How Can I Create an `std::function` from a Move-Capturing Lambda?

Creating an std::function from a Move-Capturing Lambda

Creating an std::function from a move-only type, such as a move-capturing lambda, can lead to errors due to the move constructor restriction. This restriction stems from the way std::function is defined and constructed.

Understanding the std::function Constructor

The std::function constructor can be called in two ways:

1. std::function(F f);
2. std::function(allocator_arg_t, const A& a, F f);

In both cases, the lambda (F) must be CopyConstructible. This means that you cannot move the lambda into the std::function directly.

Additionally, operator = is defined in terms of the constructor and swap, which means the same restrictions apply.

Consequences for Move-Only Types

Therefore, it is not possible to construct a std::function from a move-capturing lambda that captures a move-only type. Attempting to do so will result in a compile-time error due to the implicitly-deleted copy constructor.

Alternative Solution

To work around this issue, you can use a shared_ptr to wrap the move-only type and then capture the shared_ptr in your lambda. The shared_ptr will ensure that the type is copyable, allowing you to create an std::function from the lambda.

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