zzlljj / iface

Anonymous, non-intrusive interfaces in C++

iface

This project brings anonymous, non-intrusive interfaces to C++. Leveraging P0315R4, iface allows declaring an interface directly in a function signature - see example below. Currently tested on VS 2019 Preview only.

Examples

An interface can be declared anywhere a decltype can appear.

#include <iface.h>
#include <stdio.h>

void foo(IFACE((speak, void())(walk, void())) animal) {
    animal.speak();
    animal.walk();
}

struct Dog {
    void speak() const noexcept { puts("woof"); }
    void walk() const noexcept { puts("dog lollops along"); }
};

struct Cat {
    void speak() const noexcept { puts("meow"); }
    void walk() const noexcept { puts("tabby won't be bothered"); }
};

int main() {
    Dog dog;
    Cat cat;
    foo(dog);
    foo(cat);
}

Example of using iface at interface boundary. iface_base occupies 16B (tbl 8B + obj 8B) and is hence stored on stack, its address passed in rcx, as per the x64 calling convention.

int __declspec(noinline) foo(IFACE((f, int() const)) x) { return x.f(); }
//  mov         rax,qword ptr [rcx]        ; f (tbl SBO)
//  mov         rcx,qword ptr [rcx+8]      ; obj
//  jmp         rax

struct S { int f() const noexcept { return 42; } };
//  mov         eax,2Ah
//  ret

int calls_foo() { return foo(S{}); }
//  sub         rsp,38h
//  lea         rax,[glue::fn (...)]       ; f (tbl SBO)
//  lea         rcx,[rsp+20h]              ; iface_base at [rsp+20h]
//  mov         qword ptr [rsp+20h],rax    ; set tbl
//  call        foo (...)                  ; note no obj setup (obj SOO)
//  add         rsp,38h
//  ret

Using in your project

Please see LICENSE for terms of use.

The easiest way to use iface in your project is to include it as a submodule: git submodule add [email protected]:j-nikki/iface.git. Now discover iface in your CMake file (so you can #include <iface.h> in C++):

add_subdirectory("iface")
target_link_libraries(<target-name> ... iface)