std::experimental::scope_exit
| Defined in header <experimental/scope>
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||
template< class EF >
class scope_exit;
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(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:
| ||
-Calling an lvalue of std::remove_reference_t<EF> with no argument shall be well-formed.
| ||
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) | |
Deduction guides
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
#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
| 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 std::unique_ptr (class template) |
