何时以及如何使用模板文字运算符？ [英] When and how to use a template literal operator?

问题描述

在 cppreference 上，有人提到可以使用模板用户文字运算符，但有一些限制：

On cppreference there is a mentioning that one can have templated user-literal operators, with some restrictions:

如果文字运算符是模板，则它必须具有一个空参数列表并且可以具有只有一个模板参数，该参数必须是元素类型为 char 的非类型模板参数包，例如

If the literal operator is a template, it must have an empty parameter list and can have only one template parameter, which must be a non-type template parameter pack with element type char, such as

template <char...> double operator "" _x();

所以我在下面的代码中写了一个：

So I wrote one like in the code below:

template <char...> 
double operator "" _x()
{
    return .42;
}

int main()
{
    10_x; // empty template list, how to specify non-empty template parameters?
}

问题：

1. 该代码有效，但是如何将运算符与一些非空模板参数一起使用？ 10_x<'a'> ;; 或 10_<'a'&x; 不会编译。 li>
   
2. 您是否有使用此类模板运算符的实际示例？

1. The code works, but how can I use the operator with some non-empty template parameters? 10_x<'a'>; or 10_<'a'>x; does not compile.
2. Do you have any example of real-world usage of such templated operators?

推荐答案

10_x; // empty template list, how to specify non-empty template parameters?

那是不对的。模板参数列表不为空。

That isn't quite right. The template parameter list isn't empty. When you write:

template <char... Cs> 
??? operator "" _x()

Cs 从文字左侧的内容中填充。也就是说，当您编写以下内容时：

The Cs get populated from the stuff on the left-hand side of the literal. That is, when you write:

10_x

调用：

operator ""_x<'1', '0'>();

一个简单的例子是构建一个编译时，溢出安全的二进制文字，如下所示：

One simple example would be to build a compile time, overflow-safe binary literal such that:

template <uint64_t V>
constexpr uint64_t make_binary() {
    return V;
}

template <uint64_t V, char C, char... Cs>
constexpr uint64_t make_binary() {
    static_assert(C == '0' || C == '1', "invalid binary");

    return make_binary<2*V + C - '0', Cs...>();
}

template <char... Cs> 
uint64_t operator "" _b()
{
    static_assert(sizeof...(Cs) <= 64, "overflow");

    return make_binary<0, Cs...>();
}

uint64_t a = 101_b; // OK: a == 5
uint64_t b = 102_b; // error: invalid
uint64_t c = 11111111110000000000111111111100000000001111111111000000000011111111110000000000_b; // error: overflow

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