How can I define a recursive variant type?

Yes, it is possible to define a variant that contains a type that itself contains the same variant type. This is known as a recursive variant. However, because variants must have a fixed size, you can't directly include a variant as one of its own alternative types.

Instead, you need to wrap the recursive variant in a type that allows for dynamic allocation, such as std::unique_ptr or std::vector.

Here's an example of a recursive variant that represents a simple expression tree:

1#include <memory>
2#include <string>
3#include <variant>
4
5struct Expr;
6using ExprPtr = std::unique_ptr<Expr>;
7
8struct Expr {
9  std::variant<
10    int, // integer literal
11    std::string, // variable name
12
13    // unary operation
14    std::pair<char, ExprPtr>,
15
16    // binary operation
17    std::tuple<ExprPtr, char, ExprPtr>
18  > value;
19};

In this example, Expr is a struct that contains a variant value. The variant can hold an integer literal, a variable name (string), a unary operation (a character and a pointer to another Expr), or a binary operation (two pointers to Exprs and a character).

Here's how you could use this Expr type:

1Expr makeIntegral(int i) {
2  Expr e;
3  e.value = i;
4  return e;
5}
6
7Expr makeVariable(std::string name) {
8  Expr e;
9  e.value = std::move(name);
10  return e;
11}
12
13Expr makeUnary(char op, Expr arg) {
14  Expr e;
15  e.value = std::make_pair(
16    op, std::make_unique<Expr>(std::move(arg)));
17  return e;
18}
19
20Expr makeBinary(Expr left, char op, Expr right) {
21  Expr e;
22  e.value = std::make_tuple(
23    std::make_unique<Expr>(std::move(left)), 
24    op,
25    std::make_unique<Expr>(std::move(right))
26  );
27  return e;
28}
29
30// Example usage
31Expr exampleExpr = makeBinary(
32  makeUnary('-', makeVariable("x")),
33  '+',
34  makeIntegral(42)
35);

This creates an Expr that represents the expression -x + 42.

The key points are:

1. Use a smart pointer like std::unique_ptr to allow for recursive containment without requiring a fixed size.
2. Wrap the variant in a struct or class to allow for a named recursive type.
3. Use constructor functions (like makeIntegral, makeUnary etc in the example) to simplify the creation of complex recursive variants.

Recursive variants are a powerful tool for representing tree-like data structures in a type-safe way. They are often used in parsers, abstract syntax trees, and similar hierarchical data structures.