什么是template< unsigned int N>意思？ [英] What does template <unsigned int N> mean?

问题描述

当声明模板时，我习惯使用这种代码：

When declaring a template, I am used to having this kind of code:

template <class T>

但

But in this question, they used:

template <unsigned int N>

我检查了它的编译。但是这是什么意思？它是一个非类型的参数吗？如果是这样，我们如何可以有一个没有任何类型参数的模板？

I checked that it compiles. But what does it mean? Is it a non-type parameter? And if so, how can we have a template without any type parameter?

推荐答案

而不是类型。我们可以将模板化的值赋给一个变量，或者以任何其他整数文字的方式操作它：

It's perfectly possible to template a class on an integer rather than a type. We can assign the templated value to a variable, or otherwise manipulate it in a way we might with any other integer literal:

unsigned int x = N;

事实上，我们可以创建在编译时评估的算法（从 Wikipedia ）：

In fact, we can create algorithms which evaluate at compile time (from Wikipedia):

template <int N>
struct Factorial 
{
     enum { value = N * Factorial<N - 1>::value };
};

template <>
struct Factorial<0> 
{
    enum { value = 1 };
};

// Factorial<4>::value == 24
// Factorial<0>::value == 1
void foo()
{
    int x = Factorial<4>::value; // == 24
    int y = Factorial<0>::value; // == 1
}

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