What is a callable object in C++?

Question

I'm currently studying boost threads. And I came across that the thread class has a constructor that accepts callable objects. What are callable objects?

class CallableClass { private:     // Number of iterations     int m_iterations;  public:      // Default constructor     CallableClass()     {         m_iterations=10;     }      // Constructor with number of iterations     CallableClass(int iterations)     {         m_iterations=iterations;     }      // Copy constructor     CallableClass(const CallableClass& source)     {         m_iterations=source.m_iterations;     }      // Destructor     ~CallableClass()     {     }      // Assignment operator     CallableClass& operator = (const CallableClass& source)     {         m_iterations=source.m_iterations;         return *this;     }      // Static function called by thread     static void StaticFunction()     {         for (int i=0; i < 10; i++)  // Hard-coded upper limit         {             cout<<i<<"Do something in parallel (Static function)."<<endl;             boost::this_thread::yield(); // 'yield' discussed in section 18.6         }     }      // Operator() called by the thread     void operator () ()     {         for (int i=0; i<m_iterations; i++)         {             cout<<i<<" - Do something in parallel (operator() )."<<endl;             boost::this_thread::yield(); // 'yield' discussed in section 18.6         }     }  }; 

How does this become a callable object? Is it because of the operator overloaded or the constructor or something else?

Answer 1

A callable object is something that can be called like a function, with the syntax object() or object(args); that is, a function pointer, or an object of a class type that overloads operator().

The overload of operator() in your class makes it callable.

Answer 2

There are two steps here. In the C++ Standard, a "function object" is an object that can appear on the left-hand side of a parenthesized argument list, i.e, a pointer to function or an object whose type has one or more operator()s. The term "callable object" is broader: it also includes pointers to members (which can't be called with the normal function call syntax). Callable objects are the things that can be passed to std::bind etc. See 20.8.1 [func.def] and 20.8[function.objects]/1.