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folly/dynamic.h

folly/dynamic.h provides a runtime dynamically typed value for C++, similar to the way languages with runtime type systems work (e.g. Python). It can hold types from a predetermined set of types (ints, bools, arrays of other dynamics, etc), similar to something like boost::variant, but the syntax is intended to be a little more like using the native type directly.

Overview

Here are some code samples to get started (assumes a using folly::dynamic; was used):

    dynamic twelve = 12; // creates a dynamic that holds an integer
    dynamic str = "string"; // yep, this one is an fbstring

    // A few other types.
    dynamic nul = nullptr;
    dynamic boolean = false;

    // Arrays can be initialized with dynamic::array.
    dynamic array = dynamic::array("array ", "of ", 4, " elements");
    assert(array.size() == 4);
    dynamic emptyArray = dynamic::array;
    assert(emptyArray.empty());

    // Maps from dynamics to dynamics are called objects.  The
    // dynamic::object constant is how you make an empty map from dynamics
    // to dynamics.
    dynamic map = dynamic::object;
    map["something"] = 12;
    map["another_something"] = map["something"] * 2;

    // Dynamic objects may be initialized this way
    dynamic map2 = dynamic::object("something", 12)("another_something", 24);

Runtime Type Checking and Conversions

Any operation on a dynamic requires checking at runtime that the type is compatible with the operation. If it isn't, you'll get a folly::TypeError. Other exceptions can also be thrown if you try to do something impossible (e.g. if you put a very large 64-bit integer in and try to read it out as a double).

More examples should hopefully clarify this:

    dynamic dint = 42;

    dynamic str = "foo";
    dynamic anotherStr = str + "something"; // fine
    dynamic thisThrows = str + dint; // TypeError is raised

Explicit type conversions can be requested for some of the basic types:

    dynamic dint = 12345678;
    dynamic doub = dint.asDouble(); // doub will hold 12345678.0
    dynamic str = dint.asString(); // str == "12345678"

    dynamic hugeInt = std::numeric_limits<int64_t>::max();
    dynamic hugeDoub = hugeInt.asDouble();  // throws a folly/Conv.h error,
                                            // since it can't fit in a double

For more complicated conversions, see DynamicConverter.

Comparison Operators and Hashing

Equality Operators

Equality operators (==, !=) are supported for all types.

For dynamics of the same type, the underlying equality operator shall apply.

For comparisons between different numeric types (double and int64), numeric equality will apply - thus 2.0 == 2.

Values of any other different types will be deemed to not be equal.

Ordering operators

Ordering operators (<, <=, >, >=) are supported for all types, except dynamic::object which will throw if it is involved in an ordering operator.

For dynamics of the same type, the underlying ordering operator shall apply.

For comparisons between different numeric types (double and int64), numeric ordering will apply - thus 1.5 < 2.

Ordering of values between other different types will maintain total ordering properties and be consistent within a given binary run, and thus safe for use in e.g. std::set. The actual ordering is undefined and could change across versions, thus a dependency should not be taken outside of the total ordering property within a given binary.

Hashing

Hashing is supported by all types, and the hashes of two values will match if they are equal per dynamic::operator==.

As a result, numerical types have the same numerical hashing regardless of int64 vs double - so e.g. std::hash<dynamic>()(2) will give the same value as std::hash<dynamic>()(2.0).

Iteration and Lookup

You can iterate over dynamic arrays as you would over any C++ sequence container.

    dynamic array = dynamic::array(2, 3, "foo");

    for (auto& val : array) {
      doSomethingWith(val);
    }

You can iterate over dynamic maps by calling items(), keys(), values(), which behave similarly to the homonymous methods of Python dictionaries.

    dynamic obj = dynamic::object(2, 3)("hello", "world")("x", 4);

    for (auto& pair : obj.items()) {
      // Key is pair.first, value is pair.second
      processKey(pair.first);
      processValue(pair.second);
    }

    for (auto& key : obj.keys()) {
      processKey(key);
    }

    for (auto& value : obj.values()) {
      processValue(value);
    }

You can find an element by key in a dynamic map using the find() method, which returns an iterator compatible with items():

    dynamic obj = dynamic::object(2, 3)("hello", "world")("x", 4);

    auto pos = obj.find("hello");
    // pos->first is "hello"
    // pos->second is "world"

    auto pos = obj.find("no_such_key");
    // pos == obj.items().end()

Use for JSON

The original motivation for implementing this type was to try to make dealing with json documents in C++ almost as easy as it is in languages with dynamic type systems (php or javascript, etc). The reader can judge whether we're anywhere near that goal, but here's what it looks like:

    // Parsing JSON strings and using them.
    std::string jsonDocument = R"({"key":12,"key2":[false, null, true, "yay"]})";
    dynamic parsed = folly::parseJson(jsonDocument);
    assert(parsed["key"] == 12);
    assert(parsed["key2"][0] == false);
    assert(parsed["key2"][1] == nullptr);

    // Building the same document programmatically.
    dynamic sonOfAJ = dynamic::object
      ("key", 12)
      ("key2", dynamic::array(false, nullptr, true, "yay"));

    // Printing.  (See also folly::toPrettyJson)
    auto str = folly::toJson(sonOfAJ);
    assert(jsonDocument.compare(str) == 0);

Performance

Dynamic typing is more expensive than static typing, even when you do it in C++. ;)

However, some effort has been made to keep folly::dynamic and the json (de)serialization at least reasonably performant for common cases. The heap is only used for arrays and objects, and move construction is fully supported. String formatting internally also uses the highly performant folly::to<> (see folly/Conv.h).

A trade off to keep in mind though, is that sizeof(folly::dynamic) is 64 bytes. You probably don't want to use it if you need to allocate large numbers of them (prefer static types, etc).

Some Design Rationale

Q. Why doesn't a dynamic string support begin(), end(), and operator[]?

The value_type of a dynamic iterator is dynamic, and operator[] (or the at() function) has to return a reference to a dynamic. If we wanted this to work for strings, this would mean we'd have to support dynamics with a character type, and moreover that the internal representation of strings would be such that we can hand out references to dynamic as accessors on individual characters. There are a lot of potential efficiency drawbacks with this, and it seems like a feature that is not needed too often in practice.

Q. Isn't this just a poor imitation of the C# language feature?

Pretty much.