Struct with pre-defined constants, possible?

[Subsonics]

Is it possible to make a struct that have some members pre-defined and non editable? Let´s say I make a struct with 5 members, where only 2 is possible to asign values to while the other three is predefined. I have tried in various ways like using constants in the struct declaration without success.

[laserlight]

I suspect that it is not possible in C, since the natural solution in C++ would be to use a constructor initialisation list, which is a feature not available in C.

[matsp]

You could do something like this:

struct allmembers
{
    const struct constmembers
    {
        int x;
        int y;
    } consts;
    int z;
    int w;
};

int main()
{
  struct allmembers all = { { 3, 2 }, 0, 0 };

  all.consts.x = 1;  /* Error compiling.  */
  all.z = 5;

  return 0;
}

--
Mats

[laserlight]

The best I have been able to come up with is to place the const member variables first and then use an initialiser... but that still does not fully satisfy the "pre-defined" requirement:

struct X
{
    const int a;
    const int b;
    int c;
};

int main(void)
{
    struct X x = {1, 2};
    x.c = 3;
    return 0;
}

[Subsonics]

Thanks, it looks like I would need to initialize manualy. What I´m thinking about is a way to deal with data packets where some members are defined as a way to give identity to them. This is the struct I had in mind:

	typedef struct {
		short		header;			// F07E			
		char		channelNumber;
		char		id;			// 02
		char		data[120];			
		short		cheksum;			
		char		end;			// F7						
	} DataPacket;

The header, id and end should stay the same while channelNumber, data and checksum should be variables. I guess that I could make a loop that updates the variables and give the other members constant values.

[laserlight]

You could define say, an initDataPacket() function that is supposed to act as a constructor.

[Subsonics]

Thanks laserlight, that seems like a good solution I´ll stick to that.