C macro preprocessor mapping a numer to a type

Question

I am trying to map a number to a keyword type in C. Is there anyway to do this?

#include <stdio.h>
#include <stdlib.h>
#include "memory.h"

#define DOUBLE_TYPE 1
#define INT_TYPE 2

#define OBJECT_TYPE(Y)
        #if Y == DOUBLE_TYPE \
                double \
        #elif Y == INT_TYPE \
                int \
        #else \
                int \
        #endif \

int main(int argc, char* argv[]) {
        unsigned int type = 1;
        OBJECT_TYPE(type) value = 0.5f;
        printf("%f\n", value);

        return 0;
}
test.c:11:3: error: missing binary operator before token "double"
   double \

I want to be able to cast a void* to a specific type known by the integer code.

The C preprocessor has no concept of data types. You cannot nest directives. And C is statically typed. What you try does not make any sense. It is an XY problem. What do you want to solve? — too honest for this site
– too honest for this site, Commented Jun 13, 2017 at 17:07
The preprocessor is a compile-time thing, that runs in a separate phase before the actual compilation takes place. The preprocessor uses it's own separate language, and have no concept of C variables. — Some programmer dude
– Some programmer dude, Commented Jun 13, 2017 at 17:09
So the only way to do it parse my own files? I am trying to create my own object type system in C. If I have an unknown void* and I know the data type by integer code then I want to be able to cast the void*. — eat_a_lemon
– eat_a_lemon, Commented Jun 13, 2017 at 17:14
It's not really clear what "it" is - what's a more realistic example of the problem you're trying to solve here? — Oliver Charlesworth
– Oliver Charlesworth, Commented Jun 13, 2017 at 17:16

dbush · Accepted Answer · 2017-06-13 17:21:09Z

3

What you want to do isn't possible with macros. They only do compile time text substitution, while what you want depends on runtime values.

If you want to effectively have a variable with a variant type, use a union:

struct var_type {
    int type;
    union {
        int i;
        double d;
    } value;
};

int main(int argc, char* argv[]) {
    struct var_type value = { DOUBLE_TYPE, { d = 0.5f }};

    switch (value.type) {
    case DOUBLE_TYPE:
        printf("%f\n", value.value.d);
        break;
    case INT_TYPE:
        printf("%d\n", value.value.i);
        break;
    default:
        printf("%d\n", value.value.i);
        break;
    }

    return 0;
}

edited Jun 13, 2017 at 17:21

answered Jun 13, 2017 at 17:17

dbush

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4 Comments

eat_a_lemon Over a year ago

I want to be able to cast a void* to a specific type known by the integer code.

unalignedmemoryaccess Over a year ago

You can't do that @eat_a_lemon

dbush Over a year ago

@eat_a_lemon Then check the type and cast it to that type.

eat_a_lemon Over a year ago

@dbush yea I guess the union replaces void* with a list of known types.

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