std::visit

cppreference std::visit

fluentcpp On Design Patterns in C++ # Visitor with std::visit

C++17’s std::visit gives another example of implementing the Visitor design pattern. std::visit is a way to apply a function on a std::variant. But since, by definition, a std::variant can hold values of different type, we may need various functions to operate on it.

NOTE: 也是一种many-to-many

Consider the following example. This is one of the various techniques to create a function object that can operate on various types in C++:

struct Visitor
{
    std::string operator()(std::string const& s){ return s; }

    template<typename T>
    std::string operator()(T const& value) { return std::to_string(value);}
};

Let’s now assume that we have a function that allows to get a variant object:

std::variant<int, std::string, char> getNumber();

Then we can apply the visitor on the variant object with std::visit:

std::string s = std::visit(Visitor{}, getNumber());

Here the visited object (the variant) uses runtime polymorphism (even though without inheritance and virtual methods), and the visitor object (the Visitor) uses compile time polymorphism based on overload resolution.

bitbashing std::visit is everything wrong with modern C++