Linking a class when used inside another class - c++

Question

When I have a class I want to use somewhere else, such as Secondaryfile.cpp, I can import it fine into another file where I create a new class, as long as I do the class definition, etc inside the .cpp file, not the header file, otherwise I get linking errors. I have tried adding things like extern "C" in the Secondaryfile.cpp but to no avail. Looking at the errors, I also thought that pershaps I shouldn't use the namespace, however, that still results in the same error.
The reason why I am doing this is that I want to import a Tertiaryfile into Mainfile which has a function with the same name as a function in Secondaryfile, so I wanted to be able to refer them as, say, Secondaryfile::add1() and Tertiaryfile::add1() and avoid any issues with the same names that way - through the use of namespaces. I want to be able to declare the Mainfile class in its header file so that I can import it and use it in a different class down the line.
In the example below, running g++ Mainfile2.cpp compiles fine, however, g++ Mainfile.cpp doesn't, instead it results in the following error:
Mainfile.cpp:(.text+0x20): undefined reference to 'Secondaryfile::Secondaryfile::Secondaryfile()'
Mainfile.cpp:(.text+0x58): undefined reference to 'Secondaryfile::Secondaryfile::getX()'
Mainfile.cpp:(.text+0x7a): undefined reference to 'Secondaryfile::Secondaryfile::Secondaryfile(int)'
My code:
Mainfile.cpp
#include "Mainfile.h"
#include <cstddef>
#include <cstdio>
Mainfile::Mainfile(char* name, Secondaryfile::Secondaryfile x)
{
this->name = name;
this->x = x;
}
Mainfile::getDoubleX(){
return this->x.getX() * 2;
}
static Secondaryfile::Secondaryfile getObj(int num){
return Secondaryfile::Secondaryfile(num);
}
int main(){
}
Mainfile.h
#ifndef MAINFILE_H
#define MAINFILE_H
namespace Secondaryfile{
#include "Secondaryfile.h"
}
class Mainfile
{
public:
char* name;
Secondaryfile::Secondaryfile x;
Mainfile(char*, Secondaryfile::Secondaryfile);
int getDoubleX();
static Secondaryfile::Secondaryfile getObj(int);
};
#endif
Mainfile2.cpp
namespace Secondaryfile{
#include "Secondaryfile.h"
}
#include "Mainfile2.h"
#include <cstddef>
#include <cstdio>
class Mainfile2{
public:
char* name;
Secondaryfile::Secondaryfile x;
Mainfile2(char* name, Secondaryfile::Secondaryfile x)
{
this->name = name;
this->x = x;
}
getDoubleX(){
return this->x.getX() * 2;
}
static Secondaryfile::Secondaryfile getObj(int num){
return Secondaryfile::Secondaryfile(num);
}
};
int main(){
}
Mainfile2.h
#ifndef MAINFILE2_H
#define MAINFILE2_H
int getDoubleX();
static Secondaryfile::Secondaryfile getObj(int);
#endif
Secondaryfile.cpp
#include "Secondaryfile.h"
#include <cstddef>
#include <cstdio>
Secondaryfile::Secondaryfile(int x)
{
this->x = x;
}
Secondaryfile::Secondaryfile()
{
this->x = 0;
}
int Secondaryfile::getX()
{
return this->x;
}
int Secondaryfile::add1(int num)
{
return num+1;
}
int main(){
}
Secondaryfile.h
#ifndef SECONDARYFILE_H
#define SECONDARYFILE_H
class Secondaryfile
{
public:
int x;
Secondaryfile();
Secondaryfile(int);
int getX();
static int add1(int);
};
#endif

