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            王一偉 湖南商學院畢業 電子信息工程專業

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            The PIMPL idiom

            In C++ when anything in a header file changes, all code that includes the header (either directly or indirectly) must be recompiled. To minimalize this we use PIMPL idiom:

            // file x.h
            class X
            {
            public:
            // public members
            protected:
            // protected members
            private:
            // pointer to forward declared class
            class XImpl *pimpl_;  // opaque pointer
            };
            

            Questions: -- What should go into XImpl? Options are:

            • Put all private data but not functions into XImpl.: Not too bad, but there is better

            • Put all private members into XImpl.

            • Put all private and protected members into XImpl. Bad, protected members must be in X

            • Make XImpl entirely the class that X would have been, and write X as only the public interface made up entirely of simple forwarding functions (handle/body variant).

            -- Does XImpl require a pointer back to the X object?

            Caveats:

            • You can't hide virtual member functions in the Pimpl (here private inheritance differs from membership)

            • Functions in Pimpl may require a "back pointer" to the visible object (by convention that is called: self_.

            • Often the best compromise is to use Option 2, and in addition to put into XImpl only rhose non-private functions that need to be called by private ones.

            • 4th is better over 2nd not needed "back pointer", but X is useless for inheritance.

            PIPML has some drawbacks, like allocating/deallocating objects in the heap, which could be slow.

            What about this "optimalization"?

            // file: y.h
            class Y
            {
            //...
            static const size_t sizeofx = /* ... */;
            char x_[sizeofx];
            };
            // file: y.cpp
            #include "x.h"
            Y::Y()
            {
            assert( sizeofx >= sizeof(X) );
            new(&x_[0]) X;
            }
            Y::~Y()
            {
            (reinterpret_cast<X*>(&x_[0]))->~X();
            }
            

            Questions:

            • What is the Pimpl space overhead?

            • What is the performance overhead?

            Space overhead:

            #include <iostream>
            using namespace std;
            struct X {
            char c;
            struct XImpl *pimpl_;
            };
            struct XImpl { char c; };
            int main()
            {
            cout << sizeof(XImpl) << '\t' << sizeof(X) << endl;
            return 0;
            }
            // result: 1    8
            

            Runtime overhead:

            • allocation/deallocation cost: relativelly expensive

            • indirect access of private members (+ back pointer)

            • alignment problems: new guaranties, that object will align properly, char[] buffers doesn't!

            • X must not use the default assignmentoperator=()

            • If sizeof(XImpl) grows greater then sizeofx, we need to update the source.

            // file x.h
            class X
            {
            //...
            struct XImpl *pimpl_;
            };
            // file x.cpp
            #include "x.h"
            struct XImpl
            {
            // private stuff here ...
            static void *operator new(size_t)   { /*...*/ }
            static void *operator delete(void*) { /*...*/ }
            };
            X::X() : pimpl_( new XImpl ) { }
            X::~X() { delete pimpl_; pimpl_ = 0; }
            
            posted on 2007-07-16 14:51 @王一偉 閱讀(1307) 評論(0)  編輯 收藏 引用
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