Variadic template with member function pointer as parameter

Question

I've watched several examples online and I don't understand why this doesn't compile.. what I'm trying to do is passed a member function of say class Object, to a class that has a vector of said Objects, and have a function with templated arguments as parameters be called... example:

template <typename ...Args_t>
bool objectsDo(bool (Object::*func)(Args_t...), Args_t&&... args) {
    for (int i = 0 ; i < OBJECTS ; ++i) {
        if (!m_objects[i]->*func(std::forward<Args_t>(args)...)) {
            return false;
        }
    }
    return true;
}

but every function I try, even a parameterless one I get:

error: no instance of function template "objectsDo" matches the argument list
            argument types are: (bool ())
                objectsDo(&Object::close);

where my usage is:

            objectsDo(&Object::close);

EDIT: as suggested by Columbo, I am now sending the address to the function, but still I get errors when sending with parameters, such as :

  error: no instance of function template "objectsDo" matches the argument list
            argument types are: (bool (Object::*)(int, char), int, char)

What compiler are you using? And you're missing a & before Object::close — Columbo
– Columbo, Commented Nov 9, 2014 at 11:57
I believe in c++ you don't need that & anymore and it defaults to it, hmm using icc actually but I've done forwarding of variadic templates to c'tors before, I believe it's something with the member fucntion. — Alon
– Alon, Commented Nov 9, 2014 at 11:59
It's not about what you believe. Did you try it with an ampersand? Compiles fine for me — Columbo
– Columbo, Commented Nov 9, 2014 at 12:02

Columbo · Accepted Answer · 2014-11-09 12:16:04Z

I assume you call the function like this:

int main()
{
    int i; char c;
    objectsDo(&Object::close, i, c);
}

The problem is that the template arguments are deduced inconsequently:

template <typename ...Args_t>
bool objectsDo(bool (Object::*func)(Args_t...), Args_t&&... args)

Args_t is deduced as int, char for the first parameter and int&, char& for the second. That is because of the internal working of universal references: It works with reference collapsing.

Use another parameter-pack for the trailing parameters :

template <typename ...Args_t, typename... Args>
bool objectsDo(bool (Object::*func)(Args_t...), Args&&... args)
{ /* […] */ }

Or make the trailing parameter a non-deduced context:

template <typename T> struct identity {using type = T;};
template <typename T>
using identity_t = typename identity<T>::type;

template <typename ...Args_t>
bool objectsDo(bool (Object::*func)(Args_t...), identity_t<Args_t>... args){
    // […]
}

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