Mastering the C++17 STL

Revisiting variant

The variant data type is good at representing simple alternatives, but as of C++17, it is not particularly suitable for representing recursive data types such as JSON lists. That is, the following C++17 code will fail to compile:

    using JSONValue = std::variant<      std::nullptr_t,      bool,      double,      std::string,      std::vector<JSONValue>,      std::map<std::string, JSONValue>    >;

There are several possible workarounds. The most robust and correct is to continue using the C++11 Boost library boost::variant, which specifically supports recursive variant types via the marker type boost::recursive_variant_:

    using JSONValue = boost::variant<      std::nullptr_t,      bool,      double,      std::string,      std::vector<boost::recursive_variant_>, std::map<std::string, ...