noexcept 究竟包含什么构造函数？

Question

根据 C++ 标准，class 构造函数上的 noexcept noexcept-specification 究竟适用于什么？

函数体？
1. 在可选的ctor-initializer中初始化成员？
  1. 在可选的 mem-initializers?
  2. 在可选的 mem-initializers?
2. 复合语句?
3. 函数尝试块?
对象基的初始化class没有在ctor-initializer?
初始化对象 class 成员未在 ctor-initializer?
还有什么？

换句话说，noexcept noexcept-specification 包含以上哪些内容（即在抛出异常时触发 std::terminate() if noexcept(true))?

请提供对标准的引用。也欢迎对构造函数使用 noexcept 的任何注意事项提供提示。谢谢！

Answer 1

In other words, which of the above are encompassed by the noexcept noexcept-specification...?

异常规范（noexcept 和动态异常规范）涉及基类的构造、成员的构造和初始化以及构造函数主体中的代码。 基本上，在构造对象时执行的所有函数 - 这是有道理的，因为异常规范与对象的构造函数相关联，因此它应该涵盖构造对象期间执行的代码目的;如果构造的任何部分不在此范围内，那将是违反直觉的。

支持标准引号...

如果在构造过程中抛出异常（并且可能未处理）怎么办？

[except.spec]/9

Whenever an exception of type E is thrown and the search for a handler ([except.handle]) encounters the outermost block of a function with an exception specification that does not allow E, then,

if the function definition has a dynamic-exception-specification, the function std::unexpected() is called ([except.unexpected]),

otherwise, the function std::terminate() is called ([except.terminate]).

"outermost block of a function"是什么意思？函数体。¹

上面的 exception specification 包括 noexcept-specification.

如何在隐式声明的构造函数上确定隐式声明的异常规范？

[except.spec]/15

An implicitly-declared special member function f of some class X is considered to have an implicit exception specification that consists of all the members from the following sets:

if f is a constructor,

the sets of potential exceptions of the constructor invocations

for X's non-variant non-static data members,

for X's direct base classes, and

if X is non-abstract ([class.abstract]), for X's virtual base classes,

(including default argument expressions used in such invocations) as selected by overload resolution for the implicit definition of f ([class.ctor])...

the sets of potential exceptions of the initialization of non-static data members from brace-or-equal-initializers that are not ignored ([class.base.init]);

这对编译器将使用什么来确定（并因此考虑涵盖）异常规范提供了非常有用的说明。

¹"outermost block of a function"是什么意思？有人评论关注block的定义一个功能。该标准没有正式定义 函数块 。短语 block of a function 仅在 Exception Handling [except] 中使用。该短语早在 C++98 就包含在标准中。

为了进一步澄清这一点，我们需要寻找替代来源并得出一些合理的结论。

来自 Stroustrup C++ glossary;

function body - the outermost block of a function. See also: try-block, function definition. TC++PL 2.7, 13.

并且从 [dcl.fct.def.general]/1 开始 函数体 的语法覆盖了 ctor-initializer 和 compound-statement 和 function-try-block;

Function definitions have the form;

...

  function-body:
    ctor-initializer_opt compound-statement
    function-try-block

...

Any informal reference to the body of a function should be interpreted as a reference to the non-terminal function-body...

同样重要的是要记住，异常规范与函数相关联，而不是通用代码块（作用域块等）。

鉴于异常处理子句和 Stroustrup FAQ 中短语的存在时间，函数块 与 函数体，该标准可能会更新例外条款中使用的语言。

一些经验证据，给定下面的代码，用于构建 a1、a2 和 a3（当其他的被注释掉时），结果是 std::terminate 被召唤。结果适用于 g++, clang and MSVC.

struct Thrower { Thrower() { std::cout << "throwing..." << std::endl; throw 42; } };

struct AsMember { Thrower t_; AsMember() noexcept : t_{} { std::cout << "ctor" << std::endl; } };

struct AsBase : Thrower { AsBase() noexcept { std::cout << "ctor" << std::endl; } };

struct AsNSDMI { Thrower t_ {}; AsNSDMI() noexcept { std::cout << "ctor" << std::endl; } };

int main()
{
    std::set_terminate([](){ std::cout << "terminating..." << std::endl; });
    try {
        //AsMember a1{};
        //AsBase a2{};
        AsNSDMI a3{};
    }
    catch (...) { std::cout << "caught..." << std::endl; }
    return 0;
}

Answer 2

如果您考虑一个 noexcept 构造函数，其定义在当前翻译单元中不可用（例如，构造函数实现驻留在共享库中），您可能可以自己回答这个问题。

如果 noexcept 没有应用到整个构造函数，构造函数仍然会以某种方式发出异常，从而违背 noexcept.

的目的

在这种情况下，noexcept 可以正常工作的唯一方法是将其应用于整个构造函数，包括基础 class 和成员初始化。

noexcept 究竟包含什么构造函数？

What does noexcept exactly encompass for constructors?

c++

constructor

language-lawyer

noexcept

c++11