pImpl Idiom

爱吃鱼头 — Sat, 07 Aug 2010 14:58:00 GMT

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The Pimpl idiom, also known as the compilation firewall or Cheshire Cat technique, is a "private implementation" technique useful only in CeePlusPlus and statically compiled languages like it...

Benefits:

Changing private member variables of a class does not require recompiling classes that depend on it, thus make times are faster, and the FragileBinaryInterfaceProblem is reduced.
The header file does not need to #include classes that are used 'by value' in private member variables, thus compile times are faster.
This is sorta like the way SmallTalk automatically handles classes... more pure encapsulation.

Drawbacks:

More work for the implementor.
Doesn't work for 'protected' members where access by subclasses is required.
Somewhat harder to read code, since some information is no longer in the header file.
Run-time performance is slightly compromised due to the pointer indirection, especially if function calls are virtual (branch prediction for indirect branches is generally poor).

How to do it:

Put all the private member variables into a struct.
Put the struct definition in the .cpp file.
In the header file, put only the ForwardDeclaration of the struct.
In the class definition, declare a (smart) pointer to the struct as the only private member variable.
The constructors for the class need to create the struct.
The destructor of the class needs to destroy the struct (possibly implicitly due to use of a smart pointer).
The assignment operator and CopyConstructor need to copy the struct appropriately or else be disabled.

Code�Q?/font>

1   struct AImp;
2   class A {
3   public:
4     // Same public interface as A, but all delegated to concrete implementation.
5   private:
6     AImp * pimpl;
7   };
8

If you use a SmartPointer and you only have one implementation, there is no need to make any of the member functions virtual, except possibly the destructor. The run-time cost of non-virtual member function calls is much lower, and a compiler that does whole-program optimization can inline them even though they're in a separate translation unit. Here's an example:

 1  // foo.h
 2 
 3   class foo_impl;
 4 
 5   class foo {
 6     // Boilerplate
 7     friend class foo_impl;
 8     foo() {} // so only foo_impl can derive from foo
 9     const foo_impl * impl() const;
10     foo_impl * impl();
11   public:
12     virtual ~foo() {}
13     // Factories
14     static std::auto_ptr<foo> create(int value);
15     // Interface
16     int value() const;
17   };
18 
19   // foo.cpp
20 
21   class foo_impl : public foo {
22     friend class foo;
23     // Constructors mirroring the factory functions in foo
24     explicit foo_impl(int value) : value_(value) {}
25     // Member data
26     int value_;
27   };
28 
29   inline const foo_impl * foo::impl() const {
30     return static_cast<const foo_impl *>(this);
31   }
32   inline foo_impl * foo::impl() {
33     return static_cast<foo_impl *>(this);
34   }
35 
36   std::auto_ptr<foo> foo::create(int value) {
37     return std::auto_ptr<foo>(new foo_impl(value));
38   }
39 
40   int foo::value() const { return impl()->value_; }
41 
42

Here, the destructor needs to be declared virtual foo so that std::auto_ptr calls foo_impl's destructor. If you use boost::shared_ptr instead, even that doesn't need to be virtual, because shared_ptr remembers how to call the correct destructor. (This doesn't improve performance or memory use, because shared_ptr is larger and slower than auto_ptr, but if you need to use shared_ptr anyway you may as well eliminate the virtual destructor.) -- BenHutchings

参考阅读：

Effective C++
http://c2.com/cgi/wiki?PimplIdiom
http://en.wikipedia.org/wiki/Opaque_pointer

爱吃鱼头 2010-08-07 22:58 发表评论

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pImpl Idiom

Effective C++ - 09.�l�对不要在构造和析构�q�程中调用虚函数