Why can't "PURE" be defined as a const set to 0 to identify pure virtual classes?

Question

Trying to define keyword

PURE

as a const set to

0

, to be used as an identifier for classes that are an abstract data type. Why won't this compile? Per Meyers in "Essential C++, topic 1," I would prefer to use

const

as opposed to #define, like so:

const int PURE = 0;

virtual void myFunction() = PURE;

Alas, this throws an error (same on Apple LLVM 7.0 and gcc):

Initializer on function does not look like pure-specifier

Example follows, with three techniques labeled A, B, and C;

1. const int PURE = 0 will not compile

2. #define PURE 0 compiles and runs fine

3. Simply setting function = 0 (Stroustrup) works fine.

It's now set up use the

const

solution, and so will not compile. Simply comment/uncomment lines 4, 5, 11, and 12 appropriately to examine the three different methods:

#include <iostream>

#include <string>



const int PURE = 0;                  // Case B

// #define PURE 0                       // Case C



const float PI = 3.14159;



class Shape {

public:

//    virtual float area() = 0;     // Case A: Compiles

    virtual float area() = PURE;    // Case B: This does not compile

                                    // Case C: Compiles

};

class Circle: public Shape {

public:

    Circle(float radius):radius_(radius) {}

    float area() { return PI * radius_ * radius_; }

private:

    float radius_;

};

class Rectangle : public Shape {

public:

    Rectangle(float base, float height):base_(base),height_(height) {}

    float area() { return base_ * height_; }

private:

    float base_;

    float height_;

};

int main(int argc, const char * argv[]) {

    Circle c(3);

    Rectangle r(3,5);



    std::cout << "Circle Area:    \t" << c.area() << std::endl;

    std::cout << "Rectangle Area: \t" << r.area() << std::endl;



    std::cout << std::endl;

    return 0;

}

Recommended for you: Get network issues from WhatsUp Gold. Not end users.

Answer 1

The language grammar says:

pure-specifier:
    = 0

That is, only the tokens = 0 are allowed. You cannot put an identifier there.

#define PURE 0 works fine because macro replacement happens before translation.

Why can't "PURE" be defined as a const set to 0 to identify pure virtual classes?

Sign up