Joke Errors:
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Linker Errors
Unresolved symbols
(MSVC) error LNK2019: unresolved external symbol "__A__" (__B__) referenced in function __C__
(GNU) /usr/bin/ld: Undefined symbols:
__A__
__A__
__A__
Simple enough: this means that you try to use A (of mangled name B) in C, but the linker can't find it. Common causes include:
- Mislinked (if the function/variable belongs to a given library)
- Forgot to link against the library that implements it
- Built the library with a different calling convention than the one used in your program
- Misimplemented (if the function/variable belongs to you)
- Forgot to implement the function ("void foo();" without a "void foo() { … }")
- Forgot to implement the variable ("extern int var;" without a "int var;" (in 1 source file))
- Typeoed the name (remember, C++ is case sensitive)
- Implemented in the wrong scope ("::Foo" instead of "::Baz::Bar::Foo", for example)
- Misdeclared & Used (rare, requires multiple screwups)
- Typeoed the name ("void Foo(); void user() { Foo(); }", when the function is implemented as "void foo() { … }")
- Declared in the wrong scope ("::Foo" instead of "::Baz::Bar::Foo", for example)
- Macro Interference
- The function name is #defined in an API header to something else, and inconsistently #included in the using and defining code
- Biggest offender to check: The Win32 API. See if your function name turns up a hit in The MSDN if you're using CamelCase, or #define NOMINMAX first if trying to use the non-macro min/max es
- Second biggest offender (IMO :P): The Ruby API (if writing Ruby extensions). Redefines a lot of standard C library function names, including those reused in the C++ standard library, causing many problems. See libindustry's industry/languages/ruby/detail/wrap_retarded_ruby.hpp for that.
Multiply defined symbols
(MSVC) error LNK2005: "__A__" (__B__) already defined in __C__
Simple enough: The definition/implementation of A (of mangled name B) has been found in multiple places — wherever this error occured, as well as in C. Common causes include:
- Forgot to mark a function to be inlined as inline
- Forgot to mark a global variable decleration as extern
Compiler Errors
std::string operator confusion
(MSVC) error C2678: binary '==' : no operator found which takes a left-hand operand of type 'const std::string' (or there is no acceptable conversion)
guiddef.h(192): could be 'int operator ==(const GUID &,const GUID &)'
(MSVC) error C2678: binary '!=' : no operator found which takes a left-hand operand of type 'const std::string' (or there is no acceptable conversion)
guiddef.h(197): could be 'int operator !=(const GUID &,const GUID &)'
These are typically caused by a missing #include <string>. Other headers you've included provide the declaration for std::string (allowing you to create references to them without error), but not the definition of std::string and it's operators (hence your compiler's confusion). Including the header will provide all the missing definitions.
