Difference between revisions of "cpp/language/noexcept"
m (fmt.) |
(Expand "temporary materialization". Add a note for CWG2792 (perhaps we don't need to add it to the DR list).) |
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{{rrev|since=c++17| | {{rrev|since=c++17| | ||
| − | If {{spar|expression}} is a prvalue, {{rlpsd|implicit conversion#Temporary materialization}} is applied. | + | If {{spar|expression}} is a prvalue, {{rlpsd|implicit conversion#Temporary materialization}} is formally applied. Particularly, the destructor is required to be non-deleted and accessible if such {{spar|expression}} is of a class type or (possibly multidimensional) array thereof. |
}} | }} | ||
===Keywords=== | ===Keywords=== | ||
{{ltt|cpp/keyword/noexcept}} | {{ltt|cpp/keyword/noexcept}} | ||
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| + | ===Notes=== | ||
| + | Evaluation of a non-throwing {{spar|expression}} is allowed to throw an exception if the evaluation has undefined behavior.<!-- CWG2792 --> | ||
===Example=== | ===Example=== | ||
Revision as of 18:30, 13 November 2023
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.
Contents |
Syntax
noexcept( expression )
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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 non-throwing(since C++17), and false otherwise.
expression is an unevaluated operand.
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If expression is a prvalue, temporary materialization is formally applied. Particularly, the destructor is required to be non-deleted and accessible if such expression is of a class type or (possibly multidimensional) array thereof. |
(since C++17) |
Keywords
Notes
Evaluation of a non-throwing expression is allowed to throw an exception if the evaluation has undefined behavior.
Example
#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)
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 materialization is applied if expression is a prvalue |
it is applied in this case |
See also
noexcept specifier(C++11)
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specifies whether a function could throw exceptions |
| Dynamic exception specification(until C++17) | specifies what exceptions are thrown by a function (deprecated in C++11) |
