如何对可变参数模板的参数包中的值进行static_assert？ [英] How do you static_assert the values in a parameter pack of a variadic template?

问题描述

我正在创建可变参数模板。

假设我有这样的东西：

I'm creating a variadic template.
Let's say I have something like this:

template<typename T, T ... Numbers>
class Sequence final {

    // Unpack parameter pack into a constexpr array
    constexpr static T count = sizeof...(Numbers);        
    constexpr static T numbers[count] = { Numbers... };

    // ...
}

此实例可以实例化此类：

Instances of this class can be instantiated like:

Sequence<uint32_t, 1, 2, 3, 42, 25> seq;

我想确保在编译时使用 static_assert ，数字参数包仅包含特定数字。就本例而言，假设我只希望允许 0 或 1 。

I'd like to make sure at compile time using a static_assert that the numbers parameter pack only contains specific numbers. For the sake of this example, let's say I only want to allow 0 or 1.

所以我想做类似的事情：

So I'd like to do something like:

for (size_t i = 0; i < count; i++) {
    static_assert(numbers[i] == 1 || numbers[i] == 0, "Only ones and zeroes are allowed.");
}

但显然， static_assert 不适用于 for 循环。我很确定必须为此使用某种语法，但我无法弄清楚。

But obviously, static_assert doesn't work with a for loop. I'm pretty sure there must be some sort of syntax for this but I haven't been able to figure it out.

我更喜欢使用可编译的内容

I'd prefer to use something that compiles with a C++11 compiler (or perhaps a C++14 compiler, if it isn't doable in C++11).

推荐答案

我将抛​​出 @Columbo的 bool_pack 技巧。

I'll throw in @Columbo's bool_pack trick.

template<bool...> struct bool_pack;
template<bool... bs> 
using all_true = std::is_same<bool_pack<bs..., true>, bool_pack<true, bs...>>;

static_assert(all_true<(Numbers == 0 || Numbers == 1)...>::value, "");

如果满足以下条件，则将表达式提取到 constexpr 函数中它变得复杂。

Extract the expression into a constexpr function if it gets complex.

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