std::experimental::scope_exit

 

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Library fundamentals v3

experimental::scope_exit

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std::experimental::scope_exit

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scope_exit::scope_exit

scope_exit::~scope_exit

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scope_exit::release

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Defined in header <experimental/scope>
template< class EF > class scope_exit;		(library fundamentals TS v3)

The class template scope_exit is a general-purpose scope guard intended to call its exit function when a scope is exited.

scope_exit is not CopyConstructible, CopyAssignable or MoveAssignable, however, it may be MoveConstructible if EF meets some requirements, which permits wrapping a scope_exit into another object.

A scope_exit may be either active, i.e. calls its exit function on destruction, or inactive, i.e. does nothing on destruction. A scope_exit is active after constructed from an exit function.

A scope_exit can become inactive by calling release() on it either manually or automatically (by the move constructor). An inactive scope_exit may also be obtained by initializing with another inactive scope_exit. Once a scope_exit is inactive, it cannot become active again.

A scope_exit effectively holds an EF and a bool flag indicating if it is active.

Template parameters

EF	-	type of stored exit function
Type requirements
- EF shall be either: a Destructible FunctionObject type, an lvalue reference to FunctionObject, an lvalue reference to function.
- Calling an lvalue of std::remove_reference_t<EF> with no argument shall be well-formed.

Member functions

(constructor)	constructs a new scope_exit (public member function)
(destructor)	calls the exit function when the scope is exited if the scope_exit is active, then destroys the scope_exit (public member function)
operator= [deleted]	scope_exit is not assignable (public member function)
Modifiers
release	makes the scope_exit inactive (public member function)

Notes

Constructing a scope_exit of dynamic storage duration might lead to unexpected behavior.

If the EF stored in a scope_exit object refers to a local variable of the function where it is defined, e.g., as a lambda capturing the variable by reference, and that variable is used as a return operand in that function, that variable might have already been returned when the scope_exit's destructor executes, calling the exit function. This can lead to surprising behavior.

Example

Run this code

#include <iostream>
#include <cstdlib>
#include <string_view>
#include <experimental/scope>
 
void print_exit_status(std::string_view name, bool exit_status, bool did_throw) {
  std::cout << name << ":\n";
  std::cout << "  Throwed exception  " << (did_throw ? "yes" : "no") << "\n";
  std::cout << "  Exit status        " << (exit_status ? "finished" : "pending") << "\n\n";
}
 
// Randomly throw an exception (50% chance)
void maybe_throw() {
    if (std::rand() >= RAND_MAX / 2)
        throw std::exception{};
}
 
int main() {
  bool exit_status{false}, did_throw{false};
 
  // Manual handling at "end of scope"
  try {
    maybe_throw();
    exit_status = true; 
  } catch (...) { did_throw = true; }
  print_exit_status("Manual handling", exit_status, did_throw);
 
  // Using scope_exit: runs on scope exit (success or exception)
  exit_status = did_throw = false;
  try {
    auto guard = std::experimental::scope_exit{[&]{ exit_status = true; } };
    maybe_throw();
  } catch (...) { did_throw = true; }
  print_exit_status("scope_exit", exit_status, did_throw);
 
  // Using scope_fail: runs only if an exception occurs
  exit_status = did_throw = false;
  try {
    auto guard = std::experimental::scope_fail{[&]{ exit_status = true; } };
    maybe_throw();
  } catch (...) { did_throw = true; }
  print_exit_status("scope_fail", exit_status, did_throw);
 
  // Using scope_success: runs only if no exception occurs
  exit_status = did_throw = false;
  try {
    auto guard = std::experimental::scope_success{[&]{ exit_status = true; } };
    maybe_throw();
  } catch (...) { did_throw = true; }
  print_exit_status("scope_success", exit_status, did_throw);
}

Output:

Manual handling:
  Throwed exception  yes
  Exit status        pending
 
scope_exit:
  Throwed exception  no
  Exit status        finished
 
scope_fail:
  Throwed exception  yes
  Exit status        finished
 
scope_success:
  Throwed exception  yes
  Exit status        pending

See also

scope_fail	wraps a function object and invokes it on exiting the scope through an exception (class template)
scope_success	wraps a function object and invokes it on exiting the scope normally (class template)
default_delete (C++11)	default deleter for unique_ptr (class template)