extern keyword in c++

Abstract:
"extern" keyword has two functions:
1)  extern “C” in C++. When some code is put in extern “C” block, the C++ compiler ensures that the function names are unmangled – that the compiler emits a binary file with their names unchanged, as a C compiler would do.


2)  extend life of a variable/function to span the scope of multiple files.

case 1:

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// Save file as .cpp and use C++ compiler to compile it extern "C" {     int printf(const char *format,...); }   int main() {     printf("GeeksforGeeks");     return 0; }

case 2:

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Examples:

extern int i;

declares that there is a variable named i of type int, defined somewhere in the program.

extern int j = 0;

defines a variable j with external linkage; the extern keyword is redundant here.

extern void f();

declares that there is a function f taking no arguments and with no return value defined somewhere in the program; extern is redundant, but sometimes considered good style.

extern void f() {;}

defines the function f() declared above; again, the extern keyword is technically redundant here as external linkage is default.

extern const int k = 1;

defines a constant int k with value 1 and external linkage; extern is required because const variables have internal linkage by default.

extern statements are frequently used to allow data to span the scope of multiple files.