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jdelong October 11, 2012
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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.

To use dynamic, you need to be using gcc 4.6 or later. You'll want to include folly/dynamic.h (or perhaps also folly/json.h).


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 brackets.
    dynamic array = { "array ", "of ", 4, " elements" };
    assert(array.size() == 4);
    dynamic emptyArray = {};

    // 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 intialized 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.

Iteration and Lookup

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

    dynamic array = {2, 3, "foo"};

    for (auto& val : array) {

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

    for (auto& key : obj.keys()) {

    for (auto& value : obj.values()) {

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 programatically.
    dynamic sonOfAJ = dynamic::object
      ("key", 12)
      ("key2", { false, nullptr, true, "yay" });

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


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 is there no default constructor?

This is a bit of a limitation of std::initializer_list<> for this use case. The expression dynamic d = {} is required by the standard to call the default constructor if one exists (the reasoning for this makes sense, since {} is part of the concept of "uniform initialization", and is intended for use with things like std::vector). It would be surprising if this expression didn't leave d.isArray() true, but on the other hand it would also be surprising if dynamic d left d.isArray() as true. The solution was just to disallow uninitialized dynamics: every dynamic must start out being assigned to some value (or nullptr).

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.

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