How can I call a method of a variable, which contains in a namespace?-CodePudding

I've this C code in interface.h:

    #include <iostream>
    class A{
        void foo();
    };
    namespace interface{
        ...
        namespace Sounds{
            A val;
        };
    }

I need to call .foo method. I want to do it in interface.cpp:

#include "interface.h"

void A::foo(){
    std::cout<<1;
}

interface::Sounds::val.foo();

But Clion warns me:

No type named 'val' in namespace 'interface::Sounds'

What should I do?

CodePudding user response：

You may only declare and define types, functions and objects outside function bodies, thus the compiler looks for the type val and can't find it. You may call functions without using their returned results only from functions.

int main() {
  interface::Sounds::val.foo();
}

Above will be almost successfully compiled, for val at least. void A::foo() is declared private, thus could not be accessed at val.foo() unless it is declared public:

class A {
 public:
  void foo();
};

CodePudding user response：

There are 2 ways to solve this both of which are shown below.

Method 1: Prior C 17

First method is using the extern kewyord in the header file for the declaration of val and then define val in the source file before using it as shown below:

interface.h

#pragma once 
#include <iostream>
class A{
    public: //public added here
    void foo();
};
namespace interface{
    
    namespace Sounds{
        //note the extern here . This is a declaration
        extern A val;
    };
}

interface.cpp

#include "interface.h"

void A::foo(){
    std::cout<<1;
}

//definition 
A interface::Sounds::val;

main.cpp


#include <iostream>
#include "interface.h"
int main()
{
    //call member function foo to confirm that it works
    interface::Sounds::val.foo();
    return 0;
}

Working demo

The output of the above modified program is:

Method 2: C 17

You can use inline instead of extern with C 17 and onwards to define val in the header:

interface.h

#pragma once 
#include <iostream>
class A{
    public: //public added here
    void foo();
};
namespace interface{
    
    namespace Sounds{
        //note the inline used here
        inline A val{};
    };
}

interface.cpp

#include "interface.h"

void A::foo(){
    std::cout<<1;
}

//nothing needed here as we used inline in the header

main.cpp


#include <iostream>
#include "interface.h"
int main()
{
    //call member function foo to confirm that it works
    interface::Sounds::val.foo();
    return 0;
}

Working demo