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folly/dynamic.hfolly/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
std::variant, but the syntax is intended to be a little
more like using the native type directly.
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);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 raisedExplicit 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 doubleFor more complicated conversions, see DynamicConverter.
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 (<, <=,
>, >=) 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 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).
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()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);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).
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.