C++ – Eclipse CDT: How to reference 3rd party includes via a Relative path

Eclipse variables, such as ${workspace_loc} and ${project_loc} only work for things that are actually in the runtime Eclipse workspace. When you say you put them under the workspace in the filesystem, did you actually import the files into Eclipse (using new Project; or creating a linked folder in an existing project)?

If the files exist in the filesystem but not in Eclipse itself then Eclipse will only be able to access them with absolute paths.

BTW, CDT has variables of its own: ${WorkspaceDirPath} ${ProjDirPath} ${ProjName} which expand to the particular filesystem location.

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.