std::experimental::scope_exit - cppreference.com (original) (raw)

std::experimental::scope_exit

| | | | | ---------------------------------------- | | ---------------------------- | | 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.

Contents

• 1 Template parameters
• 2 Member functions
  • 2.1 Modifiers
• 3 Deduction guides
• 4 Notes
• 5 Example
• 6 See also

[edit] Template parameters

[edit] Member functions

| | constructs a new scope_exit (public member function) [edit] | | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | calls the exit function when the scope is exited if the scope_exit is active, then destroys the scope_exit (public member function) [edit] | | | scope_exit is not assignable (public member function) | | Modifiers | | | makes the scope_exit inactive (public member function) [edit] |

[edit] Deduction guides

[edit] 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.

[edit] Example

#include #include #include #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

[edit] See also