How to pass function in class template without use of auto?

January 23, 2024

I have following function:

bool comp(Arm l, Arm r) {
    return l.len < r.len;
}

Please, Assume Arm is appropriately declared struct.

I have following class:

template<typename T, typename F>
class Myclass {
public:
    Arm v[10];
    bool Myfunc(int i) {
        int l = i, r = i + 1;
        return F(v[l], v[s]);
    }
};

When creating object:

Myclass<Arm, decltype(comp)> obj;

I’m getting error: expression list treated as compound expression in functional cast [-fpermissive]

I have tried passing address, I have tried creating function pointer and passing that. I wish to get it worked for C++ 11, Using C++17 I got it worked using:

template<typename T, auto F>
class Myclass {
...

And creating object by:

Myclass<Arm, comp> obj;

And this was working fine.

Example in C++ 11 giving error:

struct Arm {
    int len;
};

template<typename T, typename F>
class Myclass {
public:
    Arm v[10];
    bool Myfunc(int i) {
        int l = i, r = i + 1;
        return F(v[l], v[r]);
    }
};

bool comp(Arm l, Arm r) {
  return l.len < r.len;
}

int main() {
    Myclass<Arm, decltype(comp)> obj;
    obj.Myfunc(2);
    return 0;
}

>Solution :

Prior to C++17 (which can use auto for the template parameter type), it’s usually simplest to create a functor, like a lambda or a struct with an operator() overload:

struct comp {
    bool operator()(Arm l, Arm r) const { return l.len < r.len; }
};

With that, you need to instantiate the functor when needing it (or store an instance in your class):

template<typename T, typename F>
class Myclass {
public:
    // ...
    bool Myfunc(int i) {
        int l = i, r = i + 1;
        return F{}(v[l], v[r]);
//              ^^
//     instantiate the functor
    }
};

And since comp is now a type, you can instantiate your Myclass like so:

Myclass<Arm, comp> obj;

Demo

Without a functor, you could specify the function signature in the template parameter list to make F a function pointer:

struct Arm {
    int len;
};

template <typename T, bool(*F)(T, T)>
//                    ^^^^^^^^^^^^^^
//            the function signature it accepts
class Myclass {
public:
    Arm v[10];
    bool Myfunc(int i) {
        int l = i, r = i + 1;
        return F(v[l], v[r]); // no instantiation needed
    }
};

bool comp(Arm l, Arm r) { return l.len < r.len; }

int main() {
    Myclass<Arm, comp> obj;
    obj.Myfunc(2);
}

Demo