如何在 class noexcept 之外定义默认构造函数？

Question

我知道标记为 =default 的构造函数将 "try" 尽可能地 noexcept 。但是，如果我把它定义在之外class，它就不再是noexcept了，从这段代码可以看出：

#include <iostream>
#include <utility>
#include <type_traits>

struct Bar
{
    Bar() = default;
    Bar(Bar&&) = default; // noexcept
};

struct Foo
{
    Foo() = default;
    Foo(Foo&&);
};
// moving the definition outside makes it noexcept(false)
Foo::Foo(Foo&&) = default; // not noexcept anymore

int main()
{
    Foo foo;
    Bar bar;
    std::cout << std::boolalpha;
    // checks
    std::cout << std::is_nothrow_move_constructible<Bar>::value << std::endl;
    std::cout << std::is_nothrow_move_constructible<Foo>::value << std::endl;
}

如何在 class 之外定义这样一个 =default 构造函数并使其成为 noexcept？如果定义在 class 之外，为什么这样的构造函数是 noexcept(false)？通过智能指针实现 PIMPL 时会出现此问题。

Answer 1

我现在意识到我可以做到这一点，直到现在我才想到：

struct Foo
{
    Foo() = default;
    Foo(Foo&&) noexcept;
};
Foo::Foo(Foo&&) noexcept = default; // now it is noexcept

还是第二个问题为什么默认是noexcept(false)？适用

Answer 2

§8.4.2/2 [dcl.fct.def.default][=44= 中涵盖了管理您的两个示例的异常规范的规则]

... If a function is explicitly defaulted on its first declaration,
— it is implicitly considered to be constexpr if the implicit declaration would be,
— it is implicitly considered to have the same exception-specification as if it had been implicitly declared (15.4), and
— ...

Bar 的移动构造函数是 noexcept(true) 因为在 §15.4/14 [except.spec]

An implicitly declared special member function (Clause 12) shall have an exception-specification. If f is an implicitly declared default constructor, copy constructor, move constructor, destructor, copy assignment operator, or move assignment operator, its implicit exception-specification specifies the type-id T if and only if T is allowed by the exception-specification of a function directly invoked by f's implicit definition; f shall allow all exceptions if any function it directly invokes allows all exceptions, and f shall allow no exceptions if every function it directly invokes allows no exceptions.

§8.4.2/2 中的规则不适用于在初始声明后明确默认的特殊成员函数，但析构函数除外，在 §12.4/3 中特殊情况下为 noexcept(true)除非你声明它 noexcept(false) 或者其中一个数据成员或基数类的析构函数可以抛出。

因此，除非您指定 Foo(Foo&&) 为 noexcept(true)，否则假定为 noexcept(false)。

您需要将 noexcept 规范添加到声明和后来的显式默认声明中的原因可在 §15.4
中找到

3 Two exception-specifications are compatible if:
— both are non-throwing (see below), regardless of their form,
— ...
4 If any declaration of a function has an exception-specification that is not a noexcept-specification allowing all exceptions, all declarations, including the definition and any explicit specialization, of that function shall have a compatible exception-specification. ...

如何在 class noexcept 之外定义默认构造函数？

How to make default constructor defined outside the class noexcept?

c++

default-constructor

noexcept

c++11