我需要将 constexpr 放在 else-if 之后吗?

问题描述

受到这个答案的启发，我尝试复制和粘贴(并在main()中添加测试) 这段代码:

Inspired by this answer, I tried to copy and paste (and add testing in main()) this code:

template<typename T>
std::tuple<int, double> foo(T a) {
    if constexpr (std::is_same_v<int, T>)
        return {a, 0.0};
    else if (std::is_same_v<double, T>)
        return {0, a};
    else
        return {0, 0.0};
}

int main() {
    auto [x, y] = foo("");
    std::cout << x << " " << y;
}

这很简单——如果T被推导出为int，我们想要返回一个[a, 0.0]的元组.如果将T推导出为double，我们要返回一个[0, a]的元组.否则，我们要返回[0, 0.0].

This is very straightforward - if T is deduced as int, we want to return a tuple of [a, 0.0]. If T is deduced as double, we want to return a tuple of [0, a]. Otherwise, we want to return [0, 0.0].

如您所见，在 main() 函数中，我使用 const char* 参数调用 foo，其中 应该导致 x 和 y 为 0.事实并非如此.

As you can see, in the main() function, I am calling foo with const char* argument, which should result in x and y being 0. That is not the case.

在尝试编译时，我遇到了一个奇怪的错误:

While trying to compile it, I encountered a strange error:

错误:无法将 '{0, a}' 从 '' 转换为 'std::tuple<;int, double>'

error: could not convert '{0, a}' from '<brace-enclosed initializer list>' to 'std::tuple<int, double>'

而我就像什么?.我到底为什么要这样......我专门使用 std::is_same 来启用 return {0, a} only 当类型a 的推导为 double.

And I was like what?. Why on earth would I want that... I specifically used std::is_same to enable return {0, a} only when the type of a is deduced as double.

所以我很快就在 if-constexpr 上跑到 cppreference.在页面底部，注释上方，我们可以看到这段代码:

So I quickly ran to cppreference on if-constexpr. At the bottom of the page, above Notes, we can see this snippet of code:

extern int x; // no definition of x required
int f() {
if constexpr (true)
    return 0;
else if (x)
    return x;
else
    return -x;
}

我心想哎呀..?我真的看不出原始代码有什么问题.它们使用相同的语法和语义....

但是我很好奇.我很好奇是否有一些奇怪的东西(当时)可以解决这个问题，所以我将原始代码更改为:

But I was curious. I was curious if maybe something odd (at that time) might fix that issue, so I changed the original code to:

template<typename T>
std::tuple<int, double> foo(T a) {
    if constexpr (std::is_same_v<int, T>)
        return {a, 0.0};
    else if constexpr (std::is_same_v<double, T>) // notice the additional constexpr here
        return {0, a};
    else
        return {0, 0.0};
}

int main() {
    auto [x, y] = foo("");
    std::cout << x << " " << y;
}

瞧！代码按预期编译和执行.所以，我的问题是 - 在这种情况下，我们是否需要在 if-else 语句中的每个 if 语句之后放置 constexpr? 还是只是我的编译器?我使用的是 GCC 7.3.

And voilà! The code compiled and executed as expected. So, my question is - Do we need to put constexpr after every if statement in if-else statement in these kind of situations? Or is it just my compiler? I am using GCC 7.3.

推荐答案

在这种情况下，我们是否需要在 if-else 块中的每个 if 语句之后放置 constexpr?

Do we need to put constexpr after every if statement in if-else block in these kind of situations?

是的.else-if block¹ 是个谎言:)，只有 if blocks¹ 和 else blocks¹.这是编译器看到您的代码的方式:

Yes. The else-if block¹ is a lie :), there are only if blocks¹ and else blocks¹. This is how your code is seen by the compiler:

if constexpr (std::is_same_v<int, T>)
    return {a, 0.0};
else // {
    if (std::is_same_v<double, T>)
        return {0, a};
    else
        return {0, 0.0};
// }

else if (/*...*/) 只是每个人都使用的格式约定.因此，您可以清楚地看到需要第二个 constexpr.

else if (/*...*/) is just a formatting convention that everyone uses. As such, you can clearly see that the second constexpr is needed.

¹:块"不是正确的术语.if 是一个语句(带有可选的 else 部分).一个块是 {/*...*