編譯時在C++元編程中使用運行時參數（變量）

Question

是否有機會編寫元編程功能，但不能擴展編譯中的所有參數？只是想要一些參數作爲運行時參數和一些編譯。因爲我知道其中一些將在1..10的範圍內，但另一個未知（將在運行時知道）。

允許使用標準的元編程例子：

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. 

而下面是我的情況：

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); 
} 

我如何轉換上面元編程？並運行此假設如下（或類似的東西）：

// 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：常量表達式成爲模板參數，一切沒有按「T：

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

假設你的意思是這樣的：

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); 
} 

這裏的一般概念，我覺得你描述。這其實很簡單。使函數本身爲模板。

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); 
} 

或者，您可以使用稍微更詳細的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，現在我們已經constexpr（在一些編譯器反正），該factorial容易簡化爲：

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