How to call function from multiple cpp files

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How can i include and use function in multiple cpp files?

// my_function.h
void func_i_want_to_include();

// my_function.cpp
void func_i_want_to_include()
{
    puts("hello world");
}


//main.cpp
#include "my_function.h"
#include "file_b.h"

int main()
{
     call_to_file_b();
     func_i_want_to_include();
     return 0;
}

//file_b.h
void call_to_file_b();

//file_b.cpp
#include "my_function.h"

void call_to_file_b()
{
     puts("from file b\n");
     func_i_want_to_include();
}

When i do it like this i get yield by the linker "unresolve external symbol" , i guess the linker past func_i_want_to_include() 2 times instead of understand that this is the same function.

How do i tell him ' this is the same function just call it from 2 files but don't try to make 2 copies of the same function?

Firstly, as @LuisGP mentioned, you need #ifndef if your header file is include many times. Secondly, in the associated cpp file, you need to include the header file. The header file declares the function, the cpp file describes the function. Lastly, all the cpp files have to be included when compiling (just use command line if the editor doesn't work). It goes like this:

gcc -c my_function.cpp file_b.cpp //this will make a my_function.o and a file_b.o file
gcc -o main main.cpp my_function.o file_b.o 

or for short:

gcc -o main main.cpp my_function.cpp file_b.cpp

Here is how should the files be written:

// my_function.h
#ifndef _my_function_h
#define _my_function_h

void func_i_want_to_include(); 

#endif


// my_function.cpp
#include <stdio.h>
#include "my_function.h"

void func_i_want_to_include()
{
    puts("hello world");
}


//file_b.h
#ifndef _file_b_h
#define _file_b_h

void call_to_file_b();

#endif


//file_b.cpp
#include <stdio.h>
#include "my_function.h"
#include "file_b.h"

void call_to_file_b()
{
     puts("from file b\n");
     func_i_want_to_include();
}


//main.cpp
#include "my_function.h"
#include "file_b.h"

int main()
{
     call_to_file_b();
     func_i_want_to_include();
     return 0;
}

This is my first time answering a question, if I make any mistake, tell me and I will fix it. Hope it helps.

How to call functions from one .cpp file in another .cpp file?, Basically I have multiple cpp files with all my functions I made for use with binary trees, BST, linked lists etc. What I want to do is instead of having  You can create a file called player.h declare all functions that are need by other cpp files in that header file and include it when needed. player.h #ifndef PLAYER_H // To make sure you don't declare the function more than once by including the header multiple times. #define PLAYER_H #include "stdafx.h" #include <SFML/Graphics.hpp> int

If you want to put the function definition in the header file you need to make the function inline.

// my_function.h
inline void func_i_want_to_include()
{
    puts("hello world");
}

Otherwise the compiler will create one function for each cpp file that the header file is included in, and the linker will not know which one to choose.

This is why you usually separate function declarations

void func_i_want_to_include();

from function definitions

void func_i_want_to_include()
{
    puts("hello world");
}

Where the former goes into the header file, while the later goes into the source file.

C++ Notes: Multiple Source Files, This ensures that the call and the definition are both consistent. Similarly, when using muliple source files, you want to use the same declaration where the function  The linker will connect the function call to add in main.cpp to the definition of function add in add.cpp. Using this method, we can give files access to functions that live in another file. Try compiling add.cpp and the main.cpp with the forward declaration for yourself.

As eozd said, header files are for declarations. But your problem is that your header file is being included several times, one per #include clause. You can solve that adding

#pragma once

At the top of your header, or the old fashion way:

#ifndef _MY_HEADER_FILE_H_
#define _MY_HEADER_FILE_H_

// Stuff

#endif // _MY_HEADER_FILE_H_

Multiple .cpp File Programs - C++ Forum, h file which is included from your other .cpp file. 2) You use a header file for declarations so other modules know how to call your functions in the  You can simply place a forward declaration of your second() function in your main.cpp above main(). If your second.cpp has more than one function and you want all of it in main(), put all the forward declarations of your functions in second.cpp into a header file and #include it in main.cpp. Like this-Second.h: void second(); int third(); double fourth();

You have to create a header file. Inside that file create an object and implement a public function inside of the class.

Multiple File Compilation, circle.h // header file for a class called Circle circle.cpp // implementation file for (Note:& The compiler doesn't need to match function definitions to their calls at  Just declare the C function extern "C" (in your C++ code) and call it (from your C or C++ code).

7.5, Functions provide type checking to ensure function call arguments match the multiple .cpp files, the function definition will be copied into multiple .cpp files. When you compile multiple files in the Arduno IDE the files first get concatenated together. It happens in alphabetical order, except the main INO file (the one named the same as the folder it's in) is placed first. In C the order in which things appear in the file matters.

Top 10 C++ header file mistakes and how to fix them, C++ header files is a rather mundane topic by most standards. Mistake # 9: Declaring functions shared between multiple cpp files in separate  I have a working cpp file, lets just say it performs a+b, the filename is abc.cpp. I want to call up that file, function, in another file without having to rewrite the code and simply call up the function in another cpp file, xyz.cpp and simply map my variables over to the abc.cpp file. I'm from

Modules, Separate Compilation, Using Make Files, When we separate a set of code into its own files, we call it a module. Although modules can contain sets of functions or sets of classes, we often use If a header does get included into a .cpp file more than once, we call this multiple inclusion.

Comments
  • Header files are to declare functions. You need to provide the implementation of func_i_want_to_include in a separate cpp file. (unless the function in the header is a template function)
  • "unresolve external symbol" means it can't find a symbol. How does main.cpp know about call_to_file_b()?
  • @eozd That is incorrect. They are separated by convention there is no reason that a header cannot contain implementation.
  • i have try to seperate the implementation to a .cpp , i still get the same results
  • How did you compile the code?
  • This is the most complete answer and includes all comments and previous partial answers. Should be marked as the correct answer.
  • separation didn't help here
  • i try to compile at visual studio 2015 x86 release moede
  • You have to provide more complete information, because there has to be some other issue with the setup. Did using inline work for you?
  • Are you building my_function.cpp along with the rest of cpp files? my_function.cpp includes my_function.h too?
  • yes i compile it all together and my_function.cpp includes my_function.h
  • there isn't extern keyword for function that i can use of?
  • extern is for global variables. Can you update the example with the full include files? What compiler are you using. I'm pretty sure the linker is not being able to relate your my_function.cpp with the rest of the program.