noexcept
运算符 (C++11 起)
来自 cppreference.cn
noexcept 运算符执行编译时检查,如果表达式声明为不抛出任何异常,则返回 true。
它可以在函数模板的 noexcept 说明符 中使用,以声明该函数对某些类型抛出异常,而对其他类型不抛出异常。
目录 |
[编辑] 语法
noexcept( expression ) |
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返回类型为 bool 的 prvalue。如果 expression 的 潜在异常 集合为空(C++17 前)expression 被指定为 不抛出(C++17 起),结果为 true,否则为 false。
expression 是一个 未求值操作数。
如果 expression 是一个 prvalue,则应用 临时实体化。 |
(C++17 起) |
[编辑] 注意
即使 noexcept(expr) 为 true,对 expr 的求值仍可能由于遇到未定义行为而抛出异常。
如果 expression 是类类型或其(可能是多维)数组,临时实体化要求析构函数是非删除且可访问的。 |
(C++17 起) |
[编辑] 关键词
[编辑] 示例
运行此代码
#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"; }
输出
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)
[编辑] 缺陷报告
下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。
缺陷报告 | 应用于 | 发布时的行为 | 正确的行为 |
---|---|---|---|
CWG 2722 | C++17 | 不清楚是否应用临时实体化 如果 expression 是 prvalue |
它被应用 在这种情况下 |
CWG 2792 | C++11 | 要求 noexcept 运算符确定在遇到未定义行为时是否抛出异常 在遇到未定义行为时是否可能抛出异常 |
未要求 |
[编辑] 另请参阅
noexcept 说明符(C++11) |
指定函数是否可能抛出异常 |
动态异常规范(C++17 前) | 指定函数抛出哪些异常 (C++11 中已弃用) |