Re: detecting class member offset from member itself
Jiang:
Firstly, any pointer address can be stored in a char*.
Why?
In C++, every object is nothing more than a finite sequence of bytes in
memory. The address of the first byte is taken as the address of the object
itself.
The smallest addressable unit in C++ is the byte, also known as a
"char". It
has the least strict alignment requirements (in fact, it has _no_ alignment
requirements).
The address of any byte of an object can be stored in a char*.
Are you talking about PODs?
I'm talking about all types, none are excluded.
Secondly, if two pointers point to parts of the same
object, then they can be subracted to give a distance.
Please read 5.7 again. The subtraction operation works for object
itself and you can not apply it to the object's members.
I've no reason to believe that the following wouldn't work perfectly:
#include <string>
struct MyClass {
int a;
std::string b;
void *c;
};
struct MyClassDerived : MyClass {
std::string d;
int e;
char f;
MyClass g;
};
#include <cstddef>
int main()
{
MyClassDerived obj;
std::ptrdiff_t i = (char*)&obj.g.b - (char*)&obj.c;
}
The pointers being subtracted are not unrelated, they're part of the same
object.
The Standard necessitates that the code work perfectly.
I do not see how it works perfectly. There are many ways to
break it in my mind.
For example:
int i = 1;
const int foo::a;
struct foo
{
// static member OK?
static const int a = 2;
// reference member OK?
const int &b;
foo(): b(i){}
// overloaded operator OK?
int operator&(){
return 0xDEADBEFF;
}
// inaccessible default constructor OK?
private:
foo(){}
};
If you apply your Macro to this class/members, you will find
strange results in my mind.
Okay then, let's replace the addressof operator with something like:
template<class T>
char *AddressOf(T const &obj)
{
return &(char&)obj;
}
Boost has a function like this in its library.
To repeat myself, offsetof (or Macros like it) is seldom useful
in our c++ context. Avoid them if possible please.
I use macros now and again and they come in useful.
--
Frederick Gotham
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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