Ch3.1 Lambda Expression

extra reference

在C++11標準中，新增 Lambda Expression 這個功能。Lambda Expression 做的事情類似於一個匿名的函示(Anonymous function)，即不直接以 Function name 去 call 它，而是以一個類似於 inline 的形式去 implement。

Basic Syntax

Lambda Expression 的 basic systax 如下

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[capture list] (parameter list) mutable(optional) exception attribute -> return type {
// function body
}

  • [capture list]: 可以去抓取這個scope中的 varialbe的數值，並決定將會以何種形式(call by value or call by reference)傳送
    例如，若我們後續要 capture 兩個 variable 進入這個 Lambda Expression，分別是 int x and int y

    • [] (跟傳送兩個參數的範例無關): 這個 Lambda Expression不會capture 任何 variable
    • [=]: All the variables are captured by value.
    • [&]: All the variables are captured by reference.
    • [x, &y]: Variable x are captured by value, while variable y are captured by reference.
    • [&x, y]: Variable x are captured by reference, while variable y are captured by value.
    • [=, &y]: Variable y are captured by reference, while the other variables are captured by value
    • [&, =x]: Variable x are captured by value, while the other variables are captured by reference.

  • (Parameter list): 定義這個 Lambda Expression 的參數，跟一般的 Function declaration 中的 Parameter list 雷同

    • Lambda Expression 不強制要傳入參數，如果不傳入參數的話， Parameter list 包括小括號都可以省略

  • mutable: 讓 Lambda Expression 可以直接修改外部利用 capture by value 進來的 variable

  • exception attribute: exception specification，throw。

    • 若這個 Lambda Expression 沒有用到例外功能，可以直接整個省略
    • More information about exception: w3school

  • return type: Return type of Lambda Expression.

Example

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void lambda_value_capture() {
    int value = 1;
    auto copy_value = [value] {    //capture value
        return value;
    };        //declaration of lambda expression
    value = 100;
    auto stored_value = copy_value();
    std::cout << "stored_value = " << stored_value << std::endl;
    // At this moment, stored_value == 1, and value == 100.
    // Because copy_value has copied when its was created.
}
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#include <iostream>
using namespace std;
int main() {
  auto lambda = []() { cout << "Hello, Lambda" << endl; };
  lambda();
}
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#include <iostream>
using namespace std;
int main() {
    auto sum = [](int x, int y) -> int { return x+y;};
    cout<<sum(3,5)<<endl;
}