Answer 1

There are many problems with your code.
In Mainfile.cpp you didn't specify the return type of Mainfile::getDoubleX()(it should be int)
You can only specify static member functions inside a class definition(in Mainfile.cpp, the function getObj has that qualifier). Also it should have a class scope, so the correct way of writing is Secondaryfile Mainfile::getObj(int num)
Doing namespace Secondaryfile{ #include "Secondaryfile.h"}. You shouldn't do this. Just #include "Secondaryfile.h" inside the source files is enough. You mention that you did this to prevent "linking errors", but that is probably because of the other problems in your code and in your project configuration.
You also didn't specify your compiler & IDE/cmake/makefile configs, which is essential for properly compiling and linking these header and source files.
Are you building a library or executable code? You include a main() function in your source files that doesn't do anything, which is also bad practice, because main() in C++ is an entry-point function, wich means that the compiler will look for it to create the starting point of the program1.
With these in mind, I tried to fix your code. See if this is what you had in mind:
Mainfile.cpp
//
// Created by vinicius on 21/10/2020.
//
#include "Mainfile.h"
#include <cstddef>
#include <cstdio>
Mainfile::Mainfile(char* name, Secondaryfile x)
{
this->name = name;
this->x = x;
}
int Mainfile::getDoubleX(){
return this->x.getX() * 2;
}
Secondaryfile Mainfile::getObj(int num){
return Secondaryfile(num);
}
int main() {
//Test code
printf("Hello World!\n--------\n");
Secondaryfile s = Secondaryfile();
Mainfile f = Mainfile("myname", s);
Secondaryfile myobj = f.getObj(12);
printf("Value = %d", myobj.getX());
}
Mainfile.h
//
// Created by vinicius on 21/10/2020.
//
#ifndef MAINFILE_H
#define MAINFILE_H
#include "Secondaryfile.h"
class Mainfile
{
public:
char* name;
Secondaryfile x;
Mainfile(char*, Secondaryfile);
int getDoubleX();
static Secondaryfile getObj(int);
};
#endif
Mainfile2.cpp
//
// Created by vinicius on 21/10/2020.
//
#include "Secondaryfile.h"
#include "Mainfile2.h"
#include <cstddef>
#include <cstdio>
Secondaryfile Mainfile2::getObj(int num) {
return Secondaryfile(num);
}
int Mainfile2::getDoubleX() {
return this->x.getX() * 2;
}
Mainfile2::Mainfile2(char *name, Secondaryfile x) {
this->name = name;
this->x = x;
}
Mainfile2.h
//
// Created by vinicius on 21/10/2020.
//
#ifndef MAINFILE2_H
#define MAINFILE2_H
class Mainfile2{
public:
char* name;
Secondaryfile x;
Mainfile2(char* name, Secondaryfile x);
int getDoubleX();
static Secondaryfile getObj(int num);
};
#endif
Secondaryfile.cpp
//
// Created by vinicius on 21/10/2020.
//
#include "Secondaryfile.h"
#include <cstddef>
#include <cstdio>
Secondaryfile::Secondaryfile(int x)
{
this->x = x;
}
Secondaryfile::Secondaryfile()
{
this->x = 0;
}
int Secondaryfile::getX()
{
return this->x;
}
int Secondaryfile::add1(int num)
{
return num+1;
}
Secondaryfile.h
//
// Created by vinicius on 21/10/2020.
//
#ifndef SECONDARYFILE_H
#define SECONDARYFILE_H
class Secondaryfile
{
public:
int x;
Secondaryfile();
Secondaryfile(int);
int getX();
static int add1(int);
};
#endif
And the CMakeLists.txt for the project(made in CLion):
cmake_minimum_required(VERSION 3.17)
project(secfile)
set(CMAKE_CXX_STANDARD 14)
include_directories(.)
add_executable(secfile
Mainfile.cpp
Mainfile.h
Mainfile2.cpp
Mainfile2.h
Secondaryfile.cpp
Secondaryfile.h)
After building the project with cmake, you can run it. After doing that, it outputs the following lines:
Hello World!
--------
Value = 12
As expected.
1 https://en.cppreference.com/w/cpp/language/main_function