Accessing elements in namespace defined in header file

Question

I am trying to access variables and functions defined in a namespace in a header file. However, I get the error: xor.cpp:(.text+0x35): undefined reference to "the function in header file" collect2: error: ld returned 1 exit status. It seems to me that the compilation steps are OK after reading this post, and also because I can access variables in this header file, but calling the function returns the error mentioned above. My question is: How can I access those functions in the namespace from my main.cpp ? What am I doing wrong ?

The case with a class is clear to me, but here I don't understand because I am not supposed to create an object, so just calling the namespace in front should be OK (?).

Edit

After changes suggested by Maestro, I have updated the code the following way, but it still doesn't work. The error I get is the same. If I define using NEAT::trait_param_mut_prob = 6.7; I get the error: xor.cpp:127:36: error: expected primary-expression before ‘=’ token

Main c++

#include "experiments.h"
#include "neat.h"
#include <cstring>

int main(){

  const char *the_string = "test.ne";
  bool bool_disp = true;
  NEAT::trait_param_mut_prob;
  trait_param_mut_prob = 6.7;
  NEAT::load_neat_params(the_string ,bool_disp);
  std::cout << NEAT::trait_param_mut_prob << std::endl;
  return 0;
}

neat.h

    #ifndef _NERO_NEAT_H_
    #define _NERO_NEAT_H_
    
    #include <cstdlib>
    #include <cstring>
    
    namespace NEAT {
        extern double trait_param_mut_prob;
        bool load_neat_params(const char *filename, bool output = false); //defined HERE 
    
    }
    #endif

neat.cpp

#include "neat.h"
#include <fstream>
#include <cmath>
#include <cstring>

double NEAT::trait_param_mut_prob = 0;

bool NEAT::load_neat_params(const char *filename, bool output) { 
                    //prints some stuff
                    return false;
                    };

Makefile

neat.o: neat.cpp neat.h
        g++ -c neat.cpp

Answer 1

You are breaking the "ODR rule" (One-Definition-Rule): you've defined trait_param_mut_prob and load_neat_params twice once in source file neat.cpp and second in main.cpp so simply remove those lines from main.cpp:

//double NEAT::trait_param_mut_prob; //NEAT is the namespace 
//bool NEAT::load_neat_params(const char* filename, bool output); //function defined in the namespace 

• Add #endif in you header neat.h.

• to make your function and variable available in main just use using or fully-qualify the call because as I've seen you intend to re-declare them in main to avoid fully qualifying them: in main:

  int main()
  {
  
      using NEAT::trait_param_mut_prob;
      using NEAT::load_neat_params;
  
      const char* the_string = "test.ne";
      bool bool_disp = true;
      trait_param_mut_prob = 6.7;//works perfectly 
      load_neat_params(the_string, bool_disp);
      // or fully-qualify:
  
      //NEAT::load_neat_params(the_string, bool_disp);
      //NEAT::trait_param_mut_prob = 6.7;//works perfectly 
  }
  

• Also it is erroneous that your function load_neat_params doesn't return a bool value. So make it either return true or false.