Redefined variable in the other module

Question

Why when we redefined variable in the other modules it's ok to linking this modules together? I've written two modules main.cpp and module.cpp as the following:

//--main.cpp--//
#include <stdio.h>

int main(){
    int i=5;
    printf("The value of i is %d",i);
}

and the

//--module.cpp--//
int i=7;

Now I'm compiling and linking their with g++ as the following:

g++ -c main.cpp
g++ -c module.coo
g++ -o bin main.o module.o

And it's ok. But I'm expected that the compilation will be successful only whereas linker will throw an exception like redefining varaible. When I'm running ./bin output is The value of i is 5.

UPD: I'm understand that if we declare i as global variable of module main.cpp that linker will throw an error.

Answer 1

In function main variable i is a local variable of the function. It is not visible and exists outside the function.

Consider the following example

#include <iostream>

int i = 3;

int main()
{
   int i = 10

   std::cout << "i = " << i << std::endl;
   std::cout << "::i = " << ::i << std::endl;

   i = ::i;

   std::cout << "i = " << i << std::endl;
   std::cout << "::i = " << ::i << std::endl;

   {
        int i = 20;

        std::cout << "i = " << i << std::endl;
        std::cout << "::i = " << ::i << std::endl;
   }

   std::cout << "i = " << i << std::endl;
   std::cout << "::i = " << ::i << std::endl;
}

Local variables have no linkage.