typedef

Learn function types with typedef.

typedef for function types

A typedef gives a name to a function type. Instead of writing a long function type every time, you use a short name. For example typedef IntOp = int Function(int, int); creates a new type called IntOp.

You can use this type for a variable or a parameter: IntOp op = (a, b) => a + b;. You declare the signature once and reuse the same name everywhere.

A typedef only improves readability; it does not create a new runtime type. When the compiler sees IntOp it really means int Function(int, int).

Using a typedef

After you declare a typedef you can use it as a parameter type. For example void apply(IntOp op, int x, int y) => print(op(x, y)); accepts op as an IntOp.

So apply((a, b) => a * b, 3, 4) runs multiplication and prints 12. A typedef keeps signatures short in APIs that pass functions as parameters.

You can reuse the same typedef in many places; one change propagates everywhere.

Exercise The MathOp typedef

Define typedef MathOp = int Function(int, int);. Write int add(int a, int b) => a + b;. In main assign add to a MathOp variable and print the result for 3 and 4.

Show example solution

typedef MathOp = int Function(int, int);

int add(int a, int b) => a + b;

void main() {
  MathOp op = add;
  print(op(3, 4));
}

Test yourself

What does typedef do?
- Defines a new class
- Names a function type
- Creates a new variable
- Starts a loop
Show answer

Names a function type — typedef names a function type.

The typedef keyword

____ IntOp = int Function(int, int);

Show answer

typedef IntOp = int Function(int, int);

Match the typedef parts

typedef

Definition keyword
IntOp

Type name
int Function(int, int)

Function type
Function

Function keyword

Next lesson: Type alias

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