Define rules for casting to custom datatypes in C

Question

For the built-in datatypes (e.g. int, float etc), there are built-in 'rules' for casting: e.g.

int x, float y;
x = 3;
y = x;

will produce y = 3.000000.

Is there any way to define custom 'casting rules' for my own datatypes defined by typedefs?

Say I have a struct rational representing a rational number. I would like to be able to cast int variables to rational such that the denominator field is set to 1 and the numerator is equal to the initial int. I.e. if I do:

typedef struct rational{int num; int denom;} rational;
int x, rational y;
x = 3;
y = x;

I want to get y.num = 3 and y.denom = 1.

Can arbitrary functions be defined to control the behaviour when casting to/from user-defined datatypes?

Failing that, is it possible to set a 'default' value for a subset of a struct's members (such that that member can be optionally skipped when assigning a value to a variable of that type, defaulting to the set value)?

Answer 1

Unlike C++ which allows you to overload the typecast operator and assignment operator, C doesn't allow you to define your own casts or implicit conversions.

You'll need to write a function that performs the conversion. For example:

rational int_to_rational(int x)
{
    rational r = { x, 1 };
    return r;
}