source: libcfa/src/raii.hfa@ a800a19

//
// Cforall Version 1.0.0 Copyright (C) 2023 University of Waterloo
//
// The contents of this file are covered under the licence agreement in the
// file "LICENCE" distributed with Cforall.
//
// raii.hfa -- PUBLIC
// Utilities for advanced RAII (constructor/destructor) patterns
//
// Author           : Mike Brooks
// Created On       : Fri Sep 22 15:00:00 2023
// Last Modified By :
// Last Modified On :
// Update Count     :
//
#pragma once

// Provides access to unititialized storage.
// Intended to make cheap delayed intialization possible.
// Similar to uC++ uNoCtor.
// Regardless of what constructors T offers, the declaration
//   uninit(T) x;
// makes x:
//   - assignable to T,
//   - be, at first, uninitialized, and
//   - receive a T-destructor call when x goes out of scope.
// This sitation means the user becomes responsible for making a placement constructor call
// on x before its first use, even if this first use is the implicit destructor call.
// This sitation contrasts with that of
//   T y @= {};
// in that y does not receive an implied destructor call when it goes out of scope.
// This sitation contrasts with that of
//   optional(T) z;
// in that z receives a T-destructor call conditionally upon the runtime-tracked state,
// and that z's assignability to T is guarded by the runtime-tracked state.
//
// Implementation note: the uninit RAII that follows is a parade of cfa-cpp quirk exploitations.
//
forall( T* )
struct uninit {
    inline T;
};

// Parameterless ctor: leaves bits within uninitialized.
forall( T* )
void  ?{}( uninit(T) & this ) { 

    // Implementation takes advantage of CFA-available unsoundness.
    // It could be called a bug; if it's fixed, then uninit needs an escape hatch,
    // or to find a different loophole.

    // Fundamental unsoundness: Here is a constructor for a T, whatever T is.
    // Sound compiler reaction: We don't know what fields T has,
    // so the programmer is surely failing to initialize all of T's fields,
    // for some choice of T.
    // Current compiler reaction: Ok, it initializes all the fields we know about.
    void ?{}( T & ) {}

    // Now for some ado about nothing.
    // We need to call the above constructor on the inline T field.
    //   Becasue the compiler holds us accountable for intizliing every field of uninit(T).
    //   We are happy to do so and are not trying to get out of it.
    // But the compiler doesn't recognize this form as a field initialization
    //   T & inner = this;
    //   ( inner ){};
    // And the compiler doesn't offer this feature
    //   ( (return T &) this ){};
    // It does recognize this form...

    ( (T&) this ){};

    // ...though it probably shouldn't.
    // The problem with this form is that it doesn't actually mean the Plan-9 base field.
    // It means to reinterpret `this` with type T.
    // For a plan-9 use in which the base-type field is not first,
    // this form would send the wrong address to the called ctor.
    // Fortunately, uninit has the base-type field first.
    // For an RAII use in which the constructor does something,
    // getting the wrong address would matter.
    // Fortunately, ?{}(T&) is a no-op.
}

// dtor: pass-through
forall( T* | { void ^?{}( T& ); } )
void ^?{}( uninit(T) & this) {
    // an inner dtor call is implied

    // In fact, an autogen'd dtor would have sufficed.
    // But there is no autogen'd dtor because no T-dtor is asserted on the struct declaration.
    // Adding assertions to the struct decl would make the intended ctor (implemented above)
    // a less preferred candidate than the declared, but undefined, (ugh!) autogen ctor.
}

// Optional explicit inner-ctor invoation helper.
// Generally optional, because 1 and 2 below are equivalent:
//   struct Foo;
//   void ?{}( Foo &, X, Y, Z );
//   uninit(Foo) uf;
//   ?( uf ){ x, y, z };      // 1
//   emplace( uf, x, y, z );  // 2
// Is necessary for reaching a parameterless constructor 
//   void ?{}( Foo & );
//   ?( uf ){};               // calls ?{}( uninit(Foo) & ), which does nothing
//   emplace( uf );           // calls ?{}( Foo & ), probably what you want
forall( T*, Args... | { void ?{}( T&, Args ); } )
void emplace( uninit(T) & this, Args a ) {
    T & inner = this;
    ( inner ){ a };
}