c++ - Position of a vpointer in an object -



c++ - Position of a vpointer in an object -

class c { public: c() : m_x(0) { } virtual ~c() { } public: static ptrdiff_t member_offset(const c &c) { const char *p = reinterpret_cast<const char*>(&c); const char *q = reinterpret_cast<const char*>(&c.m_x); homecoming q - p; } private: int m_x; }; int main(void) { c c; std::cout << ((c::member_offset(c) == 0) ? 0 : 1); std::cout << std::endl; std::system("pause"); homecoming 0; }

the programme above outputs 1. check addresses of c object , c's field m_x. prints out 1 means addresses not equal. guess is because d'tor virtual compiler has create vtable class , set vpointer in class's object. if i'm wrong please right me.

apparently, puts vpointer @ origin of object, pushing m_x field farther , giving different address. case? if standard specify vpointer's position in object? according wiki it's implementation-dependent. , position may alter output of program.

so can predict output of programme without specifying target platform?

there no "vpointers" in c++. implementation of polymorphism , dynamic dispatch left compiler, , resulting class layout not in way specified. object of polymorphic type have carry some state in order identify concrete type when given view of base of operations subobject.

implementations vtables , vptrs mutual , popular, , putting vptr @ origin of class means don't need pointer adjustments single inheritance , downcasts.

many c++ compilers follow (parts of) itanium abi c++, specifies class layout decisions this. popular article may provide insights.

c++ pointers memory-address vtable

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