模板 <unsigned int N> 有什么作用?意思是?

2021-12-13 00:00:00 templates c++

在声明模板时，我习惯了这样的代码:

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

template <class T>

但是在这个问题中，他们使用:

template <unsigned int N>

我检查过它可以编译.但是这是什么意思?它是非类型参数吗?如果是这样，我们怎么能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

完全可以将类模板化为整数而不是类型.我们可以将模板化的值分配给一个变量，或者以其他任何整数文字的方式对其进行操作:

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进行评估的算法(来自维基百科):

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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