Avoid capturing by reference in lambdas that will be used non-locally, including returned, stored on the heap, or passed to another thread

Reason

Pointers and references to locals shouldn't outlive their scope. Lambdas that capture by reference are just another place to store a reference to a local object, and shouldn't do so if they (or a copy) outlive the scope.

Example, bad

int local = 42;

// Want a reference to local.
// Note that after program exits this scope,
// local no longer exists, therefore
// process() call will have undefined behavior!
thread_pool.queue_work([&] { process(local); });

Example, good

int local = 42;
// Want a copy of local.
// Since a copy of local is made, it will
// always be available for the call.
thread_pool.queue_work([=] { process(local); });

Note

If a non-local pointer must be captured, consider using unique_ptr; this handles both lifetime and synchronization.

If the this pointer must be captured, consider using [*this] capture, which creates a copy of the entire object.

Enforcement

• (Simple) Warn when capture-list contains a reference to a locally declared variable
• (Complex) Flag when capture-list contains a reference to a locally declared variable and the lambda is passed to a non-const and non-local context