Datatype independent stack - C Programming

Question

Often stacks in C are dependent upon datatype used to declare them. For example,

int arr[5]; //creates an integer array of size 5 for stack use
char arr[5]; //creates a character array of size 5 for stack use

are both limited to holding integer and character datatypes respectively and presumes that the programmer knows what data is generated during the runtime. What if I want a stack which can hold any datatype?

I initially thought of implementing it as a union, but the approach is not only difficult but also flawed. Any other suggestions?

Answer 1

I would use a structure like this:

struct THolder
{
   int dataType; // this is a value representing the type
   void *val; // this is the value 
};

Then use an array of THolder to store your values.

Answer 2

This is really just a variant of Pablo Santa Cruz' answer, but I think it looks neater:

typedef enum { integer, real, other } type_t;

typedef struct {
    type_t type;
    union {
        int normal_int;     /* valid when type == integer */
        double large_float; /* valid when type == real */
        void * other;       /* valid when type == other */
    } content;
} stack_data_t;

You still need to use some way to explicitly set the type of data stored in each element, there is no easy way around that.

You could look into preprocessor magic relying on the compiler-dependent typeof keyword to do that automagically, but that will probably not do anything but ruin the portability.