How to implement closure in C++ Lambda expression?

C++ Lambda expressions support closures, which save function scope variables and make them accessible to functions. The syntax is [capture-list] (parameters) -> return-type { function-body }. capture-list defines the variables to capture, you can use [=] to capture all local variables by value, [&] to capture all local variables by reference, or [variable1, variable2, ...] to capture specific variables. Lambda expressions can only access captured variables but cannot modify the original value.

Closures in C++ Lambda expressions

A closure is a set of related variables that are stored within the function scope In addition, functions can access these variables at the same time. In C++, closures are implemented through lambda expressions to capture variables in the execution context of a function.

Syntax

The general syntax of C++ Lambda expression is as follows:

[capture-list] (parameters) -> return-type { function-body }

wherecapture-list defines the variables to be captured , can be used in the following ways:

• [=]: Capture all local variables by value
• [&]: Capture all local variables Variables by reference
• [variable1, variable2, ...]: Capture specific variables by value or reference

Practical case

Let us consider a Lambda expression that needs to access its outer function scope variable. The following code snippet demonstrates this functionality:

#include <iostream>

int main() {
  int value = 10;
  auto lambda = [value] () {
    std::cout << value << std::endl;
  };

  lambda();  // 输出 10

  return 0;
}

In this example, the Lambda expression captures the value variable by value and can be accessed within its function body.

Note

• Captured variables must be accessible when the Lambda expression is created.
• Lambda expressions can only access captured variables and cannot modify their original values.
• C++14 and later supports capturing dynamic memory in Lambda expressions.
• Use the mutable modifier to modify the captured variable by value inside a Lambda expression.

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