^{The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in RFC 2119.}

Client interface

The public interface consists of a template function to create scope guard objects, a few members of those objects, and one boolean compilation option that can be activated with a preprocessor macro definition.

Here is an outline of the client interface:

• Maker function template
• Scope guard objects
  • Invariants
  • Overview of public members
  • List of deleted public members
  • Member type callback_type
  • Member function dismiss
  • Member move constructor
  • Member destructor
• Compilation option SG_REQUIRE_NOEXCEPT_IN_CPP17

Maker function template

The free function template make_scope_guard resides in namespace sg and is the primary element of the public interface – most uses do not require anything else. It provides a way for clients to create a scope guard object that is associated with the specified callback.

Scope guards execute their associated callback when they are destroyed, unless they were meanwhile dismissed or moved. Like other variables, scope guards with automatic storage are destroyed when they go out of scope, hence the name.

This function template is SFINAE-friendly.

Function signature:

  template<typename Callback>
  /* unspecified return type */ make_scope_guard(Callback&& callback)
  noexcept(std::is_nothrow_constructible<Callback, Callback&&>::value);

Preconditions:

The template and function arguments need to respect certain preconditions. They should all be intuitive to C++ programmers, with the possible exception of precondition 2. They are summarized here and discussed in more detail elsewhere.

1. invocable with no arguments
2. void return
3. nothrow-invocable
4. nothrow-destructible if non-reference template argument
5. const-invocable if const reference
6. appropriate lifetime if lvalue reference
7. movable or copyable if non-reference

By default, precondition 3 is not enforced at compile time. Precondition 6 is not enforced at compile time. All other preconditions are enforced at compile time.

Postconditions:

A scope guard object is returned with

• the provided callback as associated callback
• active state

Exception specification:

As the signature shows, instances of this function template are noexcept iff Callback can be nothrow constructed from Callback&& (after reference collapsing). Notice this is always the case if Callback is a reference type.

Example:

const auto guard = sg::make_scope_guard([]() noexcept { /* do stuff */ });

Scope guard objects

Scope guard objects have some unspecified type that MUST NOT be used as a base class.

Invariants:

1. A scope guard object that is in an active state executes its associated callback exactly once when leaving scope.
2. A scope guard that is in inactive state does not execute its associated callback

Overview of public members:

1. Type callback_type
2. dismiss function
3. move constructor
4. destructor

List of deleted public members:

1. default constructor
2. copy constructor
3. copy assignment operator
4. move assignment operator

Note: Deleted special members cannot be used, but they participate in overload resolution. They are listed here because they are explicitly disallowed and that can be considered as part of the the client’s interface.

Member type callback_type

Template argument deduction allows the underlying callback type to be automatically derived from the function argument. That type – Callback in the signature of make_scope_guard above – is provided as the member type callback_type.

Member type declaration:

typedef Callback callback_type;

Example:

using my_cb = typename decltype(guard)::callback_type; /* where guard
                                              is a scope guard object */

Member function dismiss

Scope guards can be dismissed to cancel callback execution. Dismissed scope guards are valid but useless. They are best regarded as garbage awaiting destruction.

Member function signature:

void dismiss() noexcept;

Preconditions:

None.

Postconditions:

The dismissed scope guard is in inactive state.

Exception specification:

noexcept.

Example:

bool do_transaction()
{ // example with early returns
  if(!do_step1())
    return false;

  auto undo = sg::make_scope_guard(rollback);
  if(!do_step2());
    return false;
  if(!do_step3());
    return false;

  undo.dismiss(); // <-- using dismiss
  return true;
}

Member move constructor

Objects created with make_scope_guard can be moved. This possibility exists mainly to allow initialization with assignment syntax, as in auto g1 = make_scope_guard(f);. In general, it allows transferring scope guarding responsibility: auto g2 = std::move(g1);.

A scope-guard move transfers the callback and corresponding call responsibility (or lack thereof). Moved-from scope guards are valid but useless. They are best regarded as garbage awaiting destruction.

Member function signature:

Standard move constructor.

Preconditions:

None.

Postconditions:

• The moved-from guard is in inactive state.
• The moved-from guard is associated with an unspecified callback.
• The moved-to guard is in the same activity state as the moved-from guard was in before the move operation (active moved-to iff previously active moved-from).
• The moved-to guard is associated with the callback that was associated with the moved-from guard before the move operation;

Exception specification:

noexcept iff Callback can be nothrow-constructed from Callback&& (after reference collapse). Notice this is always the case when Callback is a reference type.

Example:

{
  auto g1 = sg::make_scope_guard([]() noexcept { std::cout << "bla"; });
  {
    auto g2 = std::move(g1);
  } // g2 leaves scope here
  std::cout << "ble";
} // g1 leaves scope here
std::cout << "bli";
// prints "blablebli"

Member destructor

Scope guards have a (non-virtual) destructor.

Member function signature:

Standard destructor.

Preconditions:

None.

Postconditions:

• the callback was executed iff the guard was in active state before destruction;
• the guard is no longer valid

Exception specification:

noexcept. This motivates two of the preconditions: nothrow-invocable and nothrow-destructible if non-reference.

Example:

Non applicable.

Compilation option SG_REQUIRE_NOEXCEPT_IN_CPP17

If ≥C++17 is used, the preprocessor macro SG_REQUIRE_NOEXCEPT_IN_CPP17 can be defined to require nothrow-invocable callbacks in make_scope_guard at compile time.

Notice however, that this restricts accepted callback types considerably.

This option is disabled by default and enabling it has no effect unless ≥C++17 is used.

Example:

#define SG_REQUIRE_NOEXCEPT_IN_CPP17 // (no effect in <C++17)
#include "scope_guard.hpp"
make_scope_guard([](){}); // ERROR: need noexcept (if >=C++17)
make_scope_guard([]() noexcept {}); // OK