Question about best practices and Macros from the book 'C++ Coding Standards'

From Herb Sutter and Andrei Alexandrescu's 'C++ Coding Standards', Item 16: Avoid Macros under Exceptions for this guideline they wrote:

For conditional compilation (e.g., system-dependent parts), avoid littering your code with #ifdefs. Instead, prefer to organize code such that the use of macros drives alternative implementations of one common interface, and then use the interface throughout.

I'm having trou开发者_如何学Cble understanding exactly what they mean by this. How can you drive alternate implementations without the use of #ifdef conditional compile macro directives? Can someone provide an example to help illustrate what's being proposed by the above paragraph?

Thanks

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What they mean is that you should abstract your code from the system-dependent part through the use of abstract base classes and use condition compilation only at the point of instantiation.

class SystemAgnosticInterface
{
public:
    virtual ~SystemAgnosticInterface() {}
    virtual void doStuff() = 0;
};

You could then for example have a Windows and a Linux specific implementations of the interface (each of which you will only be included in the compilation for its associated platform), use such as :

SystemAgnosticInterface *createFoo()
{
#ifdef _WIN32
    return new WindowsImplementation;
#else
    return new LinuxImplementation;
#endif
}

int main()
{
    SystemAgnosticInterface *foo = createFoo();
    foo->doStuff(); // No conditional compilation here
    delete foo;
}

This is obviously an over-simplified code sample, but I hope you'll understand the point : it's not about avoiding #ifdef completely, just that they should not clutter every part of your code.

---

For non-member functions:

//implementation!

#ifdef _WIN32
    void EatWindow(Food &) {}
    void DrinkWindow(Beverage &) {}
    void SleepWindow(Bed &) {}
#else
    void EatLinux(Food &) {}
    void DrinkLinux(Beverage &) {}
    void SleepLinux(Bed &) {}
#endif

//interface

#ifdef _WIN32
    #define Eat(food)       EatWindow(food)
    #define Drink(beverage) DrinkWindow(beverage)
    #define Sleep(bed)      SleepWindow(bed)
#else
    #define Eat(food)       EatLinux(food)
    #define Drink(beverage) DrinkLinux(beverage)
    #define Sleep(bed)      SleepLinux(bed)
#endif

Use the interfaces Eat(), Drink() and Sleep() throughout your program, be it windows or linux. No conditional check anywhere else!