| title | noexcept operator | ||
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import { Decl, DeclDoc } from "@components/decl-doc"; import { Desc, DescList, DocLink } from '@components/index'; import { Revision, RevisionBlock } from "@components/revision"; import { DR, DRList } from "@components/defect-report"; import { ParamDoc, ParamDocList } from "@components/param-doc";
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.
```cpp cxx-mark noexcept(/*$s:expression*/) ```Returns a prvalue of type bool. The result is true if the set of of the expression is emptyexpression is specified to be non-throwing, and false otherwise.
expression is an unevaluated operand.
If expression is a prvalue, temporary materialization is applied.
Even if noexcept(expr) is true, an evaluation of expr may still throw as the result of encountering undefined behavior.
noexcept
#include <iostream>
#include <utility>
#include <vector>
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<int> v;
};
class V
{
public:
std::vector<int> 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>().~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<T>())) << ")\n"
"T(lvalue T) is noexcept(" << noexcept(T(t)) << ")\n"
"U(rvalue U) is noexcept(" << noexcept(U(std::declval<U>())) << ")\n"
"U(lvalue U) is noexcept(" << noexcept(U(u)) << ")\n"
"V(rvalue V) is noexcept(" << noexcept(V(std::declval<V>())) << ")\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)
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
it was unclear whether temporary materialization is applied if _expression_ is a prvalue it is applied in this case the `noexcept` operator was required to determine whether exceptions may be thrown in the case of encountering undefined behavior not required noexcept specifierspecifies whether a function could throw exceptions
<Revision traits={[{ trait: "deprecated", since: "C++26" }]}>specifies what exceptions are thrown by a function</Revision>