C ++模板Turing完成？ [英] C++ templates Turing-complete?

问题描述

我被告知C ++中的模板系统在编译时是Turing-complete。这在此帖子中以及在 wikipedia 。

你能提供一个利用这个属性的计算的一个非平凡的例子吗？

这个事实在实践中有用吗？

解决方案

p>

  #include< iostream> 
 
 template< int N> struct factorial 
 {
 enum {val = Factorial< N-1> :: val * N}; 
}; 
 
模板<> 
 struct Factorial< 0> 
 {
 enum {val = 1}; 
}; 
 
 int main（）
 {
 //注意这个值是在编译时生成的。 
 //还要注意，大多数编译器对可用递归的深度有限制。 
 std :: cout<<因子4 :: val< \\\
; 
} 
  

这很有趣，但不太实用。

要回答问题的第二部分：

这个事实在实践中是否有用？

简短答案：

长答案：是的，但前提是您是模板守护程序。

使用真正有用的模板元编程来编写良好的编程是非常有用的（即库）是真的很艰难（虽然可以）。要帮助提升，还需要 MPL （元编程库）。但是，请尝试调试模板代码中的编译器错误，并且会花费很长时间。

但是一个很好的实用示例， / p>

Scott Meyers一直使用模板工具来扩展C ++语言（我用松散这个词）。您可以在这里强制实施代码功能了解他的工作

I'm told that the template system in C++ is Turing-complete at compile time. This is mentioned in this post and also on wikipedia.

Can you provide a nontrivial example of a computation that exploits this property?

Is this fact useful in practice?

解决方案

Example

#include <iostream>

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

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

int main()
{
    // Note this value is generated at compile time.
    // Also note that most compilers have a limit on the depth of the recursion available.
    std::cout << Factorial<4>::val << "\n";
}

That was a little fun but not very practical.

To answer the second part of the question:
Is this fact useful in practice?

Short Answer: Sort of.

Long Answer: Yes, but only if you are a template daemon.

To turn out good programming using template meta-programming that is really useful for others to use (ie a library) is really really tough (though do-able). To Help boost even has MPL aka (Meta Programming Library). But try debugging a compiler error in your template code and you will be in for a long hard ride.

But a good practical example of it being used for something useful:

Scott Meyers has been working extensions to the C++ language (I use the term loosely) using the templating facilities. You can read about his work here 'Enforcing Code Features'

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