Re: Alternatives to using virtuals for cross-platform development

From:
Numeromancer <tschaef@sbcglobal.net>
Newsgroups:
comp.lang.c++
Date:
Thu, 07 Jun 2007 22:54:15 GMT
Message-ID:
<r00ai.24777$YL5.14474@newssvr29.news.prodigy.net>
greek_bill wrote:

Hi,

I'm interested in developing an application that needs to run on more
than one operating system. Naturally, a lot of the code will be shared
between the various OSs, with OS specific functionality being kept
separate.

....

template<class OSImp>
class BaseFoo
{
public: void Bar() { static_cast<OSImp*>(this)->Bar(); }
};

class MyFavOSFoo : public BaseFoo<MyFavOSFoo>
{
public: void Bar() { // do some OS specific stuff }
public: void OSFunc() { // some OS-specific interface }
private: //store some OS specific data
}

Now this is more like what I want, I can have multiple
implementations, without any run-time overhead.

There are a couple of problems however (otherwise I wouldn't be here,
would I? :)


....

Many thanks,
Bill


I have used template specializations for this sort of thing (tested):

#include <iostream>

using namespace std;

enum { OS1, OS2 };

// configuration (in a system-wide header someplace)
const int OS = OS2; // or give -D to the compiler from make, or whatever

template<int> class OSClass;

template<> class OSClass<OS1> {
public: void Bar() { cout << "os1.Bar()" << endl; }
public: void OSFunc() { /* some OS1-specific interface */ }
private: //store some OS1 specific data
};

template<> class OSClass<OS2> {
public: void Bar() { cout << "os2.Bar()" << endl; m_os1.Bar(); }
public: void OSFunc() { /* some OS2-specific interface */ }
private: //store some OS2 specific data
   OSClass<OS1> m_os1; // use OS1 stuff in secret.
};

// Application code
int main (void)
{
   OSClass<OS> os;
   os.Bar();
}

You could add an abstract base class for both if you want the compiler
to check that both have the proper interface.

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