C++ header file linker error - c++

Question

I have made the following C++ program which is made up of 3 files:
The thing.h file
#ifndef THING_H
#define THING_H
class thing{
double something;
public:
thing(double);
~thing();
double getthing();
void setthing(double);
void print();
};
#endif
The thing.cpp file
#include <iostream>
#include "thing.h"
thing::thing(double d){
something=d;
}
thing::~thing(){
std::cout << "Destructed" << std::endl;
}
double thing::getthing(){
return something;
}
void thing::setthing(double d){
something = d;
}
void thing::print(){
std::cout <<"The someting is " << something << std::endl;
}
The main file
#include <iostream>
#include "thing.h"
int main(){
thing t1(5.5);
t1.print();
t1.setthing(7.);
double d=t1.getthing();
std::cout << d << std::endl;
system("pause");
return 0;
}
I had made this program previously all in one file and it ran perfectly but when I try split it into seperate files to create a header I get a linker error, here is the errors I get when I try run it from the main file:
[Linker error] undefined reference to `thing::thing(double)'
[Linker error] undefined reference to `thing::print()'
[Linker error] undefined reference to `thing::setthing(double)'
[Linker error] undefined reference to `thing::getthing()'
[Linker error] undefined reference to `thing::~thing()'
[Linker error] undefined reference to `thing::~thing()'
ld returned 1 exit status
From the above errors it seems asthough the main file doesnt recognise the functions inside the header, how do I fix this please?

Answer 1

In slightly less pedantic terms:
Your header file thing.h declares "what class thing should look like", but not its implementation, which is in the source file thing.cpp. By including the header in your main file (we'll call it main.cpp), the compiler is informed of the description of class thing when compiling the file, but not how class thing actually works. When the linker tries to create the entire program, it then complains that the implementation (thing::print() and friends) cannot be found.
The solution is to link all the files together when creating the actual program binary. When using the g++ frontend, you can do this by specifying all the source files together on the command line. For example:
g++ -o main thing.cpp main.cpp
will create the main program called "main".

Answer 2

It seems you are not linking thing.cpp into your "project".
If you are compiling using gcc:
g++ thing.cpp -o thing.o
g++ main.cpp -o main.o
g++ main.o thing.o -o my-best-application-ever
How to add the file to your project depends on the compiler/IDE/build-system you are using.

Answer 3

#sheu is right.. But you don't have to do anything if you just include thing.cpp in your main.cpp
Since you're already including thing.h in thing.cpp everything will work just fine if you include thing.cpp

Answer 4

Compiler knows about declarations of functions, but nothing about definitions. You need to say them where they are. The easiest way to do that is to create 'project' and add all files to it. Then compiler knows where to search all files.

Answer 5

Put some code in thing.cpp which let you know that it is being compiled i.e.
error ...
apparently it is not being compiled and linked...