How to pass parameters in C++ Lambda expression?

Passing parameters through the capture list, lambda expressions can access external variables. Taking int type parameters as an example, the capture list is [x](int y), where x is an external variable and y is a lambda expression parameter. Using this technique, lambda expressions can be used in various scenarios, such as array summation, where the std::accumulate function accumulates array elements into a variable with an initial value of 0, which is accumulated one by one through the lambda expression.

C++ Lambda expression: Detailed explanation of passing parameters

Lambda expression is a simplified anonymous function that is widely used in C++ to concisely and efficiently Define inline functions. In order for a lambda expression to access external variables, a capture list is used. The same goes for passing parameters.

Capture list passing parameters

The capture list precedes the lambda expression parameter list and is enclosed by [ and ]. It can capture local variables and external variables. To pass parameters, you need to specify the parameter type and name in the capture list.

// 捕获列表传递 int 参数
auto lambda = [x](int y) { return x + y; };

The above example defines a lambda expression lambda, which receives a int type parameter named y and can be accessed External local variable x.

Practical case: summing using lambda expression

Consider a scenario where all elements in an array need to be summed. This can be simplified using a lambda expression with a capture list passed as argument:

#include <vector>
#include <algorithm>

int main() {
  std::vector<int> numbers = {1, 2, 3, 4, 5};

  int sum = std::accumulate(numbers.begin(), numbers.end(), 0,
                            [](int x, int y) { return x + y; });

  std::cout << "Sum: " << sum << std::endl;
  return 0;
}

In this example, the std::accumulate function uses the provided lambda expression to ## The elements in #numbers are accumulated one by one into sum with an initial value of 0. A lambda expression takes two integer arguments x and y and returns their sum.

Notes

• Capture by value and by reference: Lambda expressions can capture variables by value or by reference. Use = to represent by value and & to represent by reference.
• Note variable lifetime: Lambda expressions can only access local variables that exist within their lifetime.
• Avoid capturing large objects: Capturing large objects may cause performance issues because they require additional memory.

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