reference (if copying is expensive and/or identity is important), or by forwarding reference (if identity is important and operator() can be non-const).

Second is operator precedence. The postfix function call operator has higher precedence than .* and ->*. container->*GetSize() is container->*(GetSize()); you want (container->*GetSize)().

I'm also not sure about this design. It's probably better to provide a uniform interface, and simply do, e.g., container.size() and container.at(i) than using this tortured system of pointer-to-member-functions.

Upvotes: 6

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