隐式生成的成员和noexcept [英] Implicit generated members and noexcept

问题描述

我最近开始添加新的 noexcept 规范，尽可能移动构造函数/赋值。现在我开始想知道隐式生成的成员函数的异常规范是什么样子。由于 noexcept 移动函数允许使用更高效的代码路径（例如，当调整向量时）那些被声明为noexcept。我有问题，了解什么标准有关这一点，因此尝试下面的代码在g ++ 4.6（与 -std = c ++ 0x ），以获得一些抓地力上：

I recently started to add the new noexcept specification to move constructors/assignments wherever possible. Now I started to wonder what the exception specification of implicit generated member functions looks like. Since having noexcept move functions allows the use of more efficient code paths(e.g. when resizing a vector) I would hope that those are declared as noexcept whenever possible. I had problems understanding what the standard has to say about that and therefore tried the following code in g++4.6 (with -std=c++0x) to get some grip on it:

struct foobar{};
int main()
{
    foobar a, b;
    std::cout<<std::boolalpha
             <<noexcept(foobar())<<", "<<noexcept(foobar(a))<<", "
             <<noexcept(a = b)   <<", "<<noexcept(a = std::move(b))<<", "
             <<noexcept(foobar(std::move(a)))<<std::endl;
}

这给了我一个输出 True，True， True，False，False ，意味着默认和复制构造函数/赋值 noexcept ，而不移动操作。

This gave me an output of True, True, True, False, False, meaning that default and copy constructor/assignment where noexcept, while move operation where not.

现在我的问题：

在什么情况下是隐式生成宣告为 noexcept ？此外， foobar 正确或仅仅是gcc4.6中的编译器错误的后续行为

Under what circumstances are implicit generated (or defaulted) member functions declared as noexcept? Furthermore is the obseved behaviour for foobar correct or simply a compiler bug in gcc4.6?

推荐答案

库错误 - 它在gcc 4.7中显示 true，true，true，true，true

Library bug — it shows true, true, true, true, true in gcc 4.7.

并且该错误是不是生成的移动构造函数不是noexcept，但 std :: move 没有标记as noexcept ，我们可以看到额外的测试：

And the bug is not that the generated move constructors aren't noexcept, but that std::move is not marked as noexcept, as we can see with the additional tests:

std::cout << noexcept(a = static_cast<foobar&&>(b)) << ", "  // true
          << noexcept(foobar(static_cast<foobar&&>(b))) << ", " // true
          << noexcept(std::move(b)) << std::endl;   // false

gcc 4.6中的大多数库函数都不是noexcept-正确的，在 gcc 4.7 中解决，

Most of the library functions in gcc 4.6 was not noexcept-correct, and this has been addressed in gcc 4.7,

对于无限生成的成员函数为noexcept，这在§15.4/ 14中有记载。基本上， noexcept 如果所有需要调用的函数都是 noexcept 。

As for when the implcitely generated member functions are noexcept, this is documented in §15.4/14. Basically, it is noexcept if all functions it will need to call are all noexcept.

隐式声明的特殊成员函数（第12条）应具有异常规范。如果 f 是
隐式声明的默认构造函数，复制构造函数，移动构造函数，析构函数，复制赋值
运算符， em> exception-specification 指定 type-id T if and only
if T由 f 的隐式定义直接调用的函数的 exception-specification 允许 f 将
允许所有异常，如果它直接调用的任何函数允许所有异常和 f 将允许没有异常
如果每个函数直接调用允许没有异常。

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.

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