CODEWITHSUNDEEP
c++cmalloclanguage-lawyernew-operator
francesco
francesco

Reputation: 7539

Using operator new in mixed c and c++ code

Consider a program with mixed C and C++ code. The C++ part contains a class which dinamically allocates a C-style typedef struct. Minimal example:

obj.h (C-code)

typedef struct _Ctype {
  int i;
} Ctype;

class.hpp (C++ code)

struct A {
  struct _Ctype *x;
  A(int i);
  ~A();
};

class.cpp (C++ code)

#include "class.hpp"

extern "C" {
#include "obj.h"
}


A::A(int i)
{
  x = new struct _Ctype;
  x->i = i;
}

A::~A()
{
  delete(x);
}

main.cpp (C++ code, main program)

#include "class.hpp"

int main()
{
  A a(3);
  return 0;
}

(The rationale for this design originates from this answer)

Is it safe (i.e., no UB) to use a new expression to allocate a C-style type struct _Ctype, as in the code above, or should one better use the C-style malloc/free?

class.cpp (C++ code, alternative)

#include <cstdlib>
#include "class.hpp"

extern "C" {
  #include "obj.h"
  }


A::A(int i)
{
  x = (struct _Ctype *)malloc(sizeof(struct _Ctype));
  x->i = i;
}

A::~A()
{
  free(x);
}

ADDITION

To clarify the question after some comments below: In the minimal example above, all the code is compiled with a C++ compiler. One can however think to use the C++ code in conjunction with a C library. The question can then be reformulated as follows:

Note that one could also think to allocate the memory for Ctype through a C function, so that the C++ code only manages a pointer to it, for example:

obj.h (C-code)

typedef struct _Ctype {
  int i;
} Ctype;

Ctype *allocate_Ctype();
void deallocate_Ctype(Ctype* p);

obj.C (C-code)

#include <stdlib.h>
#include "obj.h"

Ctype *allocate_Ctype()
{
   return (Ctype *)malloc(sizeof(Ctype));
}

void deallocate_Ctype(Ctype *p)
{
   free(p);
}

class.cpp (C++ code)

#include "class.hpp"

extern "C" {
#include "obj.h"
}


A::A(int i)
{
  x = allocate_Ctype();
  x->i = i;
}

A::~A()
{
  deallocate_Ctype(x);
}

(Note: of course the copy constructor and operator assignment of class A need to be properly defined, the code serves as illustration of the question)

Upvotes: 1

Views: 309

Answers (1)

Angew is no longer proud of SO
Angew is no longer proud of SO

Reputation: 171127

As long as the deallocation only ever happens under your control and using a delete expression, there is no problem at all. The C code interacting with the structure does not care how it was allocated.

Side note: the name _Ctype is not legal in C++, as it starts with an underscore followed by an uppercase letter. Such names (as well as names containing double underscore) are reserved for the compiler & standard library.

Upvotes: 4

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