Compiler Optimizations - Function has no address

问题

I have not used much pointers to member functions but I think that found some dangerous scenarios when using such pointers.

The problem comes when compiler decides not to assign address to function, because of some optimization. It happened with VS 2015 even in Debug, x86 (with disabled Optimization - /Od). I am refactoring one old system, moving some code in a common static library (common.lib) so to be able to be used from several projects. Even if not the best pattern, the old implementation depends heavily from function member pointers and I do not want to change this. For example, I added the interface ModuleBase to one very big old class to something like:

class ModuleBase
{
public:
    typedef void (ModuleBase::*Main)() const;  // moved from old module

    virtual void FunctionMain() const = 0;  //  Function has no address, possibly due to compiler optimizations.    
    virtual void FunctionSecondary() const = 0;  // Function has no address, possibly due to compiler optimizations.
};


class OldModule : public ModuleBase
{
public:
    virtual void FunctionMain() const {};
    virtual void FunctionSecondary() const {};
}

The idea was to move ModuleBase in the Static library, but OldModule to remain in the main EXE project. While ModuleBase was in the main project it worked fine but when I move it in the static Common.lib it start crashing! It took me about 2 days to finally notice that at several places the compiler decided (but only for the Static Library) not to assign addresses to FunctionMain, FunctionSecondary() and etc.. from ModuleBase. So when pointers to these virtual functions were passed to other routines they were zeroes.

For example in the code bellow:

new Manager::ModuleDecription(
        "Test Module",
        "Secondary Scene",
        "Description"
         PosX,
         PosY,
         Proc,
         &ModuleBase::FunctionSecondary   //contains nullptr when in static library!!!!!

The last member in the structure was zero but only when is in the static library. It was quite nasty because I had to check many other things before to notice this. Also there are other pointers which were not zero because the structure was not zeroed in the constructor so one has to notice that address value is different and crashes when trying to call the function.

So my questions are - 1) Am I seeing this right - is this valid situation (that compiler is removing functions addresses, for the same code when moved in a static library)?

2) How to force compiler always to keep the member function addresses?

回答1:

My apology, I found no problems with the addresses of pointers-to-members-functions in Visual Studio. Pointers to the base interface virtual functions are resolved Ok, even if placed in a Static Library. Reasons for my problems were:

1) Debugger sometimes shows function addresses of template classes as zeroes

2) Reason for the crashes was that the main project had the /vmg compiler option, but I missed to put it in the Static Library project. In such case one should be careful to use /vmg always in all referenced library projects (complications because of it is another topic).

Anyway, using pointers-to-members functions together with the object pointer is usually a sign of bad underlying design.

I hope this may help someone.

来源：https://stackoverflow.com/questions/54388905/compiler-optimizations-function-has-no-address