C++17 lambda capture *this

Question

C++17 will add copy capture of this object by value, with a capture specification of [*this].

How is this useful? How is it different than capturing this? Can't this already be achieved in C++14 with [tmp = *this]?

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Bonus for explaining why P0018R3 uses [=, tmp = *this] instead of [tmp = *this] in their example. If they had used [tmp = *this], all the listed downsides of the C++14 solution would be eliminated.

Answer 1

How is it useful? It's useful when you need a copy of *this - for example, when *this itself is no longer valid by the time the lambda is evaluated.

How is it different from capturing this? It makes a copy of the object, so that when the lambda is evaluated, its this pointer refers to the copy, rather than to the original object.

Can it be achieved in C++14 with [tmp = *this]? It can, but [*this] is more convenient, as code can be moved without prefixing member access with tmp.. Otherwise, especially with [=, tmp = *this], it's possible to accidentally refer to members of the original object when you meant to refer to the copy (particularly if you're in the habit of cut+paste programming). [=,*this] is a safer alternative in this case, as the original object is not accessible from inside the lambda's body (at least, not via the this pointer).

Answer 2

Imagine that *this is a handle class, which maintains a shared_ptr to some shared state.

The shared impl is (for example) a protocol handler state machine.

The handle class is passed through a series of asynchronous handlers, so itself must be copyable. Each handler mutates the shared state.

A strong use case for this might be a protocol handler for use with a custom asio service (for example, an http_protocol_socket).

[=, tmp = *this] will promiscuously capture any variables by value, including, rather dangerously, the this pointer itself, as well as specifically capturing *this into tmp.

In this use case it would be dangerous to inadvertently refer to this in the async handler, because it's likely to be a dangling pointer. This is a bug waiting to happen.

[tmp=*this] will only capture *this.