c++ - Redefined variable in the other module -

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Why do we redefine the variable in other modules, so it is OK to add this module together? I have written two modules main.cpp and module.cpp as the following: 

  // - main.cpp-- // # include & lt; Stdio.h & gt; Int main () {int i = 5; Printf ("I value is% d", i); }   
 and  
  // - module.cpp - // int i = 7;   
 I am now compiling and adding  g ++  as the following:  
  g ++ -c main .cpp g ++ - c module. cao g ++ - o bin main.o module.o   
 and it's okay. But I hope the compilation will be successful, while the linker will throw an exception like  redefining varaible  when I'm running  .bin  output then the value of  i is 5 .  
 UPD: I understand that if we use  I  as global variable module  main.cpp  that linker will throw an error.   
 
 
 The main variable in the function is a local variable of i function. It is not visible and is present outside the function is.  
 Consider the following example  
  #include & lt; Iostream & gt; Int i = 3; Int main () {int i = 10 std :: cout & lt; & Lt; "I =" & lt; & Lt; I & lt; & Lt; Std :: endl; Std :: cout & lt; & Lt; ":: I =" & lt; & Lt; :: I & lt; & Lt; Std :: endl; I = :: i; Std :: cout & lt; & Lt; "I =" & lt; & Lt; I & lt; & Lt; Std :: endl; Std :: cout & lt; & Lt; ":: I =" & lt; & Lt; :: I & lt; & Lt; Std :: endl; {Int i = 20; Std :: cout & lt; & Lt; "I =" & lt; & Lt; I & lt; & Lt; Std :: endl; Std :: cout & lt; & Lt; ":: I =" & lt; & Lt; :: I & lt; & Lt; Std :: endl; } Std :: cout & lt; & Lt; "I =" & lt; & Lt; I & lt; & Lt; Std :: endl; Std :: cout & lt; & Lt; ":: I =" & lt; & Lt; :: I & lt; & Lt; Std :: endl; }   
 Local variables have no relation.

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