我们可以在noexcept规范中引用成员变量吗? [英] Can we refer to member variables in a noexcept specification?

问题描述

请考虑以下代码段:

template<class Tuple>
class vector
{
public:
    typename Tuple::size_type size() const noexcept(noexcept(m_elements.size())) {
        return m_elements.size();
    }

private:
    Tuple m_elements;
};

class tuple
{
public:
    using size_type = std::size_t;

    size_type size() const { return 0; }
    size_type size() noexcept { return 0; }
};    

int main()
{
    vector<tuple> x;
    static_assert(noexcept(x.size()), "x.size() might throw");

    return 0;
}

在noexcept说明符内使用成员变量m_elements是否合法? GCC 5.2(C ++ 17)会产生编译器错误 在此范围内未声明.而 clang 3.6(C ++ 17)编译时没有任何错误.

Is the use of the member variable m_elements inside the noexcept specifier legal? GCC 5.2 (C++17) yields the compiler error m_elements was not declared in this scope. while clang 3.6 (C++17) compiles without any error.

如果我改用noexcept(std::declval<Tuple const&>().size())，则两个编译器都不会产生错误.但是，如您所见，我创建了一个简单的示例类tuple，其中Tuple是否具有size的合格重载至关重要.

Both compilers yield no error if I use noexcept(std::declval<Tuple const&>().size()) instead. However, as you can see, I've created a simple example class tuple where it's crucial whether or not Tuple has qualified overloads of size.

从我的角度来看，编写noexcept(m_elements.size())更直观，因为它恰好是函数体内的调用，并且考虑到vector的size方法是const限定的(这使得m_elements函数范围内的const对象.

From my point of view, it's more intuitive to write noexcept(m_elements.size()) cause it's exactly the call in the function body and it takes into account that the size method of vector is const qualified (which makes m_elements a const object in the scope of the function).

那么，合法用法是什么?如果两者相等，我应该使用哪个?在这种情况下，我应该完全使用noexcept限定词吗?问题在于vector函数是否将抛出，在大多数情况下都取决于Tuple.

So, what's the legal usage? If both are equivalent, which should I use? Should I use noexcept qualifiers at all in this scenario? The problem is that whether or not the vector functions will throw depends in all most every case on Tuple.

推荐答案

此处的lang语正确，这是gcc错误 [basic.scope.class] ，我的重点是:

Clang is correct here, this is gcc bug 52869. According to [basic.scope.class], emphasis mine:

在类中声明的名称的潜在范围不仅包括紧随其后的声明性区域. 名称的声明点，以及该类中非静态数据成员的所有函数体，默认参数，异常规范和括号或相等初始化器的声明点(包括嵌套的此类内容 类).

The potential scope of a name declared in a class consists not only of the declarative region following the name’s point of declaration, but also of all function bodies, default arguments, exception-specifications, and brace-or-equal-initializers of non-static data members in that class (including such things in nested classes).

m_elements的范围包括size()的 noexcept规范.

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