std::experimental::scope_fail
Defined in header <experimental/scope> | ||
template<class EF > class scope_fail; | (library fundamentals TS v3) | |
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.
Contents |
[edit]Template parameters
| EF | - | type of stored exit function |
| Type requirements | ||
-EF shall be either:
| ||
| -Calling an lvalue of std::remove_reference_t<EF> with no argument shall be well-formed. | ||
[edit]Member functions
constructs a new scope_fail(public member function) | |
calls the exit function when the scope is exited via an exception if the scope_fail is active, then destroys the scope_fail(public member function) | |
operator= [deleted] | scope_fail is not assignable (public member function) |
Modifiers | |
makes the scope_fail inactive (public member function) | |
[edit]Deduction guides
[edit]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.
[edit]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)throwstd::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
| wraps a function object and invokes it on exiting the scope (class template) | |
| wraps a function object and invokes it on exiting the scope normally (class template) | |
(C++11) | default deleter for unique_ptr (class template) |
