指向成员函数语法的指针 [英] Pointer to member function syntax

问题描述

我试图把头放在成员函数的指针周围，并被这个示例所困扰:

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;
}

我认为代码应该做什么:

What I think the code should do:

• 在main的第一行中，我声明一个指向class TestClass成员函数的指针，该成员函数不返回任何内容/采用两个int并被声明为const，称为 func .
• 第二行将成员函数TestMethod(属于类TestClass)分配给func，这满足这些条件.
• 第四行创建一个名为 tc 的TestClass对象.
• 第五行尝试在TestClass-对象tc上调用func指向的成员函数.

• 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.

我遇到两个编译错误:

• 而不是将TestClass::TestMethod分配给func.编译器尝试调用TestClass::TestMethod(即使不是static，因此也会引发错误):

• 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;
         ~~~~~~~~~~~^~~~~~~~~~

编译器尝试在tc上调用名为func的函数，而不是调用func指向的函数.在我看来，好像func的声明方式不正确(作为指向成员函数的指针):

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);
  ~~ ^

我在做什么错了?

推荐答案

简单的语法细节.

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.

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