Inside and Outside of Classes (struct, class), Instantiation Refer to Previous Chapter

Outside the Class	Inside the Class
Symbols modified are local during the linking phase, visible only to the compilation unit (.obj) where they are defined	This part of memory is shared by all instances of the class. Multiple instantiations will still result in only one instance of the static variable (see next chapter)

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Even if you instantiate this class or struct many times, there will only be one instance of that static variable. The same applies to static methods inside the class; these methods do not have an instance pointer (this).

Static variables are stored in the static storage area, not on the stack or heap. They exist throughout the entire execution of the program and have only one instance.

A static variable or function means that when linking to its actual definition, the linker will not find its definition outside of this compilation unit (.obj), somewhat similar to the private attribute of a class.

// Static.cpp
static int s_Variable = 5;  // s stands for static

// Main.cpp
#include <iostream>

int s_Variable = 10;

int main()
{
    std::cout << s_Variable << std::endl;   // 10
    std::cin.get();
}

// Static.cpp
int s_Variable = 5;  // remove static

// Main.cpp
#include <iostream>

int s_Variable = 10;

int main()
{
    std::cout << s_Variable << std::endl;   // LNK ERROR already defined in Main.obj
    std::cin.get();
}

Therefore, two global variables cannot have the same name.

Solution 1: extern link

extern will look for the definition of s_Variable in another compilation unit, known as external linkage or external linking.

// Static.cpp
int s_Variable = 5;

// Main.cpp
extern int s_Variable;   // It is a reference to the variable

std::cout << s_Variable << std::endl;  // 5

This means that the functions and variables you define are only "visible" to the cpp file (compilation unit) where they are declared. Using global is not recommended as it can easily lead to bugs.