如何使is_pod< T>测试在编译期间执行而不是执行？ [英] How can I make is_pod<T> tests be performed during compilation and not execution?

问题描述

这可能是一个简单的问题，我根本不掌握C ++ 11模板。

This might be an easy question, I don't master C++11 templates at all.

我有一个通用的向量类，不是 std :: vector< T> 出于性能原因（非常具体的代码）。

I have a generic vector class that is not std::vector<T> for performance reasons (very specific code).

$ c> T 是一个POD或不是，当它是，执行特殊的计算，效率要高于：

I have observed that checking whether T is a POD or not and, when it is, perform special computations, is much more efficient than not :

void vec<T>::clear() {
  if (!std::is_pod<T>::value) {
    for (int i = 0; i < size; i++) {
       data[i].~T();
    }
  }

  size = 0;
}

这里，我不调用 T 每个项目（ size 可以真的巨大），性能真的提高了。但是，一旦模板被编译，测试 if（！std :: is_pod< T> :: value）是无用的：而不是编译成：

Here, I don't call the destructor of T for each item (size can be really huge) and performance is really boosted. But the test if (!std::is_pod<T>::value) is useless once the template was compiled : rather than being compiled to :

void vec<int>::clear() {
  if (false) {
    for (int i = 0; i < size; i++) {
       data[i].~int();
    }
  }

  size = 0;
}

我想将它编译成：

I want it to be compiled to :

void vec<int>::clear() {
  size = 0;
}

编译器聪明足以跳过 if （false）阻止或 if（true）测试？

Is the compiler "clever" enough to skip if (false) blocks or if (true) tests ? Do I have to write that code somewhat differently ?

推荐答案

编译器是否聪明跳过if（false）块或if（true）测试？

Is the compiler "clever" enough to skip if (false) blocks or if (true) tests?

死代码清除是一种常规执行的简单优化。它的存在对于使许多调试库有效地工作（=在发布模式下没有运行时开销）也至关重要。

Yes, definitely. Dead code elimination is a trivial optimisation that is performed routinely. Its existence is also crucial to make many debugging libraries work efficiently (= without runtime overhead in release mode).

但是我可能还会重写这个，通过重载基于 is_pod 的函数，可以看到读取器这是一个编译时间区别：

But I would probably still rewrite this to make it visible to the reader that this is a compile-time distinction, by overloading the function based on is_pod:

void vec<T>::do_clear(std::true_type) { }

void vec<T>::do_clear(std::false_type) {
    for (int i = 0; i < size; i++) {
       data[i].~T();
    }
}

void vec<T>::clear() {
    do_clear(std::is_trivially_destructible<T>());
    size = 0;
}

在上面的代码中，我使用 is_trivially_destructible 而不是 is_pod 以使代码更加不言自明，正如Nicol在评论中所建议的。这种技术通常在标准库实现和其他库中使用。它被称为 标签分派 。

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