Instant object initialization in C++ fails with LNK2005 error - c++

Question

These are the errors that I've been receiving:
LNK2005 "class Sally TestObject" (?TestObject##3VSally##A) already defined in Source.obj Project2 c:\Users\W8User\documents\visual studio 2015\Projects\Project2\Project2\Source1.obj
and
LNK1169 one or more multiply defined symbols found Project2 c:\users\w8user\documents\visual studio 2015\Projects\Project2\Debug\Project2.exe
I don't really understand where are these errors coming from. I've tried searching in other SO threads or on the connected Microsoft's website, but none helped.
These are my files:
Source.cpp
#include <iostream>
#include "Header.h"
int main() {
std::cout << TestObject.ReturnTruth();
return 0;
}
Header.h
#pragma once
class Sally
{
public:
bool ReturnTruth();
} TestObject;
Source1.cpp
#include "Header.h"
bool Sally::ReturnTruth()
{
return 1;
}
What I know is that it's enough to move the object initialization into the Source.cpp file and not do it instantly in the header file, but since the instant initialization is a possibility then why wouldn't I use it?

Answer 1

Let's suppose that you have two different .cpp files that each include your Header.h header. Then, each one of those .cpp files gets this code incorporated into it:
class Sally
{
public:
bool ReturnTruth();
} TestObject;
As a result, each file contains the definition of an object named TestObject of type Sally. This breaks the one-definition rule, since there's only supposed to be at most one definition of each object across all translation units, and it's manifesting in your linker error.
If you do want to declare a global object of type Sally, change the header to declare an extern object of type Sally, like this:
class Sally
{
public:
bool ReturnTruth();
};
extern Sally TestObject;
This is a declaration, not a definition, and it's okay to have redundant declarations of objects.
Then, pick a .cpp file - probably the one where you implement the Sally member functions - and add this line:
Sally TestObject;
That puts the definition of the object in a single place, and so fixes the one-definition-rule issue and the linker issue.

Answer 2

Every single place you #include "Header.h" you will have an instance of TestObject, and unless your compiler can fold all of these into a singleton you're going to have several independent, unrelated instances.
That error is advising you that you've made a mistake. One way to fix this is to define in the header file:
extern Sally TestObject;
Which makes it explicitly clear that this will be instantiated elsewhere. Then make an explicit instance in Source1.cpp:
Sally TestObject;
Then your main file will know where to look for this thing.
It's not clear if you want a singleton in the first place. Why can't you just instantiate one inside of main and use it?
As a note, please, please take the time to give your source files meaningful names.

Answer 3

By appending identifier TestObject directly after the class definition Sally, you define a variable in your header file. So if different translation units include this header file, a variable with the same name is defined twice, leading to a linker error. You can resolve this by just declaring the variable, i.e. using extern-keyword in the headerfile, whereas defining the variable then in exactly one .cpp-file:
So write:
Header.h
class Sally
{
public:
bool ReturnTruth();
};
extern Sally TestObject;
Source1.cpp
#include "header.h"
Sally TestObject;
...
in all other CPP-files:
#include "header.h"
...