Why Can't I Create a `std::function` from a Move-Capturing Lambda?
Dec 17, 2024 am 01:22 AMCreating std::function from Move-Capturing Lambda Expression
Understanding the issue of creating std::function objects from lambda expressions is crucial. It is possible to create lambda expressions with move capture, but when attempting to encapsulate them within std::function, errors may arise.
Error Examples
Multiple attempts to convert a move-capturing lambda to std::function consistently lead to the error "Call to implicitly-deleted copy constructor of ' The Need for Move-Capturing Lambdas Move capture in lambda expressions becomes necessary in scenarios where sharing ownership of an object is not viable, such as in custom UI libraries. These libraries offer methods like on_mouse_down() and push_undo_action() to register event handlers using std::function. To ensure efficient ownership of resources, move capture in lambda expressions is preferred over the earlier, cumbersome "release/acquire-in-lambda" idiom. Restrictions on std::function Construction The std::function constructor for directly specifying a function object has the following signature: These guarantees include the following: Applicability to Move-Capture While lambda expressions with move capture do not violate the copyability requirement of F, they often move-capture types (such as std::unique_ptr) that do not provide the required copy constructor. As a result, the construction of std::function from these move-capturing lambdas becomes infeasible. Conclusion It is important to note that the conversion of move-capturing lambdas to std::function is not possible due to the limitations of the std::function constructor in handling move-only types. This restriction stems from the necessity of std::function to maintain a copy of the encapsulated function object. The above is the detailed content of Why Can't I Create a `std::function` from a Move-Capturing Lambda?. For more information, please follow other related articles on the PHP Chinese website!template <class F> function(F f);</p>
<p>However, another related constructor accepts an allocator and a reference to a type that provides certain guarantees:</p>
<pre class="brush:php;toolbar:false">template <class F, class A> function(allocator_arg_t, const A& a, F f);

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