convert a cpp class(.cpp file) into a c structure (.c file)

Question

I have a class named Coverage.It has some variables of datatype string and some functions with return type string.

This class is used in other header files.I want to convert the class into a C Structure. Other header file are using the class objects in there functions.I want to change the class in away that I don't have to make changes in other header files.

Like if the function were

string ABC(Coverage* coverage);

My structure should work as a class in behaviour; but unable to create a function in a structure as it is not recommended in C.

class Coverage
{
    std::string country;
    std::string KpRuntimeDir();
    std::string CRtimeDir();
    std::string RtimeDir();
};

Convert the class as, for instance:

struct Coverage n{
    char country[];
    /* [...] */
};

Answer 1

The only way that comes to my mind is to rewrite the class as the structure (only variable members) and define function (instead of methods) outside the structure. But if you have both private and public methods, some seld-defined constructor, then it seems pointless and you will never get the same safeness and functionality.

struct Coverage{
    char country[256];
    --------
    };

char* RtimeDir(Coverage* c);

But in this case it may be invoked with any pointer that is casted which will leads to runtime errors.

Answer 2

You should declare an extern "C" structure and transform an instance of Coverage into one:

extern "C" {
    struct CoverageAsACStruct
    {
        char country[SOME_DEFINED_CONSTANT_COUNTRY];
        char KpRuntimeDir[SOME_DEFINED_CONSTANT_RUNTDIR];
        char CRtimeDir[SOME_DEFINED_CONSTANT_CRTIMEDIR];
        char RtimeDir[SOME_DEFINED_CONSTANT_RTIMEDIR];
    };
}

class Coverage
{
    std::string country();
    std::string KpRuntimeDir();
    std::string CRtimeDir();
    std::string RtimeDir();
    void buildCStructure( struct CoverageAsACStruct* dst ) const;
};

void Coverage::buildCStructure( struct CoverageAsACStruct* dst ) const
{
    std::string cached_country = country();
    if (cached_country.size()+1 > sizeof(dst->country))
        // error?
    // same for other fields

    ::strcpy(dst->country, cached_country.c_str());
    // same for other fields
}

Whenever you need to pass a Coverage instance (lets call it cov) to a C function f(), pass it:

struct CoverageAsACStruct ccov;
cov.buildCStructure(&ccov);
f(&ccov);

You could even embed this into a member function of Coverage.

Answer 3

I am also not sure why you want to do it, but you can also do it by Function Pointers (but prefer conditional compilation), after all C++ is implemented in C. In the below code I have tried to do that, emulate a C++ Structure in C, by adding Functions to a C structure by using Function Pointers.

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

/*
class Someclass{
 string some_str;
public:
 string get();
 void set(string str);
};
*/
struct SomeStruct;
typedef char *(*Getter)(struct SomeStruct *);
typedef void (*Setter)(struct SomeStruct *, char *);
typedef void (*Initialize)(struct SomeStruct *);

typedef struct SomeStruct{
  char some_chr[1024];
  Getter get;
  Setter set;
  Initialize init;
}STR;

char * Get(STR *SStr){
  return SStr->some_chr;
}
void Set(STR *SStr, char *str){
  strncpy(SStr->some_chr,str,strlen(str)+1);
}
void Init(STR *STRstr){
  STRstr->get = Get;
  STRstr->set = Set;
}

int main(){

  STR SStr,Next_str;
  SStr.init = Init;
  SStr.init(&SStr);

  SStr.set(&SStr,"Example");
  printf("%s\n",SStr.get(&SStr));

  Next_str.init = Init;
  Next_str.init(&Next_str);

  Next_str.set(&Next_str,"Next_Example");
  printf("%s\n",Next_str.get(&SStr));

  return 0;
}