noexcept operator (since C++11) - cppreference.com (original) (raw)
The noexcept operator performs a compile-time check that returns true if an expression is declared to not throw any exceptions.
It can be used within a function template's noexcept specifier to declare that the function will throw exceptions for some types but not others.
[edit] Syntax
| | | | | ------------------------------ | | | | noexcept( expression ) | | | | | | |
Returns a prvalue of type bool. The result is true if the set of potential exceptions of the expression is empty(until C++17)expression is specified to be non-throwing(since C++17), and false otherwise.
expression is an unevaluated operand.
[edit] Notes
Even if noexcept(expr) is true, an evaluation of expr may still throw as the result of encountering undefined behavior.
| If expression is of a class type or (possibly multidimensional) array thereof, temporary materialization requires the destructor be non-deleted and accessible. | (since C++17) |
|---|
[edit] Keywords
[edit] Example
#include
#include
#include
void may_throw();
void no_throw() noexcept;
auto lmay_throw = []{};
auto lno_throw = noexcept {};
class T
{
public:
T(){} // dtor prevents move ctor
// copy ctor is noexcept
};
class U
{
public:
~U(){} // dtor prevents move ctor
// copy ctor is noexcept(false)
std::vector v;
};
class V
{
public:
std::vector v;
};
int main()
{
T t;
U u;
V v;
std::cout << std::boolalpha <<
"may_throw() is noexcept(" << noexcept(may_throw()) << ")\n"
"no_throw() is noexcept(" << noexcept(no_throw()) << ")\n"
"lmay_throw() is noexcept(" << noexcept(lmay_throw()) << ")\n"
"lno_throw() is noexcept(" << noexcept(lno_throw()) << ")\n"
"T() is noexcept(" << noexcept(std::declval().~T()) << ")\n"
// note: the following tests also require that ~T() is noexcept because
// the expression within noexcept constructs and destroys a temporary
"T(rvalue T) is noexcept(" << noexcept(T(std::declval())) << ")\n"
"T(lvalue T) is noexcept(" << noexcept(T(t)) << ")\n"
"U(rvalue U) is noexcept(" << noexcept(U(std::declval())) << ")\n"
"U(lvalue U) is noexcept(" << noexcept(U(u)) << ")\n"
"V(rvalue V) is noexcept(" << noexcept(V(std::declval())) << ")\n"
"V(lvalue V) is noexcept(" << noexcept(V(v)) << ")\n";
}
Output:
may_throw() is noexcept(false) no_throw() is noexcept(true) lmay_throw() is noexcept(false) lno_throw() is noexcept(true) ~T() is noexcept(true) T(rvalue T) is noexcept(true) T(lvalue T) is noexcept(true) U(rvalue U) is noexcept(false) U(lvalue U) is noexcept(false) V(rvalue V) is noexcept(true) V(lvalue V) is noexcept(false)
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| CWG 2722 | C++17 | it was unclear whether temporary materializationis applied if expression is a prvalue | it is appliedin this case |
| CWG 2792 | C++11 | the noexcept operator was required to determine whether exceptionsmay be thrown in the case of encountering undefined behavior | not required |