container_of

Type-safe inverse of pointer-to-member operators for C++11.

Motivation

In C++, one can create a pointer to non-static data member of a structure. The type of such a pointer is T X::*, where X is the type of the structure and T is the type of the member. For example,

struct point_t
{
    int x;
    int y;
};

int point_t::* mp = &point_t::y;

The pointer-to-member object effectively behaves as a type-safe offset, you can use the .* operator to move from a pointer to the structure to the pointer to its member.

point_t point;
int * coord = &point.*mp;
assert(coord == &point.y);

However, C++ doesn't provide any way to recover the pointer to the structure, given a pointer to its member and the appropriate pointer-to-member object. This library provides container_of function that does exactly that.

point_t * pt = container_of(coord, mp);
assert(pt == &point);

The function has therefore the same effect as the container_of macro in the Linux kernel or CONTAINING_RECORD in Windows, except that the member specification is an object that can be passed around.

You can now pass pointer-to-member objects around instead of member offsets.

Getting started

The library is header-only. To use it, clone the repo somewhere and add the include directory to your include path.

If you're using CMake, use the FetchContent module.

FetchContent_Declare(
    avakar.container_of
    GIT_REPOSITORY https://github.com/avakar/container_of.git
    GIT_TAG main
    GIT_SHALLOW 1
    )
FetchContent_MakeAvailable(avakar.container_of)

target_link_libraries(my_target PUBLIC avakar::container_of)

Usage

Include <avakar/container_of.h>, which provides the following function.

template <typename X, typename T>
X * avakar::container_of(T * p, T X::* mp) noexcept;

Both p and mp shall be non-null pointers. The object pointed to by p shall be a member of an object of type X corresponding to the member referred to by mp. mp shall not refer to a member of a virtual base of X (such pointers cannot be formed in standard C++, but Microsoft Visual Studio compiler allows it as an extension). Otherwise, the behavior is undefined.