As described in the documentations:
The const keyword isn’t just for declaring constant variables. You can also use it to create constant values, as well as to declare constructors that create constant values. Any variable can have a constant value.
Can someone explain the use of constant values?
This is a simple example which I hope explains clearly:
Start with two variables and a constant.
var foo = [10,11];
var bar = const [20,21];
const baz = [30,31];
Try to modify foo and it succeeds.
foo.add(12); // [10,11,12]
Try to similarly modify bar and there's an error, because even though bar is a variable, its value was declared to be a constant, thus is immutable.
bar.add(22); // ERROR!
Try to reassign bar to a different value. That works since bar itself was not declared as a constant.
bar = [40,41];
Now, try to modify bar's value again and this time it works, since its new value is not a constant.
bar.add(42) // [40,41,42]
Try to modify baz and there's an error, since baz being declared as a constant itself, its value is inherently immutable.
baz.add(32); // ERROR!
Try reassigning baz to a new value and it fails because baz is a constant and can't be reassigned to a new value.
baz = [50,51]; // ERROR!
void main() {
simpleUse();
finalUse();
constUse();
}
simpleUse() {
print("\nsimple declaration");
var x = [10];
print('before: $x');
x = [5];//changing reference allowed
x.add(10);//changing content allowed
print('after: $x');
}
finalUse() {
print("\nfinal declaration");
final x = [10];
print('before: $x');
// x = [10,20]; //nope changing reference is not allowed for final declaration
x.add(20); //changing content is allowed
print('after: $x');
}
constUse() {
print("\nconst declaration");
const x = [10];
print('before: $x');
// x = [10,20]; //nope -> changing reference is not allowed for final declaration
// x.add(20);//nope -> changing content is not allowed
print('after: $x');
}
Also, variables are simple values like x = 10;
values are instances of enums, list, maps, classes, etc.
I want to add, that another point for const is to guarantee your get the same instance of the object each time you construct it with the same parameters:
class Test {
final int value;
const Test(this.value);
}
void main() {
print(Test(5).hashCode == Test(5).hashCode); // false
print((const Test(5)).hashCode == (const Test(5)).hashCode); // true
}
This is the reason why the const constructor can be difficult to make for all objects since your need to make sure the object can be constructed on compile-time. Also, why after creation of the object no internal state can be changed as the previous answer also shows.
What is the difference between the const and final keywords in Dart?
There is a post on dart's website and it explains it pretty well.
Final:
"final" means single-assignment: a final variable or field must have an initializer. Once assigned a value, a final variable's value cannot be changed. final modifies variables.
Const:
"const" has a meaning that's a bit more complex and subtle in Dart. const modifies values. You can use it when creating collections, like const [1, 2, 3], and when constructing objects (instead of new) like const Point(2, 3). Here, const means that the object's entire deep state can be determined entirely at compile time and that the object will be frozen and completely immutable.
Const objects have a couple of interesting properties and restrictions:
They must be created from data that can be calculated at compile time. A const object does not have access to anything you would need to calculate at runtime. 1 + 2 is a valid const expression, but new DateTime.now() is not.
They are deeply, transitively immutable. If you have a final field containing a collection, that collection can still be mutable. If you have a const collection, everything in it must also be const, recursively.
They are canonicalized. This is sort of like string interning: for any given const value, a single const object will be created and re-used no matter how many times the const expression(s) are evaluated.
So, what does this mean?
Const:
If the value you have is computed at runtime (new DateTime.now(), for example), you can not use a const for it. However, if the value is known at compile time (const a = 1;), then you should use const over final. There are 2 other large differences between const and final. Firstly, if you're using const inside a class, you have to declare it as static const rather than just const. Secondly, if you have a const collection, everything inside of that is in const. If you have a final collection, everything inside of that is not final.
Final:
final should be used over const if you don't know the value at compile time, and it will be calculated/grabbed at runtime. If you want an HTTP response that can't be changed, if you want to get something from a database, or if you want to read from a local file, use final. Anything that isn't known at compile time should be final over const.
With all of that being said, both const and final cannot be reassigned, but fields in a final object, as long as they aren't const or final themselves, can be reassigned (unlike const).
Const
Value must be known at compile-time, const birthday = "2008/12/25" Can't be changed after initialized.
Final
Value must be known at run-time, final birthday = getBirthDateFromDB() Can't be changed after initialized.
Consolidated #Meyi and #faisal-naseer answers and Comparing with little programming.
const:
const keyword used to make a variable to store a compile time constant value. Compile time constant value is a value which will be constant while compiling :-)
For example 5 is a compile time constant. While DateTime.now() which is not compile time constant. Because this method will return the time when the line is getting executed at runtime. So we can't assign the DateTime.now() to a const variable.
const a = 5;
// Uncommenting below statement will cause compile time error.
