What is the difference between dynamic and Object in dart?
ObjectDartDynamictypeObject Problem Overview
They both seem like they can be used in identical cases. Is there a different representation or different subtleties in type checking, etc?
Object Solutions
Solution 1 - Object
Another perspective on dynamic
is that it's not really a type - it's a way to turn off type checking and tell the static type system "trust me, I know what I'm doing". Writing dynamic o;
declares a variable that isn't typed - it's instead marked as "not type-checked".
When you write Object o = something;
you are telling the system that it can't assume anything about o
except that it's an Object
. You can call toString
and hashCode
because those methods are defined on Object
, but if you try to do o.foo()
you will get a warning - it can't see that you can do that, and so it warns you that your code is probably wrong.
If you write dynamic o = something
you are telling the system to not assume anything, and to not check anything. If you write o.foo()
then it will not warn you. You've told it that "anything related to o
is OK! Trust me, I know what I'm doing", and so it thinks o.foo()
is OK.
With great power comes great responsibility - if you disable type-checking for a variable, it falls back on you to make sure you don't do anything wrong.
Solution 2 - Object
The section Type dynamic from the Dart Programming Language Specification, 3rd Edition states :
>Type dynamic has methods for every possible identifier and arity, with every possible combination of named parameters. These methods all have dynamic as their return type, and their formal parameters all have type dynamic. Type dynamic has properties for every possible identifier. These properties all have type dynamic.
That means you will not get warnings by calling any method on a dynamic
typed variable. That will not be the case with a variable typed as Object
. For instance:
dynamic a;
Object b;
main() {
a = "";
b = "";
printLengths();
}
printLengths() {
// no warning
print(a.length);
// warning:
// The getter 'length' is not defined for the class 'Object'
print(b.length);
}
At runtime, I think, you shouldn't see any difference.
Solution 3 - Object
To add to Alexandre's answer on the practical difference, there is also a semantic difference between the two, and using the right one will help to better communicate your intent to other programmers.
When you use Object
you are saying that you know the type that you are working with and it is Object
. For instance:
int getHashCode(Object obj) {
return obj.hashCode;
}
Since hashCode
is a property on Object
we use Object
as the parameter type to specify that the function can accept anything of type Object
.
On the other hand, using dynamic
means that the Dart system cannot properly express the type that you want to use:
void setEmail(dynamic email) {
if (email is Email) {
_email = email;
} else if (email is String) {
_email = new Email.fromString(email);
}
}
Since Dart doesn't currently support union types there is no way to express the type Email | String
so we are forced to use dynamic
to accept all types and then only handle the cases where the type is one we are interested in.
Solution 4 - Object
dynamic
is not a type, it just disables type checking. Object
is the 'union' of all non-nullable types, type checking rules still apply.
Compare these two cases:
Case 1 (dynamic)
// a 'dynamic' variable can be assigned value of any type
dynamic a = 2;
// assign 'dynamic' value to any variable and code checker will not complain
int b = a;
// even when there is a bug
String c = a;
Case 2 (Object)
// It is OK to assign a 'int' value to an 'Object' variable, because 'int' is a subtype of 'Object'
Object a = 2;
// will get type error: "A value of type 'Object' can't be assigned to a variable of type 'int'"
int b = a;
// typecast is required when assign a 'Object' value to a variale of one of its subtypes.
int c = a as int;
Solution 5 - Object
You cannot access the methods and properties of a class by using Object, but with dynamic you can access.
example:
class MyClass{
myFunction() => print("This works");
}
void main(){
dynamic a = new MyClass();
Object b = new MyClass();
a.myFunction(); // prints without error
b.myFunction(); // error comes saying myFunction isn't defined for b
}
Solution 6 - Object
In addition to the answer given by lrn, it is also worth noting, that for Dart Version 2.12 and higher with Null safety enabled, a variable declared as dynamic can be null, while a variable of type Object cannot be null.
Solution 7 - Object
also, I've noted that extension methods does not work correctly with dynamic but worked ok with Object.
// I used to have the extension on dynamic and had
// problems that didn't occur when using the same extension on Object
extension UtilExtensions on Object {
bool get isStringNotEmpty => this is String && (this as String).isNotEmpty;
String get asStringNotEmpty => isStringNotEmpty ? this as String : null;
bool get isIntNotZero => this is int && (this as int) != 0;
int get asIntNotZero => isIntNotZero ? this as int : null;
Map<String, Object> get asPair {
if (this != null && this is Map) {
return (this as Map).cast<String, Object>();
}
return null;
}
Map<String, Object> get asFullPair {
if (this != null && this is Map) {
var ret = (this as Map).cast<String, Object>();
for (var key in ret.keys) {
var val = ret[key];
if (val is Map) {
ret[key] = val.asFullPair;
}
}
return ret;
}
return null;
}
}