Pointer to member function syntax

Question

I'm trying to wrap my head around pointers to member functions and am stuck with this example:

#include <iostream>

class TestClass {
  public:
    void TestMethod(int, int) const;
};

void TestClass::TestMethod(int x, int y) const {
  std::cout << x << " + " << y << " = " << x + y << std::endl;
}

int main() {
  void (TestClass::*func)(int, int) const;
  func = TestClass::TestMethod;

  TestClass tc;
  tc.func(10, 20);

  return 0;
}

What I think the code should do:

• In the first line of main I declare a pointer to a member function of class TestClass, which returns nothing/takes two int's and is declared const, called func.
• The second line assigns the member-function TestMethod (of class TestClass) to func, which satisfies these criteria.
• The forth line creates an TestClass-object called tc.
• The fifth line tries to call the member function pointed to by func on the TestClass-object tc.

I get two compilation errors:

• Instead of assigning TestClass::TestMethod to func. The compiler tries to call TestClass::TestMethod (even though it's not static, therefore throws an error):

  testPointerToMemberFunc.cpp:14:21: error: call to non-static member function without an object argument
    func = TestClass::TestMethod;
           ~~~~~~~~~~~^~~~~~~~~~
  

• The compiler tries to call a function named func on tc, instead of the function, pointed to by func. To me this seems, like func isn't declared the right way (as a pointer to a member function):

  testPointerToMemberFunc.cpp:17:6: error: no member named 'func' in 'TestClass'
    tc.func(10, 20);
    ~~ ^
  

What am I doing wrong?

Answer 1

Simple syntax subtleties.

func = &TestClass::TestMethod;
//     ^ Missing ampersand to form a pointer-to-member

TestClass tc;
(tc.*func)(10, 20);
// ^^ Using the dot-star operator to dereference the pointer-to-member,
// while minding its awkward precedence with respect to the call operator.