Cant extern a vector if its declared in a header?

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Zeal

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Location: Colorado Springs, CO

Posted: 27th Jul 2006 03:10

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Just a general c++ question, im sure somebody here can answer.

Why is it that if you declare a vector in one cpp

+ Code Snippet

//main.cpp

std::vector<int> myVec

Then 'tell' another cpp about it via the extern command

+ Code Snippet

//other.cpp

extern std::vector<int> myVec

Everything works fine. This works for any data type (ints, floats, even custom data structs). HOWEVER! For some krazy ass reason this ONLY works IF the vector is declared in a cpp. If you declare the vector in a header, and include that header in the main cpp you get the following error...

+ Code Snippet

: error LNK2001: unresolved external symbol "class std::vector<int,class std::allocator<int> > myVec" (?myVec@@3V?$vector@HV?$allocator@H@std@@@std@@A)

The krazy part is, this is ONLY a problem with vectors. EVERY other data type can be externed just fine, if its declared in a cpp, header, doesnt matter. Why?!

All you need is zeal

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APEXnow

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Posted: 27th Jul 2006 03:15

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it's header order. Your trying to use the vector template before #include <vector> is used possibly?

Paul.

Home of the Cartography Shop - DarkBASIC Professional map importer

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Zeal

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Posted: 27th Jul 2006 03:21

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I dont see how it can be the order... The vector is declared during the app 'initialization'. The vector isnt actually 'used' until later in the main loop.

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Sephnroth

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Posted: 27th Jul 2006 03:29

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remember, headers are not compiled, they get added to cpp files which are compiled into objects and if that object does not have a link to the relevent lib that you tried to use in the header them boom.

secondly however, why would you want to declare a variable in a header?! typedefs, yes. prototypes yes, externs yes, static/constants yes - but actual generic variables in a header file? sounds like its asking for trouble to me

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Zeal

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Posted: 27th Jul 2006 03:36

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Whoops nm! Was a error on my end, looks like it DOES work from a header.

As for WHY I want to declare variables inside a header... heh its a long story.

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