openMSX
utils
outer.hh
Go to the documentation of this file.
1
#ifndef OUTER_HH
2
#define OUTER_HH
3
4
#include <bit>
5
#include <cstddef>
6
7
// Example usage:
8
// class Foo {
9
// ...
10
// struct Bar : public SomeBaseClass {
11
// ...
12
// void f() override {
13
// // transform the 'this' pointer to the 'bar' inner object
14
// // into a reference to the outer object of type Foo.
15
// Foo& foo = OUTER(Foo, bar);
16
// }
17
// } bar;
18
// };
19
//
20
// This avoids having to store a back-pointer from the inner 'bar' object to
21
// the outer 'Foo' object.
22
//
23
// The 'bar' object lives at a fixed offset inside the 'Foo' object, so
24
// changing the bar-this-pointer into a Foo-reference is a matter of
25
// subtracting this (compile-time-constant) offset.
26
//
27
// At least the offset is constant in the absence of virtual inheritance (we
28
// don't use this c++ feature in openMSX). Also strictly speaking the c++
29
// standard doesn't guarantee the offset is constant in the presence of virtual
30
// functions, though it is in all practical implementations. See here for more
31
// background info:
32
// http://stackoverflow.com/questions/1129894/why-cant-you-use-offsetof-on-non-pod-structures-in-c
33
34
35
// Adjust the current 'this' pointer (pointer to the inner object) to a
36
// reference to the outer object. The first parameter is the type of the outer
37
// object, the second parameter is the name of the 'this' member variable in
38
// the outer object.
39
40
template
<
typename
T>
int
checkInvalidOuterUsage
(T) {
return
0; }
41
42
#define OUTER(type, member) (checkInvalidOuterUsage<const decltype(std::declval<type&>().member)*>(this), \
43
*std::bit_cast<type*>(std::bit_cast<uintptr_t>(this) - offsetof(type, member)))
44
45
#endif
checkInvalidOuterUsage
int checkInvalidOuterUsage(T)
Definition
outer.hh:40
Generated on Fri Oct 11 2024 19:13:47 for openMSX by
1.9.8