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When dynamic objects contain data of a known type, it is sometimes useful to have its well-typed representation. A broad set of type-conversions are contained in DynamicConverter.h, and facilitate the transformation of dynamic objects into their well-typed format.


Simply pass a dynamic into a templated convertTo:

    dynamic d = { { 1, 2, 3 }, { 4, 5 } }; // a vector of vector of int
    auto vvi = convertTo<fbvector<fbvector<int>>>(d);

Supported Types

convertTo naturally supports conversions to

  1. arithmetic types (such as int64_t, unsigned short, bool, and double)
  2. fbstring, std::string
  3. containers and map-containers


convertTo will assume that Type is a container if

  • it has a Type::value_type, and
  • it has a Type::iterator, and
  • it has a constructor that accepts two InputIterators

Additionally, convertTo will assume that Type is a map if

  • it has a Type::key_type, and
  • it has a Type::mapped_type, and
  • value_type is a pair of const key_type and mapped_type

If Type meets the container criteria, then it will be constructed by calling its InputIterator constructor.


If you want to use convertTo to convert dynamics into your own custom class, then all you have to do is provide a template specialization of DynamicConverter with the static method convert. Make sure you put it in namespace folly.


    struct Token {
      int kind_;
      fbstring lexeme_;

      explicit Token(int kind, const fbstring& lexeme)
        : kind_(kind), lexeme_(lexeme) {}
    namespace folly {
    template <> struct DynamicConverter<Token> {
      static Token convert(const dynamic& d) {
        int k = convertTo<int>(d["KIND"]);
        fbstring lex = convertTo<fbstring>(d["LEXEME"]);
        return Token(k, lex);
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