std::experimental::scope_exit
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.
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_exit(public member function) | |
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 | |
makes the scope_exit inactive (public member function) | |
[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 <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 through an exception (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) |
