Question

I’m using static initialisation to ease the process of registering some classes with a factory in C++. Unfortunately, I think the compiler is optimising out the ‘unused’ objects which are meant to do the useful work in their constructors. Is there any way to tell the compiler not to optimise out a global variable?

class SomeClass {
    public:
        SomeClass() {
            /* do something useful */
        }
};

SomeClass instance;

My breakpoint in SomeClass’s constructor doesn’t get hit. In my actual code, SomeClass is in a header file and instance is in a source file, more or less alone.

EDIT: As guessed by KJAWolf, this code is actually compiled into a static lib, not the executable. Its purpose is to register some types also provided by the static lib with a static list of types and their creators, for a factory to then read from on construction. Since these types are provided with the lib, adding this code to the executable is undesirable.

Also I discovered that by moving the code to another source file that contains other existing code, it works fine. It seems that having a file purely consisting of these global objects is what’s causing the problem. It’s as if that translation unit was entirely ignored.

Answer 1

The compiler is not allowed to optimiza away global objects.
Even if they are never used.

Somthing else is happening in your code.
Now if you built a static library with your global object and that global object is not referenced from the executable it will not be pulled into the executable by the linker.