在编译时在C ++元编程中使用运行时参数（变量）

Question

是否有机会使元编程功能但不扩展编译中的所有参数？ 只想拥有一些参数作为运行时参数并进行一些编译。 因为我知道其中一些会在1..10范围内，而另一些则是未知的（将在运行时知道）。

让我们使用标准元编程示例：

unsigned int factorial(unsigned int n) {
    return n == 0 ? 1 : n * factorial(n - 1); 
}
template <int n>
struct factorial {
    enum { value = n * factorial<n - 1>::value };
};

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

// Usage examples:
// factorial<0>::value would yield 1;
// factorial<4>::value would yield 24.

下面是我的情况：

unsigned int cutom_imagined_function(unsigned int n, unsigned int runtime_param /* this will be given at runtime */) {
    return n == 0 ? 1 : (n + runtime_param) * cutom_imagined_function(n - 1); 
}

如何将以上转换为元编程？ 并运行如下所示（或类似内容）：

// int variable;
// variable = get_var_from_user();
// cutom_imagined_function<4>::value(variable)

Answer 1

您可以使用相同的方法：常量表达式成为模板参数，其他所有内容则不行：

template <unsigned int N>
struct cutom_imagined
{ 
    static unsigned int function(unsigned int r)
    {
        return (N + r) * cutom_imagined<N - 1>::function(r);
    }
};

template <>
struct cutom_imagined<0>
{ 
    static unsigned int function(unsigned int) { return 1; }
};

用法：

unsigned int result = cutom_imagined<N>::function(get_input());

Answer 2

假设您的意思是：

unsigned int cutom_imagined_function(
    unsigned int n, 
    unsigned int runtime_param)
{
    return n == 0 ? 1 : (n+runtime_param)*custom_imagined_function(n-1, runtime_param); 
}

这是我认为您正在描述的一般概念。 实际上非常简单。 使函数本身成为模板。

template<unsigned int in>
unsigned int custom_imagined_function(unsigned int runtime_param) {
    return (n+runtime_param)*custom_imagined_function<n-1>(runtime_param); 
}
template<> 
unsigned int custom_imagined_function<0>(unsigned int runtime_param) {
    return 1; 
}

int main() {
    int variable;
    std::cin >> variable;
    unsigned int result = custom_imagined_function<4>(variable);
}

另外，您可以使用稍微更详细的std::integral_constant 。

unsigned int custom_imagined_function(
    std::integral_constant<unsigned int,0>,
    unsigned int runtime_param) 
{
    return 1; 
}
template<unsigned int in>
unsigned int custom_imagined_function(
    std::integral_constant<unsigned int,n>,
    unsigned int runtime_param) 
{
    std::integral_constant<unsigned int,n-1> less;
    return (n+runtime_param) * custom_imagined_function(less, runtime_param); 
}

int main() {
    std::integral_constant<unsigned int,4> four;
    int variable;
    std::cin >> variable;
    unsigned int result = custom_imagined_function(four, variable);
}

此外，现在有了constexpr （无论如何在某些编译器中）， factorial容易简化：

constexpr unsigned int factorial(unsigned int n) {
    return n==0?1:n*factorial(n-1);
}