如果在纯C ++中是静态的? [英] static if in plain c++?

问题描述

简短的问题:
如何在纯c ++中实现c ++ 11中提出的static if功能?

Problem in short:
How could one implement static if functionality, proposed in c++11, in plain c++ ?

历史和原始问题:
最近我想到了这样的问题.我需要具有类似接口的类Sender

History and original problem:
Recently I came up with a problem like this. I need a class Sender with an interface like

class Sender
{
   void sendMessage( ... );
   void sendRequest( ... );
   void sendFile( ... );
   // lots of different send methods, not important actually
}

在某些情况下，我将需要创建一个 DoubleSender ，即该类的一个实例，该实例将两次调用其方法，即，当调用一个sendMessage(...)方法时，消息必须发送两次.

In some cases I will need to create a DoubleSender, i.e. an instance of this class, which would call its methods twice, i.e. when calling, let's say, a sendMessage(...) method, the same message has to be sent twice.

我的解决方案:
第一种方法:
拥有一个isDouble成员，并在每个方法调用的最后进行检查

My solutions:
First approach:
Have an isDouble member, and in the end of each method call make a check

sendMessage(...) { ... if( isDouble ) { sendMessage( ... ); }

好吧，我不希望这样，因为实际上我最近需要双重发布，并且对时间要求严格的部分中的这一部分代码将是98％被动的.

Well, I don't want this, because actually I will need double posting very recently, and this part of code in time-critical section will be 98% passive.

第二种方法:
从Sender继承类DoubleSender，并实现其方法，如:

Second approach:
Inherit a class DoubleSender from Sender, and implement its methods like:

void DoubleSender::sendMessage( ... )
{
   Sender::sendMessage(...);
   Sender::sendMessage(...);
}

这是可以接受的，但是会占用大量不愉快的代码(确实很多，因为有很多不同的send..方法.

Well, this is acceptable, but takes much space of unpleasant code (really much, because there are lots of different send.. methods.

第三种方法:
想象一下我正在使用c ++ 11 :).然后，我可以使该类通用，并使用static if根据tempalte参数生成代码的必要部分:

Third approach:
Imagine that I am using c++11 :). Then I can make this class generic and produce the necessary part of code according to tempalte argument using static if:

enum SenderType { Single, Double };
template<SenderType T>
class Sender
{
   void sendMessage(...)
   {
      // do stuff
      static if ( T == Single )
      {
         sendMessage(...);
      }
   }
};

与以前的解决方案相比，它更短，更易于阅读，并且不会生成其他代码，并且...它是c ++ 11，很遗憾，我无法在我的工作中使用它.

This is shorter, easier to read than previous solutions, does not generate additional code and... it's c++11, which I unfortunately cannot use in my work.

所以，这是我的问题所在-如何在c ++中实现static if模拟?
另外，对于任何其他有关如何解决原始问题的建议，我也将不胜感激.
预先感谢.

So, here is where I came to my question - how can I implement static if analog in c++ ?
Also, I would appreciate any other suggestions about how to solve my original problem.
Thanks in advance.

推荐答案

引用@JohannesSchaubLitb

Quoting @JohannesSchaubLitb

使用可在gcc上运行的static_if可以做到:)
以某种有限的方式

with my static_if that works on gcc one can do it :)
in some limited fashion

(另请参见此处)

此技巧涉及C ++ 11中Lambda规范的特定GCC解释.这样，它很可能会成为违反标准的缺陷报告.这将导致该技巧在GCC的最新版本中不再起作用(在4.7中已经不起作用).

有关约翰内斯的更多详细信息，请参见下面的评论主题

See the comment thread below for some more details from Johanness

http://ideone.com/KytVv :

#include <iostream>

namespace detail {
template<bool C>
struct call_if { template<typename F> void operator<<(F) { } };

template<>
struct call_if<true> {
  template<typename F>
  void operator<<(F f) { f(); }
};
}

#define static_if(cond) detail::call_if<cond>() << [&]

template<bool C, typename T>
void f(T t) {
  static_if(C) {
    t.foo();
  };
}

int main() {
  f<false>(42);
}

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