std::experimental:: scope_fail

The class template scope_fail is a general-purpose scope guard intended to call its exit function when a scope is exited via an exception.

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

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

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

A scope_fail effectively holds an EF and a bool flag indicating if it is active, alongwith a counter of uncaught exceptions used for detecting whether the destructor is called during stack unwinding.

Template parameters

Member functions

Deduction guides

Notes

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

Constructing a scope_fail is constructed from another scope_fail created in a different thread might also lead to unexpected behavior since the count of uncaught exceptions obtained in different threads may be compared during the destruction.

Example

#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);
}

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