c - How to create type safe enums?

Question

To achieve type safety with enums in C is problematic, since they are essentially just integers. And enumeration constants are in fact defined to be of type int by the standard.

To achieve a bit of type safety I do tricks with pointers like this:

typedef enum
{
  BLUE,
  RED
} color_t;

void color_assign (color_t* var, color_t val) 
{ 
  *var = val; 
}

Because pointers have stricter type rules than values, so this prevents code such as this:

int x; 
color_assign(&x, BLUE); // compiler error

But it doesn't prevent code like this:

color_t color;
color_assign(&color, 123); // garbage value

This is because the enumeration constant is essentially just an int and can get implicitly assigned to an enumeration variable.

Is there a way to write such a function or macro color_assign, that can achieve complete type safety even for enumeration constants?

Answer 1

It is possible to achieve this with a few tricks. Given

typedef enum
{
  BLUE,
  RED
} color_t;

Then define a dummy union which won't be used by the caller, but contains members with the same names as the enumeration constants:

typedef union
{
  color_t BLUE;
  color_t RED;
} typesafe_color_t;

This is possible because enumeration constants and member/variable names reside in different namespaces.

Then make some function-like macros:

#define c_assign(var, val) (var) = (typesafe_color_t){ .val = val }.val
#define color_assign(var, val) _Generic((var), color_t: c_assign(var, val))

These macros are then called like this:

color_t color;
color_assign(color, BLUE); 

Explanation:

• The C11 _Generic keyword ensures that the enumeration variable is of the correct type. However, this can't be used on the enumeration constant BLUE because it is of type int.
• Therefore the helper macro c_assign creates a temporary instance of the dummy union, where the designated initializer syntax is used to assign the value BLUE to a union member named BLUE. If no such member exists, the code won't compile.
• The union member of the corresponding type is then copied into the enum variable.

We actually don't need the helper macro, I just split the expression for readability. It works just as fine to write

#define color_assign(var, val) _Generic((var), 
color_t: (var) = (typesafe_color_t){ .val = val }.val )

---

Examples:

color_t color; 
color_assign(color, BLUE);// ok
color_assign(color, RED); // ok

color_assign(color, 0);   // compiler error 

int x;
color_assign(x, BLUE);    // compiler error

typedef enum { foo } bar;
color_assign(color, foo); // compiler error
color_assign(bar, BLUE);  // compiler error

---

EDIT

Obviously the above doesn't prevent the caller from simply typing color = garbage;. If you wish to entirely block the possibility of using such assignment of the enum, you can put it in a struct and use the standard procedure of private encapsulation with "opaque type":

color.h

#include <stdlib.h>

typedef enum
{
  BLUE,
  RED
} color_t;

typedef union
{
  color_t BLUE;
  color_t RED;
} typesafe_color_t;

typedef struct col_t col_t; // opaque type

col_t* col_alloc (void);
void   col_free (col_t* col);

void col_assign (col_t* col, color_t color);

#define color_assign(var, val)   
  _Generic( (var),               
    col_t*: col_assign((var), (typesafe_color_t){ .val = val }.val) 
  )

color.c

#include "color.h"

struct col_t
{
  color_t color;
};

col_t* col_alloc (void) 
{ 
  return malloc(sizeof(col_t)); // (needs proper error handling)
}

void col_free (col_t* col)
{
  free(col);
}

void col_assign (col_t* col, color_t color)
{
  col->color = color;
}

main.c

col_t* color;
color = col_alloc();

color_assign(color, BLUE); 

col_free(color);