GCC C++ Linker errors: Undefined reference to 'vtable for XXX', Undefined reference to 'ClassName::ClassName()'

Question

I'm setting up a C++ project, on Ubuntu x64, using Eclipse-CDT. I'm basically doing a hello world and linking to a commerical 3rd party library.

I've included the header files, linked to their libraries, but I still get linker errors. Are there some possible problems here other than the obvious (e.g. I am 99% sure I'm linking to the correct library).

1. Is there a way to confirm the static libraries I am linking to are 64bit?
2. Is there a way to confirm that the library has the class (and methods) I am expecting it to have?

Eclipse says:

 Building target: LinkProblem Invoking: GCC C++ Linker g++ -L/home/notroot/workspace/somelib-3/somelib/target/bin -o"LinkProblem"  ./src/LinkProblem.o   -lsomelib1 -lpthread -lsomelib2 -lsomelib3 ./src/LinkProblem.o: In function `main': /home/notroot/workspace/LinkProblem/Debug/../src/LinkProblem.cpp:17: undefined reference to `SomeClass::close()' ./src/LinkProblem.o: In function `SomeOtherClass': /home/notroot/workspace/somelib-3/somelib/include/sql/somefile.h:148: undefined reference to `SomeClass::SomeClass()' /home/notroot/workspace/somelib-3/somelib/include/sql/somefile.h:148: undefined reference to `vtable for SomeOtherClass' /home/notroot/workspace/somelib-3/somelib/include/sql/somefile.h:151: undefined reference to `SomeClass::~SomeClass()' ./src/LinkProblem.o: In function `~SomeOtherClass': /home/notroot/workspace/somelib-3/somelib/include/sql/somefile.h:140: undefined reference to `vtable for SomeOtherClass' /home/notroot/workspace/somelib-3/somelib/include/sql/somefile.h:140: undefined reference to `SomeClass::~SomeClass()' /home/notroot/workspace/somelib-3/somelib/include/sql/somefile.h:140: undefined reference to `SomeClass::~SomeClass()' collect2: ld returned 1 exit status make: *** [LinkProblem] Error 1 

Answer 1

This linker error usually (in my experience) means that you've overridden a virtual function in a child class with a declaration, but haven't given a definition for the method. For example:

class Base {     virtual void f() = 0; } class Derived : public Base {     void f(); } 

But you haven't given the definition of f. When you use the class, you get the linker error. Much like a normal linker error, it's because the compiler knew what you were talking about, but the linker couldn't find the definition. It's just got a very difficult to understand message.