Using struct, functions and pointers, I am trying to convert the follwoing C code to C, but its no-CodePudding

I am trying to convert this C code to C. The C class needs to be converted to struct.

Also, we need to use pointers.

Following is the C code:

#include <iostream>
class rectangle {
private:
    double width,height;
public:
    rectangle(double w, double h) {
        width = w;
        height = h;
    }
    double get_area() {
        return width * height;
    }
    bool compare(rectangle* rf) {
        return (this->get_area() > rf->get_area());
    }
};
int main() {
    rectangle* r1=new rectangle(1.5,2.6);
    rectangle* r2=new rectangle(2.5,1.6);
    bool ans=r1->compare(r2);
    printf("%s\n", ans?"true":"false");
    return 0;
}

My code in C does not raise any errors but does not print anything. Here is my code. Cannot seem to find the issue. Can you please help out?

#include <stdio.h>
#include <stdbool.h>

struct Rectangle
{
    double width;
    double height;
    
    void (*new_rectangle)(double, double);
    double (*get_area)();
    bool (*compare)(struct Rectangle*);
    
};

void new_rectangle(struct Rectangle* self, double w, double h) 
{
    self->width = w;
    self->height = h;
}

double get_area(struct Rectangle* self)
{
    return self->width*self->height;
}   

bool compare(struct Rectangle* this, struct Rectangle* other) {
    return (this->get_area() > other->get_area());
}


int main()
{   
    struct Rectangle* r1;
    new_rectangle(r1, 1.5, 2.6);
    struct Rectangle* r2;
    new_rectangle(r2, 2.5, 1.6);
    bool ans = r1->compare(r2);
    printf("%s\n", ans?"true":"false");
    
    // printf("%c", );
//  bool ans=r1->compare(r2);
//  printer(r1)
//  printf("%s\n", ans?"true":"false");
    return 0;
}

CodePudding user response：

Your C code is not setting up the function pointers in the struct, and is not using pointers correctly in main().

Try this instead:

#include <stdio.h>
#include <stdbool.h>

struct Rectangle
{
    double width;
    double height;
    
    double (*get_area)(struct Rectangle*);
    bool (*compare)(struct Rectangle*);
};

double get_area(struct Rectangle* this)
{
    return this->width * this->height;
}   

bool compare(struct Rectangle* this, struct Rectangle* other) {
    return this->get_area(this) > other->get_area(other);
}

void new_rectangle(struct Rectangle* this, double w, double h) 
{
    this->width = w;
    this->height = h;
    this->get_area = &get_area;
    this->compare = &compare;
}

int main()
{   
    struct Rectangle r1;
    new_rectangle(&r1, 1.5, 2.6);
    struct Rectangle r2;
    new_rectangle(&r2, 2.5, 1.6);
    bool ans = r1.compare(&r1, &r2);
    printf("%s\n", ans ? "true" : "false");
    return 0;
}

Alternatively:

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>

struct Rectangle
{
    double width;
    double height;
    
    double (*get_area)(struct Rectangle*);
    bool (*compare)(struct Rectangle*);
};

double get_area(struct Rectangle* this)
{
    return this->width * this->height;
}   

bool compare(struct Rectangle* this, struct Rectangle* other) {
    return this->get_area(this) > other->get_area(other);
}

struct Rectangle* new_rectangle(double w, double h) 
{
    struct Rectangle* r = malloc(sizeof(struct Rectangle));
    if (!r) return NULL;
    r->width = w;
    r->height = h;
    r->get_area = &get_area;
    r->compare = &compare;
    return r;
}

void delete_rectangle(struct Rectangle* this)
{
    free(this);
}

int main()
{   
    struct Rectangle* r1 = new_rectangle(1.5, 2.6);
    struct Rectangle* r2 = new_rectangle(2.5, 1.6);
    bool ans = r1->compare(r1, r2);
    printf("%s\n", ans ? "true" : "false");

    delete_rectangle(r1);
    delete_rectangle(r2);

    return 0;
}

That being said, it doesn't really make sense to have get_area() and compare() be members of Rectangle in C:

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>

struct Rectangle
{
    double width;
    double height;
};

double get_area(struct Rectangle* this)
{
    return this->width * this->height;
}   

bool compare(struct Rectangle* this, struct Rectangle* other) {
    return get_area(this) > get_area(other);
}

struct Rectangle* new_rectangle(double w, double h) 
{
    struct Rectangle* r = malloc(sizeof(struct Rectangle));
    if (!r) return NULL;
    r->width = w;
    r->height = h;
    return r;
}

void delete_rectangle(struct Rectangle* this)
{
    free(this);
}

int main()
{   
    struct Rectangle* r1 = new_rectangle(1.5, 2.6);
    struct Rectangle* r2 = new_rectangle(2.5, 1.6);
    bool ans = compare(r1, r2);
    printf("%s\n", ans ? "true" : "false");

    delete_rectangle(r1);
    delete_rectangle(r2);

    return 0;
}

CodePudding user response：

Your translation is not semantically correct: the Rectangle structure does not need to have function pointers for the methods. In the C code, the methods are not virtual, so the compiler determines which functions to call from the type of the arguments: translating this to C puts this burden on the programmer, who must name the methods more explicitly and call them appropriately.

Here is a closer translation (ie: closer to what the c compiler actually generates):

#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>

typedef struct rectangle {
    double width, height;
} rectangle;

rectangle *rectangle_construct(rectangle *this, double w, double h) {
    this->width = w;
    this->height = h;
    return this;
}

rectangle *rectangle_new(double w, double h) {
    return rectangle_construct(malloc(sizeof(rectangle)), w, h);
}

void rectangle_destroy(rectangle *this) {
    // no members need destroying
}

void rectangle_delete(rectangle *this) {
    rectangle_destroy(this);
    free(this);
}

double rectangle_get_area(rectangle *this) {
    return this->width * this->height;
}

bool compare(rectangle *this, rectangle *that) {
    return rectangle_get_area(this) > rectangle_get_area(that);
}

int main() {
    rectangle *r1 = rectangle_new(1.5, 2.6);
    rectangle *r2 = rectangle_new(2.5, 1.6);
    bool ans = rectangle_compare(r1, r2);
    printf("%s\n", ans ? "true" : "false");
    // in your code, you should add these lines
    //delete r1;
    //delete r2;
    //  emulated as
    rectangle_delete(r1);
    rectangle_delete(r2);
    return 0;
}