How extern works with header files? (C++ VS2022)

Question

Whenever I try using extern in my header file, the program fails to build. If I don't use extern, the value of the variable is not saved across .cpp files. I've made absolutely sure that the header file is included and the variable name is spelled correctly (there's also no error without the externkeyword).

This is what I've been doing, based on what I've read from numerous sources:

**
globals.h

#pragma once
#include <string>
extern std::wstring name;

void Start();

globals.cpp

#include <string>
#include 'globals.h'

void Start()
{
	std::wstring name = L"Bob";
}

source1.cpp

#include <string>
#include 'globals.h'

Start();

std::wstring someone = name; // LNK2001 unresolved external symbol ...
std::wstring bob = L"Bob";
name = bob;  // error C4430: missing type specifier ...


void myFunction()
{
	wstring bob = L"Bob";

	name = bob; // LNK2001 unresolved external symbol ...
	::name = bob; // LNK2001 unresolved external symbol ...


	wstring someone = name; // LNK2001 unresolved external symbol ...
	wstring someone = ::name; // LNK2001 unresolved external symbol ...
}

How do I correctly use a global variable shared across multiple .cpp files?

Thanks

Answer 1

Hi, @Nikolas Fraser

The snippet needs the following modifications:
std:: is missing
Supplement the variable type when providing a definition for a global variable. Please check the doc: extern linkage for non-const globals
//fileA.cpp

int i = 42; // declaration and definition
Use Start() inside function.
Variable names are duplicated
Updated:

main.cpp

#include "globals.h"
#include"source.h"
#include<iostream>
//Start();
std::wstring someone = name;
std::wstring bob = L"Bob";
std::wstring name = bob;

void myFunction()
{
	std::wstring bob = L"Bob";

	name = bob;
	//::name = bob; 


	std::wstring someone = name;
	//std::wstring someone = ::name;
}
int main()
{
std::wcout << name<<std::endl;
	foo();
	std::wcout << name;
}

Source.h

#pragma once
void foo();

Source.cpp

#include<string>
extern std::wstring name;
void foo()
{
	name = L"aaa";
}

Best regards,

Minxin Yu

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Answer 2

It is mandatory to declare the variable at the global scope inside globals.cpp to resolve properly.

globals.h

#pragma once
#include <string>

extern std::wstring name; // declare for external use

void Start();

globals.cpp

#include <string>

#include 'globals.h'

std::wstring name;  // **** mandatory to declare here ****

void Start()
{
    name = L"Bob"; // now we can define it
}

source.h

#include <string>

#include 'globals.h'

extern std::wstring zoneName; // declare as an external

source.cpp

#include <string>
#include <iostream>

#include 'globals.h'

int main()
{
	Start(); // value is set in this function
	std::wcout << name;  // name set in globals is output here in source
}