std::experimental::scope_fail
| Defined in header <experimental/scope>
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template< class EF >
class scope_fail;
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(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.
Template parameters
| EF | - | type of stored exit function |
| Type requirements | ||
-EF shall be either:
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-Calling an lvalue of std::remove_reference_t<EF> with no argument shall be well-formed.
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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) | |
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);
}
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