// Because we can't able to assign a runtime value to a const variable
// const b = DateTime.now();
Should be initialized at the same line.
const a = 5;
// Uncommenting below 2 statement will cause compilation error.
// Because const variable must be initialized at the same line.
// const b;
// b = 6;
All statements mentioned below are acceptable.
// Without type or var
const a = 5;
// With a type
const int b = 5;
// With var
const var c = 6;
Class level const variable should be initialized like below.
Class A {
static const a = 5;
}
Instance level const variable is not possible.
Class A {
// Uncommenting below statement will give compilation error.
// Because const is not possible to be used with instance level
// variable.
// const a = 5;
}
The another major use of const is used to make the object immutable. To make a class object immutable we need to use the const keyword with constructor and make all the fields as final like mentioned below.
Class A {
final a, b;
const A(this.a, this.b);
}
void main () {
// There is no way to change a field of object once it's
// initialized.
const immutableObja = const A(5, 6);
// Uncommenting below statement will give compilation error.
// Because you are trying to reinitialize a const variable
// with other value
// immutableObja = const A(7, 9);
// But the below one is not the same. Because we are mentioning objA
// is a variable of a class A. Not const. So we can able to assign
// another object of class A to objA.
A objA = const A(8, 9);
// Below statement is acceptable.
objA = const A(10, 11);
}
we can use const keyword to a list.
const a = const [] - A variable a initialized as const which contains a list of const objects(i.e., The list should contain only compile time constant and immutable objects). So we can't able to assign a with another list.
var a = const [] - A variable a initialized as var which contains a list const objects. So we can able to assign another list to the variable a.
Class A {
final a, b;
const A(this.a, this.b);
}
class B {
B(){ // Doing something }
}
void main() {
const constantListOfInt = const [5, 6, 7,
// Uncommenting below statement give compilation error.
// Because we are trying to add a runtime value
// to a constant list
// DateTime.now().millisecondsSinceEpoch
];
const constantListOfConstantObjA = const [
A(5, 6),
A(55, 88),
A(100, 9),
];
// Uncommenting below 2 statements will give compilation error.
// Because we are trying to reinitialize with a new list.
// constantListOfInt = [8, 9, 10];
// constantListOfConstantObjA = const[A(55, 77)];
// But the following lines are little different. Because we are just
// trying to assign a list of constant values to a variable. Which
// is acceptable
var variableWithConstantList = const [5, 6, 7];
variableWithConstantList = const [10, 11, 15];
var variableOfConstantListOfObjA = const [A(5, 8), A(7, 9), A(10, 4)];
variableWithConstantList = const [A(9, 10)];
}
final:
final keyword also used to make the variable to hold a constant value. Once initialized we can't change the value.
final a = 5;
// Uncommenting below statement will give compilation error.
// Because a is declared as final.
// a = 6;
All statements mentioned below are acceptable.
// Without type or var
final a = 5;
// With a type
final int b = 5;
// Can't use var along with final keyword. Uncommenting below line cause compilation issue.
// final var c = 6;
Able to assign a runtime value.
// DateTime.now() will return the time when the line is getting
// executed. Which is a runtime value.
final a = DateTime.now();
var b = 5;
final c = b;
Class level final variable must be initialized in the same line.
Class A {
static final a = 5;
static final b = DateTime.now();
}
Instance level final variable must be initialized in the same line or in the constructor initialization. The value will be put into memory when the object is created.
Class A {
final a = 5;
}
// Constructor with a parameter.
Class B {
final b;
B(this.b);
}
// Constructor with multiple parameter.
Class C {
final c;
C(this.c, int d) {
// Do something with d
}
}
void main() {
A objA = new A();
B objB = new B(5);
C objC = new C(5, 6);
}
Assigning a list.
final a = [5, 6, 7, 5.6, A()];
// Uncommenting Below statement will give compilation error.
// Because we are trying to reinitialize the object with another list.
// a = [9.9, 10, B()];
Extending the answer by #Meyi
final variable can only be set once and it is initialized when
accessed.(for example from code section below if you use the value of biggestNumberOndice only then the value will be initialized and memory will be assigned).
const is internally final in nature but the main difference is that
its compile time constant which is initialized during compilation
even if you don't use its value it will get initialized and will take
space in memory.
Variable from classes can be final but not constant and if you want a
constant at class level make it static const.
