Is there a C++ standard class to set a variable to a value at scope exit

Question

Within the scope of a member function, I want to temporarly set a member variable to a certain value.

Then, when this function returns, I want to reset this member variable to a given known value.

To bo safe against exceptions and multiple returns, and I've done it with a simple RAII like class. It's defined within the scope of the member function.

void MyClass::MyMemberFunction() {
    struct SetBackToFalse {
        SetBackToFalse(bool* p): m_p(p) {}
        ~SetBackToFalse() {*m_p=false;}
    private:
        bool* m_p;
    };

    m_theVariableToChange = true;
    SetBackToFalse resetFalse( &m_theVariableToChange ); // Will reset the variable to false.

    // Function body that may throw.
}

It seems so obviously commonplace, that I was wondering if there was any such template class doing this in the C++ standard library?

Answer 1

Not yet (there have been proposals for this). But implementing a generic one is simple enough;

struct scope_exit {
  std::function<void()> f_;
  explicit scope_exit(std::function<void()> f) noexcept : f_(std::move(f)) {}
  ~scope_exit() { if (f_) f_(); }
};
// ...
m_theVariableToChange = true;
scope_exit resetFalse([&m_theVariableToChange]() { m_theVariableToChange = false; });

For simplicity above, I've redacted the copy and move constructors etc...

Marking them as = delete will make the above a minimal solution. Further; moving could be allowed if desired, but copying should be prohibited.

---

A more complete scope_exit would look like (online demo here);

template <typename F>
struct scope_exit {
  F f_;
  bool run_;
  explicit scope_exit(F f) noexcept : f_(std::move(f)), run_(true) {}
  scope_exit(scope_exit&& rhs) noexcept : f_((rhs.run_ = false, std::move(rhs.f_))), run_(true) {}
  ~scope_exit()
  {
    if (run_)
      f_(); // RAII semantics apply, expected not to throw
  }

  // "in place" construction expected, no default ctor provided either
  // also unclear what should be done with the old functor, should it
  // be called since it is no longer needed, or not since *this is not
  // going out of scope just yet...
  scope_exit& operator=(scope_exit&& rhs) = delete;
  // to be explicit...
  scope_exit(scope_exit const&) = delete;
  scope_exit& operator=(scope_exit const&) = delete;
};

template <typename F>
scope_exit<F> make_scope_exit(F&& f) noexcept
{
  return scope_exit<F>{ std::forward<F>(f) };
}

Notes on the implementation;

• std::function<void()> can be used to erase the type of the functor. std::function<void()> offers exception guarantees on the move constructors based on the exception specific of the held function. A sample of this implementation is found here
• These exception specifications are consistent the C++ proposal and GSL implementations
• I've redacted most of the motivation for the noexcept, more substantial detail is found in the C++ proposal
• The "usual" RAII semantics of the destructor, hence the "scope exit" function is applicable; it will not throw, this is also consistent with the C++11 specification on the default exception specification for a destructor. See cppreference, SO Q&A, GotW#47 and HIC++

Other implementations can be found;

• The C++ proposal mentioned above and its revision (as of this writing)
• Boost.ScopeExit
• The Microsoft GSL implementation

Answer 2

You could 'abuse' shared_ptr for this:

m_theVariableToChange = true;
std::shared_ptr<void> resetFalse(nullptr, [&](void*){ m_theVariableToChange = false; });

---

If there are concerns about using void as template parameter T, I found the following in the C++ standard:

20.8.2.2§2:

... The template parameter T of shared_ptr may be an incomplete type.

This indicates that T is only used as a pointer, therefore using void should be fine.

Answer 3

There is no standard version of this.

The CppGoreGuidelines Support Library (GSL) has a generalized version of this called finally but that library is not production quality yet. Its definitely recommended practice.

E.19: Use a final_action object to express cleanup if no suitable resource handle is available

Reason

finally is less verbose and harder to get wrong than try/catch.

Example

void f(int n)
{
    void* p = malloc(1, n);
    auto _ = finally([p] { free(p); });
    // ...
}

Note

finally is not as messy as try/catch, but it is still ad-hoc. Prefer proper resource management objects.

Answer 4

Similar question: The simplest and neatest c++11 ScopeGuard

On that thread is described a similar guard for invoking an arbitrary function. To solve your problem, invoke a lambda that resets your variable.

For example, the solution from this answer for your code would be:

scope_guard guard1 = [&]{ m_theVariableToChange = false; };

Another answer on that thread notes that a similar concept has been proposed for C++17 standardization; and there is also a C++03 solution presented.