这段代码如何成为constexpr？ （std :: chrono） [英] How can this code be constexpr? (std::chrono)

问题描述

Howard Hinnant在标准论文P0092R1中写道：

In the standards paper P0092R1, Howard Hinnant wrote:

template <class To, class Rep, class Period,
          class = enable_if_t<detail::is_duration<To>{}>>
constexpr
To floor(const duration<Rep, Period>& d)
{
    To t = duration_cast<To>(d);
    if (t > d)
        --t;
    return t;
}

此代码如何工作？问题是 std :: chrono :: duration 上的操作符-不是constexpr操作。定义为：

How can this code work? The problem is that operator-- on a std::chrono::duration is not a constexpr operation. It is defined as:

duration& operator--();

但是此代码可以编译，并在编译时给出正确的答案：

And yet this code compiles, and gives the right answer at compile time:

static_assert(floor<hours>(minutes{3}).count() == 0, "");

这是怎么回事？

推荐答案

答案是，并非编译时例程中的所有操作都必须是constexpr；只有那些在编译时执行的操作。

The answer is that not all operations in a compile-time routine have to be constexpr; only the ones that are executed at compile time.

在上面的示例中，操作为：

In the example above, the operations are:

hours t = duration_cast<hours>(d);
if (t > d) {} // which is false, so execution skips the block
return t;

所有这些都可以在编译时完成。

all of which can be done at compile time.

另一方面，如果您要尝试：

If, on the other hand, you were to try:

static_assert(floor<hours>(minutes{-3}).count() == -1, "");

它会给出一个编译时错误，说（使用clang）：

it would give a compile-time error saying (using clang):

error: static_assert expression is not an integral constant expression
        static_assert(floor<hours>(minutes{-3}).count() == -1, "");
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: non-constexpr function 'operator--' cannot be used in a constant expression
                        --t;
                        ^
note: in call to 'floor(minutes{-3})'
        static_assert(floor<hours>(minutes{-3}).count() == -1, "");

在编写constexpr代码时，必须考虑通过所有路径代码。

When writing constexpr code, you have to consider all the paths through the code.

PS您可以这样修改建议的 floor 例程：

P.S. You can fix the proposed floor routine thusly:

template <class To, class Rep, class Period,
          class = enable_if_t<detail::is_duration<To>{}>>
constexpr
To floor(const duration<Rep, Period>& d)
{
    To t = duration_cast<To>(d);
    if (t > d)
        t = t - To{1};
    return t;
}

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