What is the meaning of closure in C++ lambda expression?

In C, a closure is a lambda expression that can access external variables. To create a closure, capture the outer variable in the lambda expression. Closures provide advantages such as reusability, information hiding, and delayed evaluation. They are useful in real-world situations such as event handlers, where the closure can still access the outer variables even if they are destroyed.

C Closures in Lambda expressions

In C, a closure refers to one that has access to its declaration scope. lambda expression for external variables. In other words, a closure is a function that contains a snapshot of a specific variable.

How to create a closure

To create a closure, use a lambda expression and capture the surrounding variable to be accessed.

int outerVar = 10;

auto lambda = [outerVar](int x) { return x + outerVar; };

In the above example, the lambda expression captures the variable outerVar. This means that even if lambda leaves its declaration scope, it can still access the value of outerVar.

Advantages of closures

Closures provide the following advantages:

• Reusability: Yes Use closures in different contexts, even if the surrounding variables accessed change.
• Information hiding: Closures can hide the external variables they access, thereby improving encapsulation.
• Delayed evaluation: A closure can defer the evaluation of its parameters until a function is called.

Practical case: event handler

In GUI programming, closures are often used to implement event handlers. For example, the following lambda expression creates a click event handler that prints the captured button ID:

QPushButton* button = new QPushButton("Button");

button->clicked.connect([id = button->objectName()](bool) {
    qDebug() << "Clicked button: " << id;
});

Even after button is destroyed, the closure can still access it ID because captured variables are always stored in closures.

Conclusion

Closures in lambda expressions are powerful tools in C that allow a function to access variables outside its declaration scope. They provide advantages such as reusability, information hiding, and lazy evaluation. By understanding the concept of closures, developers can create C code that is more flexible, powerful, and maintainable.

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