C++ Linking to separate .cpp file says "multiple definitions"

Question

I'm very new to C++ and could use some help. I'm trying to link a file my_help_fxns.cpp to my main.cpp file so i can use those functions in main.cpp, but when i try linking I get the following error for each function in my_help_fxns:

C:/TDM-GCC-64/bin/../lib/gcc/x86_64-w64-mingw32/9.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: C:\Users\Geoff\AppData\Local\Temp\ccaPL79E.o:data_vars_class.cpp:(.text+0x0): multiple definition of `my_help_fxns::print_vector_items_int_type(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::vector<int, std::allocator<int> >)'; C:\Users\Geoff\AppData\Local\Temp\cc0mRP1w.o:main.cpp:(.text+0x0): first defined here

So it says I'm defining twice, but I don't know how to get around this. I have a class called data_vars_class. i include my_help_fxns at the top of data_vars_class.cpp, and use the helper fxns successfully in methods for that class. An instance of the class is created at the top of main.cpp. however if i try to use the helper functions in main() in main.cpp, without declaring "my_help_fxns.cpp" at the top of main.cpp, it says functions arent found, and if i do declare it at the top of main.cpp, i get the duplication error its been declared twice. How can I fix this, thanks!

this is the structure of my project

main.cpp ==>

#include "data_vars_class.hpp"
#include <iostream>
#include <chrono>
#include "my_help_fxns.cpp"   <--- including here gives duplication error, but if i dont, its functions not found error

DataVars dataVars;

int main () {

    my_help_fxns::pause_program();

    return 0; 
}

data_vars_class.hpp ==>

#include <string>
#include <vector>
#include <unordered_map>
#include <unordered_set>

class DataVars
{
    private:
         ...
    public:
         ...
}

data_vars_class.cpp ==>

#include <iostream>
#include <vector>
#include <string>
#include <unordered_set>
#include <array>

#include "data_vars_class.hpp"
#include "my_help_fxns.cpp"

...i can use my_help_fxns here with no problem, as an instance of this class is created before main() in main.cpp

my_help_fxns.cpp ==>

#include <iostream>
#include <fstream>
#include <string>
#include <sstream>
#include <vector>
#include <algorithm>
#include <map>

using namespace std;

namespace my_help_fxns
{
    void pause_program() {
        std::string dummy;
        std::cout << "Enter to continue..." << std::endl;
        std::getline(std::cin, dummy);            
    }
}

And here is the build command for the file in Geany:

g++ main.cpp data_vars_class.cpp -o a.out

Thanks for your help!

The way you've minimized your code isn't good. We would never know where the compiler is exactly conflicting since there's no function defined anywhere. — Rohan Bari
– Rohan Bari, Commented Jul 3, 2020 at 4:18
Also, you can't use a .cpp extension to implement the code insider it as a header file (given in data_vars_class.cpp). — Rohan Bari
– Rohan Bari, Commented Jul 3, 2020 at 4:20
i added the "pause_program" function I'm trying to call in my_help_fxns.cpp. I'm not sure what you mean by "consider it as a header", and how would I implement that? thanks — Geoff L
– Geoff L, Commented Jul 3, 2020 at 4:21

Daniel Junglas · Accepted Answer · 2020-07-03 04:20:51Z

1

Don't include the my_help_fxns.cpp into the other CPP files since that will effectively define those functions in all the CPP files. This violates the one definition rule.

Instead

create a header file that declares (but not defines) those functions
include that header file in all the CPP files
add my_help_fxns.cpp to the compilation command line

answered Jul 3, 2020 at 4:20

Daniel Junglas

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Rohan Bari · Accepted Answer · 2020-07-03 04:36:10Z

0

Make changes to your files as described:

main.cpp

#include "data_vars_class.hpp"
#include <iostream>
#include <chrono>
#include "my_help_fxns.hpp" // change file extension from cpp -> hpp

DataVars dataVars;

int main () {

    my_help_fxns::pause_program();

    return 0; 
}

data_vars_class

#include <iostream>
#include <vector>
#include <string>
#include <unordered_set>
#include <array>

#include "data_vars_class.hpp"
// #include "my_help_fxns.cpp" --> Not required here

And then you may simply run:

g++ -o a.out main.cpp; ./a.out

Gives here:

Enter to continue...
sdfsdfsdfsd    // --- INPUT

answered Jul 3, 2020 at 4:36

Rohan Bari

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