JUCE/modules/juce_core/misc/juce_ScopeGuard.h

/*
  ==============================================================================

   This file is part of the JUCE library.
   Copyright (c) 2022 - Raw Material Software Limited

   JUCE is an open source library subject to commercial or open-source
   licensing.

   The code included in this file is provided under the terms of the ISC license
   http://www.isc.org/downloads/software-support-policy/isc-license. Permission
   To use, copy, modify, and/or distribute this software for any purpose with or
   without fee is hereby granted provided that the above copyright notice and
   this permission notice appear in all copies.

   JUCE IS PROVIDED "AS IS" WITHOUT ANY WARRANTY, AND ALL WARRANTIES, WHETHER
   EXPRESSED OR IMPLIED, INCLUDING MERCHANTABILITY AND FITNESS FOR PURPOSE, ARE
   DISCLAIMED.

  ==============================================================================
*/

namespace juce
{

/** An easy way to ensure that a function is called at the end of the current
    scope.

    Usage:
    @code
    {
        if (flag == true)
            return;

        // While this code executes, flag is true e.g. to prevent reentrancy
        flag = true;
        // When we exit this scope, flag must be false
        const ScopeGuard scope { [&] { flag = false; } };

        if (checkInitialCondition())
            return; // Scope's lambda will fire here...

        if (checkCriticalCondition())
            throw std::runtime_error{}; // ...or here...

        doWorkHavingEstablishedPreconditions();
    } // ...or here!
    @endcode

    @tags{Core}
*/
template <typename Fn> struct ScopeGuard : Fn { ~ScopeGuard() { Fn::operator()(); } };
template <typename Fn> ScopeGuard (Fn) -> ScopeGuard<Fn>;

/**
    A ScopeGuard that uses a std::function internally to allow type erasure.
    This can be handy; it allows lots of ErasedScopeGuards, all with different
    callbacks, to be stored in a homogeneous container.

    An instance of this type will automatically call its callback when it is destroyed.

    ErasedScopeGuard has a few similarities with std::unique_ptr:
    - Calling reset() on a unique_ptr destroys the object if it hasn't been destroyed yet
      and puts the unique_ptr back into a default/null state; calling reset() on an
      ErasedScopeGuard calls the callback if it hasn't been called yet and puts the Guard
      back into a default/null state.
    - Calling release() on a unique_ptr returns the unique_ptr back to a default state
      without destroying the managed object; calling release() on an ErasedScopeGuard
      returns the Guard back to a default state without calling the callback.
    - Moving a unique_ptr transfers the responsibility of destroying the managed object
      to another unique_ptr instance; moving an ErasedScopeGuard transfers the
      responsibility of calling the callback to another Guard instance.
*/
class [[nodiscard]] ErasedScopeGuard
{
public:
    /** Constructs an ErasedScopeGuard with no callback. */
    ErasedScopeGuard() = default;

    /** Constructs an ErasedScopeGuard that will call the provided callback
        when the Guard is destroyed.
    */
    explicit ErasedScopeGuard (std::function<void()> d);

    /** Constructs an instance that assumes responsibility for calling other's callback. */
    ErasedScopeGuard (ErasedScopeGuard&& other) noexcept;

    /** Calls the stored callback, if any, then assumes responsibility for calling
        other's callback. After this call, other will be reset to its default state.
    */
    ErasedScopeGuard& operator= (ErasedScopeGuard&& other) noexcept;

    /** Destructor, calls the callback assigned to this ScopeGuard.
    */
    ~ErasedScopeGuard() noexcept;

    /** Calls the stored callback, if any, then resets this instance to its
        default state.
    */
    void reset();

    /** Resets this instance to its default state without calling the stored
        callback.
    */
    void release();

    JUCE_DECLARE_NON_COPYABLE (ErasedScopeGuard)

private:
    std::function<void()> detach;
};

} // namespace juce