openMSX
outer.hh
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1#ifndef OUTER_HH
2#define OUTER_HH
3
4#include <cstddef>
5
6// Example usage:
7// class Foo {
8// ...
9// struct Bar : public SomeBaseClass {
10// ...
11// void f() override {
12// // transform the 'this' pointer to the 'bar' inner object
13// // into a reference to the outer object of type Foo.
14// Foo& foo = OUTER(Foo, bar);
15// }
16// } bar;
17// };
18//
19// This avoids having to store a back-pointer from the inner 'bar' object to
20// the outer 'Foo' object.
21//
22// The 'bar' object lives at a fixed offset inside the 'Foo' object, so
23// changing the bar-this-pointer into a Foo-reference is a matter of
24// subtracting this (compile-time-constant) offset.
25//
26// At least the offset is constant in the absence of virtual inheritance (we
27// don't use this c++ feature in openMSX). Also strictly speaking the c++
28// standard doesn't guarantee the offset is constant in the presence of virtual
29// functions, though it is in all practical implementations. See here for more
30// background info:
31// http://stackoverflow.com/questions/1129894/why-cant-you-use-offsetof-on-non-pod-structures-in-c
32
33
34// Adjust the current 'this' pointer (pointer to the inner object) to a
35// reference to the outer object. The first parameter is the type of the outer
36// object, the second parameter is the name of the 'this' member variable in
37// the outer object.
38
39template<typename T> int checkInvalidOuterUsage(T) { return 0; }
40
41#define OUTER(type, member) (checkInvalidOuterUsage<const decltype(std::declval<type&>().member)*>(this), \
42 *reinterpret_cast<type*>(reinterpret_cast<uintptr_t>(this) - offsetof(type, member)))
43
44#endif
int checkInvalidOuterUsage(T)
Definition outer.hh:39