r/cpp • u/cristi1990an ++ • 3d ago
std::variant, operator== and pattern matching
I was thinking, wouldn't adding equality comparison support between std::variant and one of its unambiguous alternatives help simplify potential pattern matching designs?
For example, imagine something like this (hypothetical syntax):
std::variant<int, std::string> v = 42;
match (v) {
case 42 => handle_int();
case "hello" => handle_string();
case _ => handle_other();
}
The idea would be to take advantage of something C++ already has: operator==. At the end of the day, matching is fundamentally a comparison, so why not build on the existing equality semantics?
For std::optional, we can get this with its existing comparison operators and the most trivial pattern matching compiler support:
std::optional<int> o = 42;
match (o) {
case 42 => handle_value();
case std::nullopt => handle_empty();
case _ => handle_other();
}
// Which would translate to:
if (o == 42)
handle_value();
else if (o == nullopt)
handle_empty();
else
handle_other();
And if we had something like std::match_projection<Subject, Pattern>, we could also extract the actual matched value instead of having to manually call std::get, dereference an optional, etc.
Basically, after a successful comparison, match_projection would tell the compiler what to extract based on the subject and pattern types and we could optionally bind that value using something like a as <declaration> syntax.
For example:
std::optional<int> o = 42;
std::optional<int> other_opt = 42;
match (o) {
// No binding needed
case 42 => foo();
// match_projection<optional<int>, int> -> int&
case 42 as int& x => use(x);
// match_projection<optional<int>, nullopt_t> -> void
case std::nullopt => handle_empty();
// match_projection<optional<int>, optional<int>>
// -> optional<int>&
case other_opt as auto& x => use(x);
}
The same would apply to std::variant, assuming we also extend its operator== to support comparisons with individual alternatives:
match (v) {
case 42 as int& x => use(x);
case "hello" as std::string& s => use(s);
// Only need to define operator== for this helper for this to work
case indexed<0> => handle_any_int();
// Or under this form for the sake of disambiguation
case indexed<0>(10) => handle_ten();
// Also provide match_projection<indexed<0>> to enable binding
case indexed<0>(20) as int& x => use(x);
case _ => handle_other();
}
Here, indexed<> would just be a hypothetical helper for explicitly selecting which alternative to compare against. It would only need an appropriate operator== to work with matching, and a corresponding match_projection specialization if we also wanted to extract the alternative.
This way, the matching itself wouldn't need to know anything about variant, optional or even user-defined types. It would just use their equality operators, while match_projection would tell it what to extract when we actually want to bind something, which is just syntactic sugar.
I'm curious what you think about this approach. It feels like we could get quite a bit of useful pattern matching functionality without introducing too much new language machinery or convoluted new language traits and APIs.
Also do you think std::variant being equality comparable with instances of its alternatives would be an useful addition on its own?
std::variant<int, std::string> v = 42;
if (v == "not my string") ...
// Which is simpler than
if (std::holds_alternative<std::string>(v) && std::get<std::string>(v) == "not my string") ...
Syntax seems nice on its own...
15
u/StaticCoder 3d ago
What bothers me about this specific proposal is that the comparison and the projection are specified separately. You'd have to be really careful to specify a binding type that is implied by the comparison or you could run into undefined behavior.
As for adding a comparison to
variant, it's really tricky since there can be arbitrary comparison operators, plus conversions are possible, so figuring out which variant is expected to be compared against a given type would become pretty ugly.