C++17: The two line visitor explained

If you have ever used an “idiomatic” C++ variant datatype like Boost.Variant or the new C++17 std::variant, you probably wished you could assemble a visitor to dispatch on the type by assembling a couple of lambda expressions like this:

auto my_visitor = visitor{
  [&](int value) { /* ... */ },
  [&](std::string const& value) { /* ... */ },

The code in question

While reading through the code for lager I stumbled upon a curious way to to make this happen. And it is just two lines of code! Wow, that is cool.

template<class... Ts> struct visitor: Ts... { using Ts::operator()...; };
template<class... Ts> visitor(Ts...) -> visitor<Ts...>;

A comment in the code indicated that the code was copied from cppreference.com where I quickly found the source on the page for std::visit, albeit with the different name “overloaded”. There were, however, no comments as to how this code worked.

Multiple inheritance to the rescue

Lambda expressions in C++ are just syntactic sugar for callables, pretty much like a struct with an operator(). As such, you can derive from them. which is what the first line does.
It uses variadic templates and multiple inheritance to assemble the types of the lambdas into one type. Without the content in the struct body, an instantiation with our example would be roughly equivalent to this:

struct int_visitor {
  void operator()(int value)
  {/* ... */}

struct string_visitor {
  void operator()(std::string const& value)
  {/* ... */}

struct visitor : int_visitor, string_visitor {

Using all of it

Now this cannot yet be called, as overload resolution (by design) does not work across different types. Hence the using in the structs body. It pulls the operator() implementations into the visitor type where overload resolution can work across all of them.
With it, our hypothetical instantiation becomes:

struct visitor : int_visitor, string_visitor {
  using int_visitor::operator();
  using string_visitor::operator();

Now an instance of that type can actually be called with both our types, which is what the interface for, e.g. std::visit demands.

Don’t go without a guide

The second line intruiged me. It looks a bit like a function declaration but that is not what it is. The fact that I had to ask in the (very helpful!) C++ slack made me realize that I did not keep up with the new features in C++17 as much as I would have liked.
This is, in fact, a class template argument deducation (CTAD) guide. It is a new feature in C++17 that allows you do deduce template arguments for a type based on constructor parameters. In a way, it supercedes the Object Generator idiom of old.
The syntax is really quite straight-forward. Given a list of constructor parameter types, resolve to a specific template instance based on those.


The last piece of the puzzle is how the visitor gets initialized. The real advantage of using lambdas instead of writing the struct yourself is that you can capture variables from your context. Therefore, you cannot just default-initialize most lambdas – you need to transport its values, its bound context.
In our example, this uses another new C++17 feature: extended aggregate initialization. Aggregate initialization is how you initialized structs way back in C with curly-brackets. Previously, it was forbidden to do this with structs that have a base class. The C++17 extension now lifts this restriction, thus making it possible to initialize this visitor with curly brackets.

Edit 2018/04/16: The people on r/cpp rightfully pointed out that using the “other name” in the code snippet was confusing – so the visitor is now called “visitor”.


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