Difference between revisions of "cpp/language/noexcept"
(Added explanation and examples about the C++17 change for non-throwing lambdas) |
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}} | }} | ||
{{rev end}} | {{rev end}} | ||
+ | |||
+ | As a consequence, the evaluation of simple {{rlp|lambda|lambdas}} without {{rlp|except_spec|exception specification}}, whose body potentially does not throw exceptions, changed in C++17. All of the followings were evaluated as {{ttb|false}} before C++17, but {{ttb|true}} thereafter: | ||
+ | |||
+ | {{example | ||
+ | | code= | ||
+ | #include <iostream> | ||
+ | |||
+ | auto simple_1 = []{}; | ||
+ | |||
+ | auto simple_2 = []{ constexpr auto c = 3.5; return 1+c; }; | ||
+ | |||
+ | auto simple_3 = [](int a) { | ||
+ | if( a>0 ) return simple_2(); | ||
+ | |||
+ | ++a; | ||
+ | return a+3.14; | ||
+ | }; | ||
+ | |||
+ | int i; | ||
+ | auto simple_4 = []{ return &i; }; | ||
+ | |||
+ | int main() | ||
+ | { | ||
+ | |||
+ | std::cout << std::boolalpha | ||
+ | << "Is simple_1() noexcept? " << noexcept(simple_1()) << '\n' | ||
+ | << "Is simple_2() noexcept? " << noexcept(simple_2()) << '\n' | ||
+ | << "Is simple_3() noexcept? " << noexcept(simple_3(1)) << '\n' | ||
+ | << "Is simple_4() noexcept? " << noexcept(simple_4()) << '\n'; | ||
+ | } | ||
+ | |output= | ||
+ | Is simple_1() noexcept? true | ||
+ | Is simple_2() noexcept? true | ||
+ | Is simple_3() noexcept? true | ||
+ | Is simple_4() noexcept? true | ||
+ | }} | ||
+ | |||
===Keywords=== | ===Keywords=== | ||
Revision as of 08:47, 25 June 2018
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.
Explanation
The noexcept
operator does not evaluate expression.
The result is
In all other cases the result is |
(until C++17) |
The result is |
(since C++17) |
As a consequence, the evaluation of simple lambdas without exception specification, whose body potentially does not throw exceptions, changed in C++17. All of the followings were evaluated as false
before C++17, but true
thereafter:
#include <iostream> auto simple_1 = []{}; auto simple_2 = []{ constexpr auto c = 3.5; return 1+c; }; auto simple_3 = [](int a) { if( a>0 ) return simple_2(); ++a; return a+3.14; }; int i; auto simple_4 = []{ return &i; }; int main() { std::cout << std::boolalpha << "Is simple_1() noexcept? " << noexcept(simple_1()) << '\n' << "Is simple_2() noexcept? " << noexcept(simple_2()) << '\n' << "Is simple_3() noexcept? " << noexcept(simple_3(1)) << '\n' << "Is simple_4() noexcept? " << noexcept(simple_4()) << '\n'; }
Output:
Is simple_1() noexcept? true Is simple_2() noexcept? true Is simple_3() noexcept? true Is simple_4() noexcept? true
Keywords
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 << "Is may_throw() noexcept? " << noexcept(may_throw()) << '\n' << "Is no_throw() noexcept? " << noexcept(no_throw()) << '\n' << "Is lmay_throw() noexcept? " << noexcept(lmay_throw()) << '\n' << "Is lno_throw() noexcept? " << noexcept(lno_throw()) << '\n' << "Is ~T() 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 << "Is T(rvalue T) noexcept? " << noexcept(T(std::declval<T>())) << '\n' << "Is T(lvalue T) noexcept? " << noexcept(T(t)) << '\n' << "Is U(rvalue U) noexcept? " << noexcept(U(std::declval<U>())) << '\n' << "Is U(lvalue U) noexcept? " << noexcept(U(u)) << '\n' << "Is V(rvalue V) noexcept? " << noexcept(V(std::declval<V>())) << '\n' << "Is V(lvalue V) noexcept? " << noexcept(V(v)) << '\n'; }
Output:
Is may_throw() noexcept? false Is no_throw() noexcept? true Is lmay_throw() noexcept? false Is lno_throw() noexcept? true Is ~T() noexcept? true Is T(rvalue T) noexcept? true Is T(lvalue T) noexcept? true Is U(rvalue U) noexcept? false Is U(lvalue U) noexcept? false Is V(rvalue V) noexcept? true Is V(lvalue V) noexcept? false
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) |