What are the writing specifications for function lambda expressions in C++?

C The standard syntax for writing function Lambda expressions is: [capture](parameters) -> return_type { // Function body}, where capture is the capture of external variables, parameters is the function parameters, and return_type is the function return value type. Its types are divided according to the capture list and parameter list: capture all external variables, no parameters: auto type; capture specific external variables, no parameters: auto type (can be reduced); no capture, parameters: function type. Lambda expressions are used to create temporary function objects, which can be assigned to variables or function pointers, or passed directly as arguments.

C function Lambda expression writing specification

Syntax:

[capture](parameters) -> return_type {
  // 函数体
}

Among them:

• ##capture: Capture external variables, optional, default is [&] (capture all external variables)
• parameters: function parameters, optional
• return_type: function return value type, optional

Lambda expression type:

Lambda expressions belong to the anonymous function type. Its type depends on the capture list and parameter list:

Capture all external variables, no parameters:
• auto Type
Capture specific external variables, no parameters:
• auto Type (can be reduced)
No capture, with parameters:
• function Type

Usage specifications:

Lambda expression is used to create a temporary function object.
It can be assigned to a variable or function pointer, or passed directly as a parameter.
Captured external variables can be accessed in a Lambda expression, but they cannot be modified (unless captured by reference).
The
• this pointer in Lambda expressions points to the context in which they were created.

Actual case:

// 捕获所有外部变量，无参数
auto lambda1 = []() {
  // 可以访问外部变量
  std::cout << "Lambda 1: " << x << std::endl;
};

// 捕获特定外部变量，无参数
int x = 10;
auto lambda2 = [x]() {
  // 只可以访问捕获的外部变量 x
  std::cout << "Lambda 2: " << x << std::endl;
};

// 无捕获，有参数
auto lambda3 = [](int y) {
  // 没有捕获外部变量，y 为函数参数
  std::cout << "Lambda 3: " << y << std::endl;
};

int main() {
  lambda1();
  lambda2();
  lambda3(20);

  return 0;
}

Output:

Lambda 1: 10
Lambda 2: 10
Lambda 3: 20

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