Initialization of virtual function table pointer

From: DaKoadMunky (dakoadmunky_at_aol.com)
Date: 04/30/04

  • Next message: Peter Hermann: "Re: OOP Language for OS Development" 
    Date: 30 Apr 2004 13:20:31 GMT

    I was recently looking at some assembly code generated by my compiler. When
    examining the assembly code associated with constructors I noticed that the
    dynamic-dispatch mechanism was enabled before member initialization but after
    base initialization.

    To speak in implementation details the virtual function table pointer for
    objects being constructed was set before member initialization but after base
    initialization.

    Curiousity then led me to the following toy program...

    <CODE>

    struct Base
    {
            Base(int) {}
    };

    class Derived : public Base
    {
            public:

            Derived():
            Base(ProxyForVirtualFunction()),
            member(ProxyForVirtualFunction())
            {
            }

            private:

            int ProxyForVirtualFunction()
            {
                    return VirtualFunction();
            }

            virtual int VirtualFunction()
            {
                    return 0;
            }

            int member;
    };

    int main()
    {
            Derived();

            return 0;
    }

    </CODE>

    In the example code class "Derived" has a base class "Base" which must be
    initialized with an integer. It also has an int member that is to be
    initialized. In both cases the value used for initialization is produced by
    indirectly calling a virtual function. The reason for the indirect call is
    because my compiler optimizes away direct calls to virtual functions within
    constructors.

    The call sequence that produces the value to be used as the base class
    initializer results in a program crash because the virtual function table
    pointer has not yet been set. The attempt to dispatch the virtual function
    call accesses an out-of-bounds memory location and all hell breaks loose.

    The call sequence that produces the value to be used as the member initializer
    results in no fault because at that point the virtual function table pointer
    has been set.
     
    I am wondering if the C++ language standard states when a dynamic dispatch
    mechanism is expected to be associated with an object under construction and
    what assurances the language standard makes regarding when it is okay to assume
    that dynamic dispatch is safe.

    Any insight would be appreciated.

  • Next message: Peter Hermann: "Re: OOP Language for OS Development"

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