Thread: How to define a variant record type in C

Use C++ and build a class. Or, use C and enums. You cannot enforce the behaviour you want in C easily. Yes it can be done. No, it's not easy. Use C++.
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Code:

/* type POWER is (GAS, STEAM, DIESEL, NONE); */
typedef enum { NONE, GAS, STEAM, DIESEL } POWER;

/*
   type VEHICLE (Engine : POWER) is
      record
         Model_Year : INTEGER range 1888..1992;
         Wheels     : INTEGER range 2..18;
         case Engine is
            when GAS    => Cylinders   : INTEGER range 1..16;
            when STEAM  => Boiler_Size : INTEGER range 5..22;
                           Coal_Burner : BOOLEAN;
            when DIESEL => Fuel_Inject : BOOLEAN;
            when NONE   => Speeds      : INTEGER range 1..15;
         end case;
      end record;
*/
typedef struct {
	unsigned char Model_Year;
	unsigned char Wheels;
	POWER Type;
	union
	{
		struct
		{
			unsigned char Cylinders;
		} Gas;
		struct
		{
			unsigned char Boiler_Size;
			unsigned char Coal_Burner;
		} Steam;
		struct
		{
			unsigned char Fuel_Inject;
		} Diesel;
		struct
		{
			unsigned char Speeds;
		}
	} Engine;
} VEHICLE;

/*   Ford    : VEHICLE(GAS); */
VEHICLE Ford;

SetWheels( &Ford, 4 ); /* This would assign the value. */
SetYear( &Ford, 1983 ); /* This would assign the value. */
SetEngine( &Ford, GAS ); /* This would prompt for the correct values. */

It is up to you to enforce the values correctly. You really want C++.
[/edit]

Quzah.