one or more re definition in c++?

how to write partial code in differenct cpp files? If I write some in .h file. I can only use it

in main.cpp? can not write the code in othere cpp files of the .h file?

in .h file:

void GetDevice();

void GetBitmap();

can i write the GetDevice in dev.cpp and GetBitmap in bit.cpp?

can i write the GetDevice in dev.cpp and GetBitmap in bit.cpp?

Yes.

There's a difference between function declarations and variable definitions.

Try building an application with the following examples -

File Test.h

#pragma once

void GetDevice();
void GetBitmap();

// Uncomment to causes multiple definition error
//int non_const_variable;

const int const_variable = 42; // const variable is OK

extern int extern_variable;  // declaration as extern is OK, definition in one .cpp file

File main.cpp

#include <stdio.h>
#include "Test.h"

int extern_variable = 100;

int main()
{
    GetBitmap();
    GetDevice();

    return 0;
}

File dev.cpp

#include <stdio.h>

#include "Test.h"

void GetDevice()
{
    printf("In GetDevice(), const variable is %d, extern variable is %d\n", const_variable, extern_variable);
}

File bit.cpp

#include <stdio.h>

#include "Test.h"

void GetBitmap()
{
    printf("In GetBitmap(), const variable is %d, extern variable is %d\n", const_variable, extern_variable);
}

extern ID2D1Factory *factory=NULL in header.h but still error. already definition in game.obj.