从另一个类访问受保护的函数c ++

Question

I am trying to access a protected function in class Test but I have no idea how to do it.

Ok I admit one fact that I'm asking this because there's one part of my homework which I am tasked to put a function as protected: and access it instead of putting it to public:

and I am not sure how should I do it.

the code below is how I normally access a function but of course, it doesn't work since it's protected:

Test.h

#ifndef Test_Test_h
#define Test_Test_h

class Test {

protected:
      void sampleOutputMethod();
};

#endif

Test.cpp

 #include "Test.h"

 void Test::sampleOutputMethod()  {
    std::cout << "this is a test output method" << std::endl;
 }

main.cpp

 #include Test.h
 #include<iostream>

 int main() {
    Test test;
    test.sampleOutputMethod();
 }

Answer 1

A protected function is just as good as private function if you try to access it from a class that is not part of your hierarchy. You either have to make the class trying to access it a subclass of Test, or you have to declare it as friend class. I bet you need the first option.

Answer 2

You can delegate the function as well. Create a public function that calls the protected one.

This can be accomplished through either deriving Test or creating another class, Test2, and declaring Test a friend of Test2 and having Test2 contain an instance of Test.

Seeing all the instructions of your homework would help.

Answer 3

There are essentially two ways of accessing a protected member: 1) Create a class that inherits from your class Test:

class Test2 : public Test {
public:
    void sampleOutputMethod() { ::sampleOutputMethod; }
}

2) Create another class, and modify class Test to make the other class a friend of your Test class:

class Test;  // Forward declaration of Test
class Test2 {
public:
    void output( Test& foo ) { foo.sampleOutputMethod(); }
}

class Test {
protected:
    void sampleOutputMethod();
}

Answer 4

如果允许您修改类Test，则可以添加一个调用“ sampleOutputMethod”的公共函数，也可以使用此技巧返回“ sampleOutputMethod”的函数指针https://stackoverflow.com/a/6538304/1784418