Code:
void main() {
// final demonstration
final biggestNumberOndice = '6';
// biggestNumberOndice = '8'; // Throws an error for reinitialization
// const
const smallestNumberOnDice = 1;
}
class TestClass {
final biggestNumberOndice = '6';
//const smallestNumberOnDice = 1; //Throws an error
//Error . only static fields can be declared as constants.
static const smallestNumberOnDice = 1;
}
const means its initial value is must be fixed, can not be a dynamic value;
final means its initial value is must be fixed but can be a dynamic value, equal to the var with a fixed value.
code demos
const
void main() {
const sum = 1 + 2;
// ✅ const can not change its value
print("sum = ${sum}");
// ⚠️ Const variables must be initialized with a constant value.
const time = new DateTime.now();
// ❌ Error: New expression is not a constant expression.
print("time = ${time}");
}
final
// new DateTime.now();
// dynamic timestamp
void main() {
final sum = 1 + 2;
// ✅ final can not change its value
print("sum = ${sum}");
final time = new DateTime.now();
// ✅ final === var with fixed value
print("time = ${time}");
}
Screenshots
refs
https://dart.dev/guides/language/language-tour#final-and-const
final and const in dart are confusing to the level we think both of them are the same.
Let's see their differences :
P.S. I included image instead of text as I couldn't tabulate the info
in Stackoverflow .md format easily.
Both final and const prevent a variable from being reassigned (similar to how final works in Java or how const works in JavaScript).
The difference has to do with how memory is allocated. Memory is allocated for a final variable at runtime, and for a const variable at compile-time. The final modifier should be the more commonly used, because many program variables won't need any memory since the program logic won't call for them to be initialized. With a const variable you are basically telling the computer, "Hey, I need memory for this variable up front because I know I'm going to need it."
Thinking of them in this way makes it easier to understand differences in their syntactical usage. Mainly that a final variable may be an instance variable, but a const must be a static variable on a class. This is because instance variables are created at runtime, and const variables--by definition--are not. Thus, const variables on a class must be static, which means simply that a single copy of that variable exists on a class, regardless of whether that class is instantiated.
This video breaks it down fairly simply:
https://www.youtube.com/watch?v=9ZZL3iyf4Vk
This article goes into more depth and explains a very important semantic difference between the two, i.e. final modifies variables and const modifies values, which essentially boils down to only being able to initialize const values which are derivable at compile-time.
https://news.dartlang.org/2012/06/const-static-final-oh-my.html
Anything that isn't known at compile time should be final over const.
If you are coming from C++ then const in Dart is constexpr in C++ and final in Dart is const in C++.
The above applies to primitive types only.
However in Dart objects marked final are mutable in terms of it's members.
Simple words:
Const
Value must be known at compile-time, i.e. values coming from internal files.
Sample: API keys, supported languages of your app or any variables in your i.e. helper file, basically anything which you ship with your app.
Final
Value must be known at run-time.
It can be data like above but also i.e. device info which will be checked when the app is starting or data that is loaded from API's or servers when the app starts, but before the app is ready to use i.e. you need to check if the user is logged in or not, your app would load or check a session token from the server.
The answer is in the image itself.
const is a Compile-time constant.
final is a Run-time constant.
All these answers I can describe in a concise way.
const list = [1, 2, 3];
Both variable/identifier & value are const. Like - const list = const [1, 2, 3]
That's why they are not allowed to be re-assigned.
Good fit for global variables.
It is possible to use it as a class variable but have to set static. Like - static const list = [1, 2, 3].
vs:
final list = [1, 2, 3];
Variable/Identifier is const but value is not. Like - const list = [1, 2, 3]
That's why we can perform like - list.add(4)
You can't initialise a const using a final. For example :
final myConst = 1;
const myFinal = 2;
final a = myConst; // possible
final b = myFinal; // possible
const c = myConst; // this is not possible
const d = myFinal; // possible
When to use which keyword?
A simple example for both:
Use final: If you don’t know what it’s value will be at compile-time. For example, when you can need to get data from an API, this happens when running your code.
Use const: If you are sure that a value isn’t going to be changed when running your code. For example, when you declare a sentence that always remains the same.
https://itnext.io/difference-between-const-and-final-in-dart-78c129d0c573
A variable with the final keyword will be initialized at runtime and can only be assigned for a single time.
A variable with the const keyword is initialized at compile-time and is already assigned when at runtime.
Use final: If you don’t know what it’s value will be at compile-time. For example, when you can need to get data from an API, this happens when running your code.
Use const: If you are sure that a value isn’t going to be changed when running your code. For example, when you declare a sentence that always remains the same.
dynamic x = 2;
This doesn't compile. But:
final int n = 6; /* and */
final y = "Hello world!"
both compile.
Is it possible and how to declare variables explicitly as of type dynamic?
