Namespace with multiple files

Question

Inside FileThree.h

#ifndef FILETHREE
#define FILETHREE
namespace blue{}
class Filethree
{
public:
    Filethree(void);
    ~Filethree(void);
};
#endif

Inside FileThree.cpp

#include "Filethree.h"
#include<iostream>
using namespace std ;
namespace blue{
     void blueprint(int nVar){
         cout<<"red::"<<nVar<<endl;
     }
}
Filethree::Filethree(void)
{
}

Filethree::~Filethree(void)
{
}

Inside FileFour.h

#ifndef FILEFOUR
#define FILEFOUR
namespace red{}
class FileFour
{
public:
    FileFour(void);
    ~FileFour(void);
};
#endif

Inside FileFour.cpp

#include "FileFour.h"
#include<iostream>
using namespace std; 
 namespace red{
     void redprint(double nVar){
         cout<<"red::"<<nVar<<endl;
     }
}
FileFour::FileFour(void)
{
}

FileFour::~FileFour(void)
{
}

Inside main.cpp

#include "FileFour.h"
 #include "Filethree.h"
using namespace red ;
using namespace blue ;

int main()
{
    blueprint(12);
return 0;
}

When i compile the above file it gives me the following error .

 error C3861: 'blueprint': identifier not found

Can anyone tell me why i am getting this error ?

Answer 1

Compiler can't find functions when they are not declared in header files. You need to declare blueprint function in namespace blue in FileThree.h

FileThree.h:

namespace blue{
    void blueprint(int nVar);
}

Same to redprint function, need to declare it in FileFour.h inside namespace red

FileFour.h

namespace red{
   void redprint(double nVar);
}

Answer 2

When you separate your code into multiple files, you’ll have to use a namespace in the header and source file.

add.h

#ifndef ADD_H
#define ADD_H

namespace basicMath
{
    // function add() is part of namespace basicMath
    int add(int x, int y);
}

#endif
add.cpp


#include "add.h"

namespace basicMath
{
    // define the function add()
    int add(int x, int y)
    {
        return x + y;
    }
}
main.cpp


#include "add.h" // for basicMath::add()

#include <iostream>

int main()
{
    std::cout << basicMath::add(4, 3) << '\n';

    return 0;
}

source : https://www.learncpp.com/cpp-tutorial/user-defined-namespaces/comment-page-4/#comment-464049