Undefined reference error refers to missing declaration or the definition or both?
Compiler: GCC on Linux
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1Without the actual error it's hard to guess, but probably it's a missing definition. – Some programmer dude Jan 03 '12 at 09:11
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Since compiler/linker errors are tool specific, to get a precise answer you should tell us what compiler you are using. – Péter Török Jan 03 '12 at 09:11
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@JoachimPileborg Well, in my case it was a missing definition. but I wish to know if the declaration is missing isn't the error same? – Aquarius_Girl Jan 03 '12 at 09:12
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@PéterTörök GCC on Linux – Aquarius_Girl Jan 03 '12 at 09:13
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1If the declaration is missing, the error usually says something like "XXX was not declared in this scope". – jrok Jan 03 '12 at 09:13
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@jrok You are correct, I just verified it. Post that as an answer, I'll select it. – Aquarius_Girl Jan 03 '12 at 09:14
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@jrok Yup. It sounds like a linking error when definition of the function is not provided - usually, when you forget to link the library. Missing declaration would yield `undefined symbol`. And it would be compiler error, not linker. – lapk Jan 03 '12 at 09:16
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1"undefined symbol" is typically a linker error as well – Gerald Jan 03 '12 at 09:27
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While it is technically compiler specific, "undefined reference" almost always means - as the error message states - that the definition is missing. I don't know of any compiler that would complain of an "undefined reference" when an unDECLARED symbol is being referenced.
Gerald
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i had the same problem using the library with Qt.
the solution is to specify the path which leads to the -lblob file in the .pro file:
LIBS += -L/user/local/lib \
-lblob
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1This is a very special case, not an answer to the question whether "undefined reference" is caused by missing declarations or definitions. – jogojapan Jun 03 '13 at 08:43
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@jogojapan From the rest of the answers anyone could get an idea of what's going on, it doesn't hurt to add a QT specific answer. Actually, it helps. Also, it is invisible but I suppose the most people who do find this useful are the same people who will not login and not upvote normally... Also, at times when people come here with a mind that usually don't seem to have the luxury of actually getting the picture at their disposal, letting then them have a chance to find the QT specific version might be what they need at the moment... – n611x007 Mar 22 '17 at 09:37
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Usualy (if not always?) it's a linker error and it means missing definition. Compilation errors for missing declarations are similar to: "XXX was not declared in this scope."
jrok
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