Capture *this with initializer

We propose to allow the lambda by-value capture of *this to rebind this to arbitrary objects: [*this = std::move(x)](){}

Ordinary capture	`*this` capture
struct X { void f(int n); void g() { // Capture copy of current object X that = this; [that] (int n) { that.f(n); } // C++11 [self = this] (int n) { self.f(n); } // C++14 } void h() { // Capture copy of different object X that = X(); [that] (int n) { that.f(n); } // C++11 [self = X()] (int n) { self.f(n); } // C++14 [self = std::move(*this)](int n) { self.f(n); } // C++14 } };	struct X { void f(int n); void g() { // Capture copy of current object [this] (int n) { f(n); } // C++17 } void h() { // Capture copy of different object [this = X()] (int n) { f(n); } // New! [this = std::move(this)](int n) { f(n); } // New! } };

In particular, this feature “adds move semantics to C++17’s value-capture of *this”. Situations that motivated value-capture (see P0018R3), such as asynchronous dispatch, arise when then current class instance does not outlive the desired lambda, in which case the lambda needs a copy of the current instance. However, in typical cases where only one such “asynchronous hand-off” happens, a copy is not actually required, and it suffices to move the instance data along into the lambda.

Discussion

C++11 introduced lambda expressions that could capture named entities from their environment by value. C++14 extended this facility to allow capture by value of arbitary expressions. C++17 introduced the ability for lambda expressions that appear in the definition of a non-static class member function to rebind the meaning of the this keyword inside the lambda body: The rebound this would refer to a private copy of the class instance that is captured by the lambda expression and stored within the closure object (P0018R3):

The extension provided by P0018R3 looks simple, but is conceptually profound: For the first time, the meaning of this can vary inside a single class’s scope and be rebound by the user. Once we are comfortable with this situation, it is only natural to ask for ability to rebind this to any captured value of the right type. Just as C++14 added value capture from expressions to C++11 value capture, we propose to add *this capture from expressions to C++17 *this capture.

Unlike the simple-capture “*this”, which would be equivalent under this proposal to “*this = *this”, the init-capture form is not constrained to appear in a non-static class member, since it does not require a prototypical instance to copy from, and we can allow it in static member functions just as well.

struct X { std::unique_ptr<int> p_; void g(int n) const; auto f() && { return [*this = std::move(*this)](int n) { g(n + *p_); }; // move the current object } void f(X x) { return [*this = std::move(x), m = *p_]() { g(m + *p_); }; // move some other object } static auto CallMyX(int m) { return [*this = X{std::make_unique<int>(m)}](int n) { g(n + *p_); } // use a newly created instance } };

Proposed wording

[…] It shall not appear before the optional cv-qualifier-seq and it shall not appear within the declaration of a static member function (although its type and value category are defined within a static member function as they are within a non-static member function), except in the body of a lambda expression that captures *this [Note: necessarily via init-capture – end note]. [Note: This is because […]

init-capture:
identifier initializer
& identifier initializer
* this initializer

An init-capture that is not * this initializer behaves as if it declares and explicitly captures a variable of the form “auto init-capture ;”. An init-capture * this initializer behaves as if it declares and explicitly captures a variable of the form “auto __this initializer ;”. The ~~whose~~ declarative region of this variable is the lambda-expression’s compound-statement, except that:

if the capture is by copy (see below), the non-static data member declared for the capture and the variable are treated as two different ways of referring to the same object, which has the lifetime of the non-static data member, and no additional copy and destruction is performed, and
if the capture is by reference, the variable’s lifetime ends when the closure object’s lifetime ends.

[Note: This enables an init-capture like “x = std::move(x)”; the second “x” must bind to a declaration in the surrounding context. – end note]

[…] If *this is captured by a local lambda expression, its nearest enclosing function shall be a non-static member function. If __this is captured, the nearest enclosing function shall be a member function and the type of __this shall be the same as that of the enclosing class. If a lambda-expression […]

[…] If *this is captured by copy or __this is captured, each odr-use of this is transformed, respectively, into a pointer to the corresponding unnamed data member or to the __this member of the closure type, cast (8.4) to the type of this. […]

Non-proposal

This proposal is limited to value capture of *this. A natural question is why we would not also allow reference capture of *this to rebind this. The reason is that such an extension seems much more difficult, and it seems less useful to the author.

Let us briefly examine the problems. A reference init-capture currently starts with &. The natural syntax for reference init-capture of *this would be “[&*this = x]”. This syntax may look surprising at first sight, but it is natural if we think of *this as the fundamental object. But then we would end up with two separate syntaxes, [this] for the simple-capture and [&*this = x] for the init-capture, and [this] would be equivalent to [&*this = *this]. If we were tempted to replace &*this with just this for backwards “consistency”, we would end up with paradoxical syntax like [this = *this]. Alteratively, or additionally, we could consider a separate pointer capture [this = &x] that reseats this. We believe that such extensions should be discussed in depth in a separate proposal.

Capture `*this` with initializer

Contents

Revision history

Summary

Discussion

Proposed wording

Non-proposal