stdfunction

std::function is a class template in the C++ standard library that provides a type-erased wrapper for any callable object with a given signature. It is defined in the header <functional> and is typically declared as std::function<R(Args...)>, meaning it can store any callable that can be invoked with arguments of types Args and returns a value of type R.

A std::function object can be constructed from a function pointer, a lambda, a function object (functor), or the result of std::bind, as long as the stored callable is compatible with the signature R(Args...). It stores the target object in a type-erased way and invokes it via operator() when called.

Key features include:

- operator() to invoke the stored callable with the appropriate arguments.

- emptiness checks via operator bool, empty(), or by testing if it holds a target.

- reset() or assignable semantics to replace or clear the stored target.

- Optional access to the stored target type through member functions such as target_type and target<T>().

Constraints and behavior:

- std::function is CopyConstructible and CopyAssignable if the stored target is CopyConstructible; the underlying callable object must be CopyConstructible to be stored.

- It may allocate dynamic memory to hold large callables; some implementations employ small object optimization, but this is not guaranteed by the standard.

- It is not inherently thread-safe; concurrent use requires external synchronization.

Common uses include storing callbacks, event handlers, and generic function wrappers where a uniform, copyable callable interface is desirable. Its flexibility comes at the cost of some performance overhead compared to direct function pointers or simple lambdas, due to type erasure and possible heap allocation.