为什么用成员数组调用constexpr函数不是常量表达式？ [英] Why is calling a constexpr function with a member array not a constant expression?

问题描述

我具有以下帮助函数：

template<typename T, std::size_t N>
constexpr std::size_t Length(const T(&)[N]) {
    return N;
}

返回静态数组的长度。在过去，这一直有效，但是当我这样做时：

Which returns the length of a static array. In the past this always has worked but when I do this:

struct Foo
{
    unsigned int temp1[3];
    void Bar()
    {
        constexpr std::size_t t = Length(temp1); // Error here
    }
};

使用MSVS 2017时出现错误：

I get an error when using MSVS 2017:

error C2131: expression did not evaluate to a constant

note: failure was caused by a read of a variable outside its lifetime

note: see usage of 'this'

我希望有人可以阐明我做错了。

I was hoping someone can shed light on what I'm doing wrong.

推荐答案

MSVC是正确的。 Length（temp1）不是常量表达式。来自 [expr.const] p2

MSVC is correct. Length(temp1) is not a constant expression. From [expr.const]p2

表达式 e 是一个核心常量表达式，除非按照抽象机的规则对 e 的求值将对其中一个求值。以下表达式：

An expression e is a core constant expression unless the evaluation of e, following the rules of the abstract machine, would evaluate one of the following expressions:

• this ，但在constexpr函数或constexpr构造函数中被评估为 e 的一部分；

• this, except in a constexpr function or a constexpr constructor that is being evaluated as part of e;

temp1 隐式评估 this （因为您指的是 this-> temp1 ），所以您不会有一个常数表达式gcc和clang接受它是因为它们支持VLA作为扩展（尝试使用 -Werror = vla 或 -pedantic-errors ）。

temp1 evaluates this implicitly (because you are referring to this->temp1), and so you don't have a constant expression. gcc and clang accept it because they support VLAs as an extension (try compiling with -Werror=vla or -pedantic-errors).

为什么不允许这样做？好了，您可以访问基础元素并可能对其进行修改。如果您要处理 constexpr 数组或要作为常量表达式求值的数组，这是完全可以的，但是如果不是这样，则可能无法拥有常量表达式，因为您将要处理在运行时设置的值。

Why isn't this allowed? Well, you could access the underlying elements and potentially modify them. This is completely fine if you are dealing with a constexpr array or an array that is being evaluated as a constant expression, but if you are not, then you cannot possibly have a constant expression as you will be manipulating values that are set at run time.

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