为什么使用非类型模板参数? [英] Why are non-type template arguments used?

问题描述

我已经阅读了很多问题和答案，但是这个问题最吸引我的眼球；它及其答案很有帮助，但我仍然觉得我没有完全理解使用非类型模板参数背后的用途和理论.它们提供了许多关于何时使用它的有用示例，但没有一个真正阐明非类型模板参数背后的理论.

I have read many questions and answers, but this question caught my eye the most; it and its answers are helpful but I still feel that I do not fully understand the uses and the theories behind using non-type template arguments. They provide many useful examples as to when it is used, but none really shed light on the theories behind the non-type template arguments.

我感兴趣的不是在示例中具体是什么时候，而是更广泛地了解为什么我倾向于使用非类型模板参数而不是常规函数参数.

I am interested in knowing not specifically when in examples, but more broadly why I would be inclined to use a non-type template argument over just a regular function argument.

我知道它们是在编译时确定的，并且每个新调用都会创建一个新函数，该函数具有非类型模板参数的确定值.那么，当我可以将我想要的参数输入到一个函数中并得到相同的结果——据说——只有一个编译函数时，为什么我要为同一个函数创建许多不同的实例.

I understand that they are determined at compile-time and that every new call creates a new function with the determined value for the non-type template argument. So why exactly would I want to create many different instances of the same function when I could just input the parameter I want into a function and have the same result - supposedly - with only one compiled function.

本质上，根据 cplusplus.com的="noreferrer">本页?

In essence, why should I be inclined to do #1, instead of #2, as according to the final section of this page of cplusplus.com?

#1:

template <class T, int N>
T fixed_multiply (T val)
{
  return val * N;
}

int main() {
  std::cout << fixed_multiply<int,2>(10) << '\n';
  std::cout << fixed_multiply<int,3>(10) << '\n';
}

#2:

template <class T>
T fixed_multiply (T val, int N)
{
  return val * N;
}

int main() {
  std::cout << fixed_multiply<int>(10, 2) << '\n';
  std::cout << fixed_multiply<int>(10, 3) << '\n';
}

另外，是否有任何性能优势或其中之一?是否有任何常见应用程序可以从使用非类型模板参数中受益，或者这是在特定应用程序中使用的技术性很强的参数来产生特定结果?

In addition, would there be any performance benefits or the such of either one? Are there any common applications that could benefit from using non-type template arguments, or is this a highly technical parameter used in specific applications to yield specific results?

由于某种原因，这被标记为重复，第一段解释了我提出类似问题的原因.

This got marked as a duplicate for some reason, the first paragraph explains my reasons for a similar question.

推荐答案

当您希望(或需要)编译器在保持代码良好分解的同时进行某些编译时优化时，它们非常有用.我将简要列出几个示例:

They are useful when you want (or need) the compiler to make certain compile-time optimizations while keeping code well-factored. I'll briefly list a few examples:

分支消除

void doSomething(bool flag) {
  if (flag) {
    //whatever
  }
}

template <bool flag>
void doSomething() {
 if (flag) {
   //whatever
}

}

在上面的例子中，如果你在编译时调用doSomething时总是知道flag的值，那么你就可以避免分支的开销，而无需手动创建doSomethingTrue 和 doSomethingFalse 函数可能需要你重复很多代码.例如，当您想在网络代码的发送端和接收端之间分解代码，并且您的代码位于性能关键堆栈的深处时，此技巧非常有用.

In the above example, if you always know the value of flag at compile time when you call doSomething, then you can avoid the cost of a branch without manually creating doSomethingTrue and doSomethingFalse functions which may require you to repeat a lot of code. This trick is useful when you want to factor code between send and recv sides in network code, for example, and your code sits deep down in a performance-critical stack.

避免动态内存管理

void someFunction(size_t size, float* inout) {
  std::vector<float> tmp(size);
  //do something with inout and tmp and then store result in inout
}

template <size_t N>
void someFunction(float *inout) {
  float tmp[N]; //or std::array if you like
  //do something with inout and tmp and store result in inout
}

在上面的例子中，如果第二个版本卡在关键循环中，它的性能会好得多.

In the above example, the second version will perform much better if it is stuck inside a critical loop.

允许特殊优化

考虑循环:

for (size_t i = 0; i < N; ++i) {
  //some arithmetic
}

如果编译器知道 N 是 4 或 8，它可能能够用等效的 SIMD 指令替换您的算术，或者至少比 N 是运行时参数时更智能地展开您的循环.这种事情在 GPU 编程 (CUDA) 世界中司空见惯.

If the compiler knows that N is 4 or 8, it may be able to replace your arithmetic with equivalent SIMD instructions, or at least unroll your loop more intelligently than it could if N were a runtime argument. This sort of thing is commonplace in the GPU programming (CUDA) world.

模板元编程

当您使用模板生成一些非平凡的代码时，有时您必须在编译时迭代事物.您需要模板非类型参数才能做到这一点.参数包和 std::tuple 是一种常见的情况，但还有更多，这里很难举例说明.

When you do some non-trivial code generation with templates, sometimes you must iterate over things at compile time. You need template non-type arguments to be able to do this. Argument packs and std::tuple are one common case, but there are many more, and hard to exemplify here.

回答您的问题，如果您有选择，何时将某些内容作为模板参数:采用动态(运行时)参数的代码总是更灵活，因此如果可以，您应该只将某些内容转换为模板参数用性能的显着提高来证明它是合理的.另一方面，如果您发现自己在一组枚举上编写了大量重复的代码，您可能应该尝试使用模板进行分解.

To answer your question of when you'd make something a template parameter if you have a choice: code that takes dynamic (runtime) arguments is always more flexible, so you should only convert something to be a template argument if you can justify it with a substantial gain in performance. On the other hand if you find yourself writing much duplicated code over a set of enumerations, you should probably try and factor with templates.

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