Using multiple .cpp files in c++ program?

Question

I recently moved from Java for C++ but now when I am writing my application I'm not interested in writing everything of the code in the main function I want in main function to call another function but this other function is in another .cpp file.

Let me explain better if you wouldn't understand:
I have one file: main.cpp inside it I have main function.

I have the second file: second.cpp inside I have a function called second() I want to call this function called second() from my main function..

Any help?

Well header files are the answer, so since you know about those it's not clear what you are doing wrong. Post the code you have and the error you get, otherwise we are just guessing. — john
– john, Commented Aug 9, 2011 at 11:41
I know this is an old question, but learncpp.com recommends doing this by using forward declarations. Which technique is better? — Tyrcnex
– Tyrcnex, Commented Feb 1, 2023 at 12:01
@Tyrcnex This is basically what the accepted answer talks about and its standard way of splitting things into multiple files in C++. Which way is better is mainly how you prefer to structure your project. — User
– User, Commented Mar 12, 2024 at 8:03

Zandor Smith · Accepted Answer · 2019-04-01 02:43:35Z

107

You must use a tool called a "header". In a header you declare the function that you want to use. Then you include it in both files. A header is a separate file included using the #include directive. Then you may call the other function.

other.h

void MyFunc();

main.cpp

#include "other.h"
int main() {
    MyFunc();
}

other.cpp

#include "other.h"
#include <iostream>
void MyFunc() {
    std::cout << "Ohai from another .cpp file!";
    std::cin.get();
}

edited Apr 1, 2019 at 2:43

Zandor Smith

5926 silver badges26 bronze badges

answered Aug 9, 2011 at 11:40

Puppy

147k40 gold badges271 silver badges481 bronze badges

Sign up to request clarification or add additional context in comments.

9 Comments

James Well Over a year ago

May I ask that the purpose of the .h file be further explained ? It seems to me that it contains no info unmentioned in the other.cpp file

Blake Over a year ago

I believe, it is not necessary to #include "other.h" in other.cpp

Lightness Races in Orbit Over a year ago

@MichaelD.Blake In this example, you are correct. However, real projects are often more complicated, and other.h would define types that are used inside other.cpp. Usually X.cpp will include X.h so you may as well get used to it now. Other simplifications include lack of header guards in other.h.

jackw11111 Over a year ago

A note to people who are receiving undefined reference to myFunc(); you need to include other.cpp in the compilation command, so the linker knows the implementation of myFunc().... g++ main.cpp other.cpp.

Hareen Laks Over a year ago

@jackw11111, yeah. There are issues with undefined reference. It means, in case of having multiple cpp files should I include all of them as arguments?

|

Creat · Accepted Answer · 2011-08-09 11:44:21Z

25

You should have header files (.h) that contain the function's declaration, then a corresponding .cpp file that contains the definition. You then include the header file everywhere you need it. Note that the .cpp file that contains the definitions also needs to include (it's corresponding) header file.

// main.cpp
#include "second.h"
int main () {
    secondFunction();
}

// second.h
void secondFunction();

// second.cpp
#include "second.h"
void secondFunction() {
   // do stuff
}

answered Aug 9, 2011 at 11:44

Creat

5241 gold badge4 silver badges9 bronze badges

Comments

Ataur Rahman Munna · Accepted Answer · 2016-05-12 07:59:15Z

8

In C/C++ you have header files (*.H). There you declare your functions/classes. So for example you will have to #include "second.h" to your main.cpp file.

In second.h you just declare like this void yourFunction(); In second.cpp you implement it like

void yourFunction() { 
   doSomethng(); 
}

Don't forget to #include "second.h" also in the beginning of second.cpp

Hope this helps:)

edited May 12, 2016 at 7:59

Ataur Rahman Munna

3,9151 gold badge25 silver badges35 bronze badges

answered Aug 9, 2011 at 11:41

m_pGladiator

8,6808 gold badges46 silver badges61 bronze badges

Comments

Blake · Accepted Answer · 2019-03-31 23:48:56Z

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();

main.cpp:

#include <iostream>
#include "second.h"
int main()
{
    //.....
    return 0;
}

second.cpp:

void second()
{
    //...
}

int third()
{ 
    //...
    return foo;
}

double fourth()
{ 
    //...
    return f;
}

Note that: it is not necessary to #include "second.h" in second.cpp. All your compiler need is forward declarations and your linker will do the job of searching the definitions of those declarations in the other files.

Collectives™ on Stack Overflow

Using multiple .cpp files in c++ program?

4 Answers 4

other.h

main.cpp

other.cpp

9 Comments

Comments

Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

other.h

main.cpp

other.cpp

9 Comments

Comments

Comments

Comments

Linked

Related