Modern C++ Programming Cookbook

How it works...

A visitor is a callable object (a function, a lambda expression, or a function object) that accepts every possible alternative from a variant. Visitation is done by invoking std::visit() with the visitor and one or more variant objects. The variants do not have to be of the same type, but the visitor must be able to accept every possible alternative from all the variants it is invoked for. In the examples earlier, we have visited a single variant object, but visiting multiple variants does not imply anything more than passing them as arguments to std::visit().

When you visit a variant, the callable object is invoked with the value currently stored in the variant. If the visitor does not accept an argument of the type stored ...