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Smaug compiling using g++

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David Haley USA (3,881 posts) Bio

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Reply #15 on Tue 11 Dec 2007 06:50 AM (UTC)

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Ah yes, good old extern, I didn't notice those were coming from a .h file.

OK... well, the thing is that whenever the compiler sees something of the form:

VAR_TYPE VAR_NAME;

it creates memory of size sizeof(VAR_TYPE) and all future occurrences of VAR_NAME will refer to that block of memory.

But sometimes you want to let several source files know that a given variable (i.e. named block of memory) exists. You could do something like VAR_TYPE VAR_NAME; in each source file, but that would be incorrect: that would create one block per file. That is the error you were getting, actually: the compiler was telling you that something was happening you didn't want.

The extern keyword tells the compiler not to allocate the memory, but to defer looking up that variable's memory address until the link phase. That way, all of your source files contain references to the variable name, but not the location in code; during link, the linker will go in and fix all those references to point to the right place.

So, you still need somewhere to have VAR_TYPE VAR_NAME in order for the compiler to allocate the memory. But you only want that in one spot: everywhere else where you want to [i]refer[/i] to that variable you need to have the extern keyword in front.

The intuition behind the keyword is that you are declaring that a variable is external with respect to the file being compiled, that is, you are assuring the compiler that the variable has memory out there somewhere and that it should defer looking that up until it has the external object files to link against.

David Haley aka Ksilyan
Head Programmer,
Legends of the Darkstone

http://david.the-haleys.org

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