在gnu c ++中有__if_exists的等价物吗？ [英] Is there an equivalent for __if_exists in gnu c++?
问题描述
__ if_exists是用于在编译时测试标识符存在的微软特定关键字：
__if_exists is a microsoft specific keyword for testing existence of identifiers at compile time:
在faked模板专门化中，它可能非常方便，因为在某些情况下，比其他方法，如真正的专门化或重载或其他方法，真的更简单，可读性更好的方式。
It can come in very handy at "faked" template specialization as in some cases it provides a really much more simple, readable and better performing way than other methods like "real" specialization or overloading or whatever.
但现在我必须移植一个大项目到gnu c我想我会开始一点哭泣，如果我必须找到其他方式（当然很少）的场合，我使用它
But now I have to port a big project to gnu c++ and I think I would start a little bit of crying if I would have to find other ways for the (admittedly few) occasions I used it
推荐答案
这是一个crappy关键字在我看来...
That's a crappy keyword in my opinion...
不幸的是，它不存在于gcc就我所知，但然后我可能根本不了解它。
Unfortunately, it doesn't exist in gcc as far as I know, but then I may simply not know about it.
正确的C ++方式来处理这个问题是通过使用概念，即根据一些要求调整类型上的操作。
The proper C++ way to handle this is through the use of Concepts, ie adapt the operations carried on the type depending on some requirements.
通常，它使用 traits 而不是真正的概念来执行，因为它更容易放置：
Usually, it's carried out with traits rather than real concepts, because it's easier to put in place:
template <typename T>
struct has_dump: boost::mpl::false_ {};
然后通过专门化 has_dump 结构。
最简单的是定义3个方法，一个路由，另外两个执行不同的分支：
The simplest is to define 3 methods, one to route, the two others to execute the different branches:
template <typename T>
void dump(T& t, boost::mpl::true_ const& dummy)
{
t.Dump();
}
template <typename T>
void dump(T& t, boost::mpl::false_ const& dummy)
{
std::cout << typeid(T).name() << " does not have Dump\n";
}
template <typename T>
void dump(T& t) { dump(t, has_dump<T>()); }
类型traits的另一个使用将与 enable_if 设施：
Another use of the type traits would be in conjunction with the enable_if facilities:
template <typename T>
typename boost::enable_if< has_dump<T> >::type dump(T& t)
{
t.Dump();
}
// disable_if exists too...
这里，如果类型没有启用 has_dump ，而不是运行时错误消息，您可能会收到编译时错误。
Here, instead of a runtime error message, you can get a compile-time error if the type does not have has_dump enabled, not sure if that's you want.
然而，这两种方法都相当麻烦，因为检测不是自动的。这就是为什么有Boost.Concept库。
However both those methods are quite cumbersome, since the detection isn't automated. That is why there is the Boost.Concept library.
这个想法是，检查将由一个Concept对象执行，被创建来测试需求，因此你不再需要专门化特性，这使得开发更容易。然而，我总是发现Boost.Concept的文档有点欠缺。
The idea is that the check will be performed by a Concept object, created to test the requirements, and thus you don't have to specialize traits any longer, which make easier on the development. However I have always found the documentation of Boost.Concept to be somewhat lacking.
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