How to define a private member function only in cpp-CodePudding

So consider I have a class with a private member variable and a private function which I do not want to define in the header file, because I want to "hide" it from the user.

How can I make this? I can not access the private variable without the declaration of the function in the header.

So what works is something like this:

// header file
class Testclass { 
  public:
    // ...
  private:
    const int m_i;
    void func() const;
}


// cpp file
#include "TestClass.h"

Testclass::func() const {
  int j = m_i; //access to a private member variable
}

// ...

But I want something like this:

// header file
class Testclass{ 
  public:
    //...
  private:
    const int m_i;
}

// cpp file
#include "TestClass.h"

Testclass::func() const {
  int j = m_i; //access to a private member variable
}

// ...

Which possibilities do I have? I read something about the PIMPL Idiom, but I am not sure if this is what I want, since it looks a bit cumbersome coding-wise.

CodePudding user response：

Normally one achieves this through the PIMPL (Pointer to IMPLementation) idiom. In your header file you have:

class MainClass
{
    public:
    void public_function();
    private:
    class Impl;
    Impl* impl;
};

Note that the header file does not contain the definition of the Impl class, only its declaration.

You then define that class in your cpp file and forward the calls from your public interface to the functions of the impl class:

class MainClass::Impl
{
    void actualfunc()
    {
        //do useful stuff here
    }

};

void MainClass::public_function()
{
    return impl->actualfunc();
}

Apart from your indended hiding of unwanted members from your class users, the PIMPL idiom provides the additional benefit that if no changes are made to the interface of the class, the users of the class need not be recompiled.

CodePudding user response：

You can have non-member helper functions in the cpp file, that the class members can use. However, they would have to pass the private variable as a parameter.

// header file
class Testclass{ 
  public:
    //...
  private:
    const int m_i;
}

// cpp file
#include "TestClass.h"

void func(int m_i) {
  int j = m_i; //private member variable supplied by caller
}

// ...