“未解决的重载函数类型”错误，并将运算符作为函数参数传递

Question

I'd would like to make a method that would perform a certain task plus make a simple calculation from within, like addition, subtraction or multiplication. Correct me if I'm wrong, it seems that I cannot pass the operator of such an operation directly and I need to define an intermediary method (like one in my example called operator_add ). I try to accomplish my task with the following code :

struct A {
  typedef int T;
  /* (...) */
  struct element {
    /* (...) */
    inline T value() const { /* something simple */ };
    element& comp_assign( const T r, T (*operation)(const T, const T) ) { // line # 40
      T res = operation( value(), r );
      return modif_aux( res );
    } /* (...) */
    inline T operator_add( const T a, const T b ) { return a + b; }
    inline element& operator+=( const T r ) { return comp_assign( r, operator_add ); } // line # 64
  };
};

But I get the following error :

A.h:64: error: no matching function for call to ‘A::element::comp_assign(const int&, <unresolved overloaded function type>)’
A.h:40: note: candidates are: A::element& A::element::comp_assign(int, int (*)(int, int))

Answer 1

operator_add is a member function and so you can't use a normal function pointer to refer to it. Making it a static function or free function would fix this, although I'll recommend using templates instead as then it can use any callable object:

template<typename Operation>
element& comp_assign( const T r, Operation operation) { // line # 40
  T res = operation( value(), r );
  return modif_aux( res );
}

inline element& operator+=( const T r ) { return comp_assign( r, std::plus<T>() ); }