Advanced Programming Techniques in D/

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Delegates

Delegates

Learn about the delegates in this lesson.

We'll cover the following...

Delegates: Definition

A delegate is a combination of a function pointer and the context that it should be executed in. Delegates also support closures in D. Closures are a feature supported by many functional programming languages.

As we discussed in the lifetimes and fundamental operations chapter, the lifetime of a variable ends upon leaving the scope that it is defined in:

{
    int increment = 10; 
    // ...
} // ← the life of 'increment' ends here

That is why the address of such a local variable cannot be returned from a function.

Let’s imagine that increment is a local variable of a function that itself returns a function. Let’s make it so that the returned lambda happens to use that local variable:

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alias Calculator = int function(int);



Calculator makeCalculator() { 

    int increment = 10;

    return value => increment + value; // ← compilation ERROR 

}



void main() {

    makeCalculator();

}

That code is in error because the returned lambda makes use of a local variable that is about to leave the ...