C++: String and unions

Question

I'm having a (design) problem: I'm building an interpreter and I need some storage for variables. There are basically two types of content a variable here can have: string or int.

I'm using a simple class for the variables, all variables are then stored in a vector.

However, as a variable can hold a number or a string, I don't want C++ to allocate both and consume memory for no reason.

That's why I wanted to use unions:

union
{
 string StringValue;
 int IntValue;
}

However, strings don't work with unions.

• Is there any workaround so no memory gets eaten for no reason?

Answer 1

You may check out Boost.Variant

typedef boost::variant<std::string, int> Variable;

(Although that "wastes" extra memory - the size of a variant is something like the size of the largest type plus a word or so for bookkeeping - can't see how you could do with less anyway.)

Answer 2

However, as a variable can hold a number or a string, I don't want C++ to allocate both and consume memory for no reason.

Why not? How does four or eight or sixteen extra bytes hurt?

Unless you have a good reason, I don't think wasting 1KB is a lot.

If you want to use a union with a string, you can instead use pointers. Either use string *, int, string *, int *, or const char *, int or const char *, int *, as the union member types.

Answer 3

I would not worry about the extra storage of a single integer. The overhead of correctly managing a variant or discriminated union with a string pointer is going to be much worse than using the extra storage associated with a single field.