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c++variablesreferenceextern

Why does "extern int &c;" working fine?

In C++, reference variable must be initialized. int &a; // Error

static int &b; // Error

But 

extern int &c; // No error

Why Compiler doesn't give an error for extern specifier reference?

Solution

  • The extern keyword is a directive for the compiler that you are now declaring a symbol that will be filled in during linking, taken from another object file. The initialization is EXPECTED to happen where the actual symbol is defined.

    If you have an a.c file with

    int foo;
int &bar = foo;

    And a b.c file with

    extern int &bar;

    When you compile the file b.c into b.o the compiler will leave the symbol for bar empty. When linking the program, the linker will need to find the exported symbol bar in a.o and will then replace the blank symbol in b.o with the bar from a.o

    If the linker can't find the required symbol anywhere in the linked object files - a Linker error (not compiler error) will be issued.