Question

The format of define preprocessor directives are:

#ifndef  SIZE
#define SIZE 10
int hello[SIZE];
#endif

But when I look at the following code, there is no replacement for the preprocessor directives:

#ifndef CREDIT_CARD_H                    // Avoid repeated expansion
#define CREDIT_CARD_H

#include <string>                        // Provides string
#include <iostream>                      // Provides ostream

class CreditCard
{
    public:
        CreditCard(const std::string& no,    // Constructor
                   const std::string& nm, int lim, double bal = 0);

        // Accessor functions
        std::string    getNumber()const    { return number; }
        std::string    getName() const     { return name; }
        double         getBalance() const  { return balance; }
        int            getLimit() const    { return limit; }

        bool chargeIt(double price);       // Make a charge
        void makePayment(double payment);  // Make a payment

    private:                               // Private member data
        std::string    number;             // Credit card number
        std::string name;                  // Card owner's name
        int            limit;              // Credit limit
        double        balance;             // Credit card balance
};

std::ostream& operator<<(std::ostream& out, const CreditCard& c);
#endif

What does this mean?

Answer 1

You can say #define FOO, which means that #ifdef FOO will be true, but FOO doesn’t have any replacement text. This is useful precisely for conditional checks like the include guards.

It can also be useful for platform-specific extensions, which you want to be empty in the general case:

#ifdef WIN32
#  define API __declspec(dllexport)
#else
#  define API
#endif

API void foo();