Dart Programming - Typedef

A typedef, or a function-type apas, helps to define pointers to executable code within memory. Simply put, a typedef can be used as a pointer that references a function.

Given below are the steps to implement typedefs in a Dart program.

Step 1: Defining a typedef

A typedef can be used to specify a function signature that we want specific functions to match. A function signature is defined by a function’s parameters (including their types). The return type is not a part of the function signature. Its syntax is as follows.

typedef function_name(parameters)

Step 2: Assigning a Function to a typedef Variable

A variable of typedef can point to any function having the same signature as typedef. You can use the following signature to assign a function to a typedef variable.

type_def  var_name = function_name

Step 3: Invoking a Function

The typedef variable can be used to invoke functions. Here is how you can invoke a function −

var_name(parameters) 

Example

Let’s now take an example to understand more on typedef in Dart.

At first, let us define a typedef. Here we are defining a function signature. The function will take two input parameters of the type integer. Return type is not a part of the function signature.

typedef ManyOperation(int firstNo , int secondNo); //function signature

Next, let us define the functions. Define some functions with the same function signature as that of the ManyOperation typedef.

Add(int firstNo,int second){ 
   print("Add result is ${firstNo+second}"); 
}  
Subtract(int firstNo,int second){ 
   print("Subtract result is ${firstNo-second}"); 
}  
Divide(int firstNo,int second){ 
   print("Add result is ${firstNo/second}"); 
}

Finally, we will invoke the function via typedef. Declare a variable of the ManyOperations type. Assign the function name to the declared variable.

ManyOperation oper ;  

//can point to any method of same signature 
oper = Add; 
oper(10,20); 
oper = Subtract; 
oper(30,20); 
oper = Divide; 
oper(50,5); 

The oper variable can point to any method which takes two integer parameters. The Add function s reference is assigned to the variable. Typedefs can switch function references at runtime

Let us now put all the parts together and see the complete program.

typedef ManyOperation(int firstNo , int secondNo); 
//function signature  

Add(int firstNo,int second){ 
   print("Add result is ${firstNo+second}"); 
} 
Subtract(int firstNo,int second){ 
   print("Subtract result is ${firstNo-second}"); 
}
Divide(int firstNo,int second){ 
   print("Divide result is ${firstNo/second}"); 
}  
Calculator(int a, int b, ManyOperation oper){ 
   print("Inside calculator"); 
   oper(a,b); 
}  
void main(){ 
   ManyOperation oper = Add; 
   oper(10,20); 
   oper = Subtract; 
   oper(30,20); 
   oper = Divide; 
   oper(50,5); 
} 

Add result is 30 
Subtract result is 10 
Divide result is 10.0 

typedef ManyOperation(int firstNo , int secondNo);   //function signature 
Add(int firstNo,int second){ 
   print("Add result is ${firstNo+second}"); 
}  
Subtract(int firstNo,int second){
   print("Subtract result is ${firstNo-second}"); 
}  
Divide(int firstNo,int second){ 
   print("Divide result is ${firstNo/second}"); 
}  
Calculator(int a,int b ,ManyOperation oper){ 
   print("Inside calculator"); 
   oper(a,b); 
}  
main(){ 
   Calculator(5,5,Add); 
   Calculator(5,5,Subtract); 
   Calculator(5,5,Divide); 
} 

Inside calculator 
Add result is 10 
Inside calculator 
Subtract result is 0 
Inside calculator 
Divide result is 1.0