It is possible to declare variables explicitly as being type dynamic. The code
dynamic x = 2;
compiles and is equivalent to the code
var x = 2;
var is shorthand for dynamic when declaring variables. Omitting a type annotation is equivalent to making the type annotation dynamic.
The difference between var and dynamic is that var is for declaring variables and is not a type; it cannot be the return type of a function (since that is not declaring a variable) and function arguments can omit the keyword var (the declaration f(x){} is equivalent to the declarations f(dynamic x){} and f(var x){}).
You only need to explicitly use dynamic in type parameters for generic classes where at least one but not all type parameters are dynamic, such as Map<String, dynamic>.
var x = 2; defines a variable without explicit type, which is the same as dynamic.
I'm more of an Android developer, but i'm beginning to see the light at the end of the tunnel on iOS development.
There is, however, one coding pattern I can't seem to find an equivalent for.
The use of static fields as flags.
Android :
public final static int ERROR_EMPTY = 1;
public final static int ERROR_NO_CONNECTION = 2;
public final static int ERROR_WRONG_USER = 4;
...
if (error == MyClass.ERROR_EMPTY) {//do things}
What would be the proper way to achieve this on iOS ?
Thanks.
Using Objective-C and C
i often use prefixes:
typedef enum MyClass_Error {
// never use MyClass_Error_Undefined
// or you may favor MyClass_Error_None for a valid error code
MyClass_Error_Undefined = 0,
MyClass_Error_Empty = 1,
MyClass_Error_NoConnection = 2,
MyClass_Error_WrongUser = 4
// ...
} MyClass_Error;
for these value collections. then you get benefits such as typesafety and switch value checking.
for non-type constants:
enum { MyClass_ConstantName = 4 };
and feel free to hide these in the *.m when private.
also note that C enums may have gaps in their defined values (unlike Java's).
Update: there's an even better way to declare an enum, as demonstrated in Abizern's answer -- if you're sticking with the most recent toolchains. the big reason to use this extension is for binary compatibility and encoding (although i favor fixed-width types for these purposes).
There are a few other variations, for the cases when you want to use existing types:
Private Constant
MyClass.m
static const NSRange MyClass_InputRange = {1,1};
Public Constant
MyClass.h
extern const NSRange MyClass_InputRange;
MyClass.m
const NSRange MyClass_InputRange = {1,1};
Using C++
You would likely favor introducing a new scope for these values -- either in a class or a namespace, rather than simulating the scope using prefixes.
Common Mistakes
Use of #define for constants (unless definition is mandatory when preprocessing)
Use of short identifiers, and identifiers which are not prefixed
Use of static values in headers
Not using const when possible
Declaring them in the header, when they could be in the *.m source.
Just to add to Justin's excellent answer - the Modern Objective-C definition for the enum would be:
typedef enum MyClass_Error : NSUInteger {
// never use MyClass_Error_Undefined
// or you may favor MyClass_Error_None for a valid error code
MyClass_Error_Undefined = 0,
MyClass_Error_Empty = 1,
MyClass_Error_NoConnection = 2
// ...
} MyClass_Error;
I just want have a final List with colors :
final List<String> COLORS = ['#cefbe4', '#81ec72', '#5cd646'];
final num MAX = 90;
class Square {
// ...
It's ok for MAX but it does not compile for list :
Initializer must be a compile time constant
I also tried :
static final num MAX = 90;
class Square {
final List<String> COLORS = ['#cefbe4', '#81ec72', '#5cd646'];
New error : "initializer must be a compile time constant"
The only solution, I found is to initialise in constructor...
I don't like this solution : I don't want a list instance by Square object.
How can I do ?
I don't get the same errors as you. In the first case I get a compile time warning "Expected constant expression" and in the second case another compile-time warning and a runtime error. Try the latest SDK which you can find here.
static final num MAX = 90;
class Square {
final List<String> COLORS = ['#cefbe4', '#81ec72', '#5cd646'];
In this case static it doesn't make any sense because static denotes class variables which are the same for all instantiated objects of the same class. And in this case MAX is outside a class boundary. Hence a runtime error is shown and a compile time warning ("Top-level field cannot be static").
I guess both MAX and COLORS are supposed to be the same for all instances of the Square class. So it makes sense to do this:
class Square {
static final List<String> COLORS = const ['#cefbe4', '#81ec72', '#5cd646'];
static final num MAX = 90;
}
The list itself is the part that is final, not the contents. The contents inside a final list can still be added and removed.
final List<String> COLOURS = new ArrayList<String>();
This would create your list. The actual filling of the list would be done in the constructor, or for that matter anywhere in your code.
It would be more appropriate to use an enum for this in my opinion.