Why is it a linker error to #include this file more than once?
c++
Solution
Think about if from the code perspective - there is this symbol `superGlobal` that points at an integer.
You have a bunch of .o files to link together into a single executable. Each of the .o files has it's own `superGlobal` symbol. Which should the linker use?
The `extern` declaration says: another of the compilations units (.o files) is going to declare this symbol, so I can use it. Thus there is only one copy so the linker knows what to do.
Problem
This question isn't so much a 'how to solve' question as its a question about why doesn't this work? In C++ I can define a bunch of variables that I want to use across multiple files in a few ways. I can do it like this: ``` int superGlobal; #include "filethatUsesSuperglobal1.h" int main() { // go. } ``` That way ONLY works if "filethatUsesSuperglobal1.h" has its entire implementation there in the header and no attached .cpp file. Another way (the "morer correcter" way) is to use extern: externvardef.h ``` #ifndef externvardef_h #define externvardef_h // Defines globals used across multiple files. // The only way this works is if these are declared // as "extern" variables extern int superGlobal; #endif ``` externvardef.cpp ``` #include "externvardef.h" int superGlobal; ``` filethatUsesSuperglobal1.h ``` #include "externvardef.h" #include <stdio.h> void go(); ``` filethatUsesSuperglobal1.cpp ``` #include "filethatUsesSuperglobal1.h" void go() { printf("%d\n", superGlobal ); } ``` main.cpp ``` #include <stdio.h> #include "externvardef.h" #include "filethatUsesSuperglobal1.h" int main() { printf( "%d\n", superGlobal ) ; go() ; } ``` This is a small point and a somewhat nit picky question, but Why do I need to declare it extern - should not the #include guards on "externvardef.h" avoid redefinition? Its a linker error, even though that header file has #include guards around it. So its not a compiler error, its a linker error, but why.