How to use a C header.h in multiple C++ files without getting multiple definition error?

Question

I want to use a function that from the "func.h" file in the Wireshark open source project. I need to use the funct() function in multiple .cpp files, but I get the a multiple definition error.

func.h:

    #ifndef func_h
    #define func_h
    #include<string>
    
     void *funct(char *cName)
     {
         std::string name = cName;
         cName+= ".extension";
     }

In the .cpp files I include the func.h:

    #include "func.h"

And call the funct() function from 2 .cpp files:

    funct("program");

What should I do so I don't get the multiple definition error? A workaround is to copy and paste the function defition in every .cpp file and change the function name, but this is ugly.

Many thanks.

Put the definition in its own `.cpp` file, or mark it `inline` in the `.h`. — BoBTFish, Jul 01 '21 at 08:28
Declare the function [static](https://stackoverflow.com/questions/572547/what-does-static-mean-in-c) — Jan Gabriel, Jul 01 '21 at 08:31
@JanGabriel That won't help. `static` only guards a definition from being accessed from other translation units, but here we have definitions in multiple TUs — Yksisarvinen, Jul 01 '21 at 08:34
I'm not sure if this is the best duplicate, but here's one: [Is it a good practice to define C++ functions inside header files?](https://stackoverflow.com/questions/25274312/is-it-a-good-practice-to-define-c-functions-inside-header-files) — Yksisarvinen, Jul 01 '21 at 08:37
@Yksisarvinen, you are completely correct and also that is the reason why it would work. The `funct()` is defined in the header and by including the header file in each source file and making it static then each TU gets its own version. For this specific `funct()` it should be fine. — Jan Gabriel, Jul 01 '21 at 08:50
@JanGabriel I think it still violates One Definition Rule (unless it would be `inline`), if it is, then it's not a good solution. It may work tho. — Yksisarvinen, Jul 01 '21 at 09:02
@Yksisarvinen Actually, it will help. With `static`, yes, we have definitions in multiple TUs. But this does not break the ODR, because each definition defines a separate function. Live demo: https://wandbox.org/permlink/506cFfJKnuMniOrT. — Daniel Langr, Jul 01 '21 at 09:11

score 5 · Accepted Answer · answered Jul 01 '21 at 08:39

You have two options:

You move the implementation of the function in a separate .c/.cpp file, leaving in the header file only the declaration. Then you compile this source file, and link it with the rest of the program. Example:

func.h

#ifndef func_h
#define func_h
#include<string>

void *funct(char *cName);
#endif

func.cpp

#include "func.h"

void *funct(char *cName)
{
    std::string name = cName;
    cName+= ".extension";
}

You compile and link as usual, e.g.

g++ -c -o func.o func.cpp
g++ -c -o main.o main.cpp
g++ -o prog main.o func.o

You specify the keyword inline in the definition of the function. This allows appearance of the function in multiple compile units. See this question.

Hi @AlexSeceleanu if this or any answer has solved your question please consider [accepting it](https://stackoverflow.com/help/someone-answers) by clicking the check-mark. This indicates to the wider community that you've found a solution and gives some reputation to both the answerer and yourself. There is no obligation to do this. — francesco, Jul 01 '21 at 12:09

score 2 · Answer 2 · answered Jul 01 '21 at 08:37

func.cpp:

#include "func.h"

void *funct(char *cName)
{
  std::string name = cName;
  cName+= ".extension";
}

func.h:

#ifndef func_h
#define func_h
#include<string>
    
void *funct(char *cName);
#endif

Then in your build script (CMakeLists.txt, or a simple Makefile), compile both .cpp files and link them into one executable.

You may want to put using namespace std; or using std::string; (check the latter syntax; I usually use the former), in which case you don't have to write std::.

Please **don't** recommend `using namespace std;`, it is [widely considered to be a bad practice](https://stackoverflow.com/q/1452721/1171191). — BoBTFish, Jul 01 '21 at 08:46

How to use a C header.h in multiple C++ files without getting multiple definition error?

2 Answers2