C++11中Lambda的使用

C++11中Lambda的使用

Lambda functions: Constructs a closure, an unnamed function object capable of capturing variables in scope.

In C++11, a lambda expression----often called a lambda----is a convenient way of defining an anonymous function object right at the location where it is invoked or passed as an argument to a function. Typically lambdas are used to encapsulate a few lines of code that are passed to algorithms or asynchronous methods.

A lambda function is a function that you can write inline in your source code (usually to pass in to another function, similar to the idea of a functor or function pointer). With lambda, creating quick functions has become much easier, and this means that not only can you start using lambda when you'd previously have needed to write a separate named function, but you can start writing more code that relies on the ability to create quick-and-easy functions.

Lambda表达式语法:[capture ] ( params ) mutable exception attribute -> return-type { body }

其中capture为定义外部变量是否可见(捕获),若为空,则表示不捕获所有外部变量,即所有外部变量均不可访问,= 表示所有外部变量均以值的形式捕获,在body中访问外部变量时,访问的是外部变量的一个副本,类似函数的值传递,因此在body中对外部变量的修改均不影响外部变量原来的值。& 表示以引用的形式捕获,后面加上需要捕获的变量名,没有变量名,则表示以引用形式捕获所有变量,类似函数的引用传递,body操作的是外部变量的引用,因此body中修改外部变量的值会影响原来的值。params就是函数的形参,和普通函数类似,不过若没有形参,这个部分可以省略。mutalbe表示运行body修改通过拷贝捕获的参数,exception声明可能抛出的异常,attribute修饰符,return-type表示返回类型,如果能够根据返回语句自动推导,则可以省略,body即函数体。除了capture和body是必需的,其他均可以省略。

Lambda表达式是用于创建匿名函数的。Lambda 表达式使用一对方括号作为开始的标识,类似于声明一个函数,只不过这个函数没有名字,也就是一个匿名函数。Lambda 表达式的返回值类型是语言自动推断的。如果不想让Lambda表达式自动推断类型,或者是Lambda表达式的内容很复杂,不能自动推断,则这时必须显示指定Lambda表达式返回值类型,通过”->”。

引入Lambda表达式的前导符是一对方括号,称为Lambda引入符(lambda-introducer):

(1)、[] // 不捕获任何外部变量

(2)、[=] //以值的形式捕获所有外部变量

(3)、[&] //以引用形式捕获所有外部变量

(4)、[x, &y] // x 以传值形式捕获,y 以引用形式捕获

(5)、[=, &z] // z 以引用形式捕获,其余变量以传值形式捕获

(6)、[&,x] // x 以值的形式捕获,其余变量以引用形式捕获

(7)、对于[=]或[&]的形式,lambda 表达式可以直接使用 this 指针。但是,对于[]的形式,如果要使用 this 指针,必须显式传入,如:this { this->someFunc(); }();

下面是从其他文章中copy的测试代码,详细内容介绍可以参考对应的reference:

#include "Lambda.hpp"
#include <iostream>
#include <algorithm>
#include <functional>
#include <vector>
#include <string>

// Blog: http://blog.csdn.net/fengbingchun/article/details/52653313

///////////////////////////////////////////////////
// reference: http://en.cppreference.com/w/cpp/language/lambda
int test_lambda1()
{
    /*
    []      Capture nothing (or, a scorched earth strategy?)
    [&]     Capture any referenced variable by reference
    [=]     Capture any referenced variable by making a copy
    [=, &foo]   Capture any referenced variable by making a copy, but capture variable foo by reference
    [bar]       Capture bar by making a copy; don't copy anything else
    [this]      Capture the this pointer of the enclosing class
    */
    int a = 1, b = 1, c = 1;

    auto m1 = [a, &b, &c]() mutable {
        auto m2 = [a, b, &c]() mutable {
            std::cout << a << b << c << '\n';
            a = 4; b = 4; c = 4;
        };
        a = 3; b = 3; c = 3;
        m2();
    };

    a = 2; b = 2; c = 2;

    m1();                             // calls m2() and prints 123
    std::cout << a << b << c << '\n'; // prints 234

    return 0;
}

///////////////////////////////////////////////////
// reference: http://en.cppreference.com/w/cpp/language/lambda
int test_lambda2()
{
    std::vector<int> c = { 1, 2, 3, 4, 5, 6, 7 };
    int x = 5;
    c.erase(std::remove_if(c.begin(), c.end(), [x](int n) { return n < x; }), c.end());

    std::cout << "c: ";
    std::for_each(c.begin(), c.end(), [](int i){ std::cout << i << ' '; });
    std::cout << '\n';

    // the type of a closure cannot be named, but can be inferred with auto
    auto func1 = [](int i) { return i + 4; };
    std::cout << "func1: " << func1(6) << '\n';

    // like all callable objects, closures can be captured in std::function
    // (this may incur unnecessary overhead)
    std::function<int(int)> func2 = [](int i) { return i + 4; };
    std::cout << "func2: " << func2(6) << '\n';

    return 0;
}

///////////////////////////////////////////////////////
// reference: https://msdn.microsoft.com/zh-cn/library/dd293608.aspx
int test_lambda3()
{
    // The following example contains a lambda expression that explicitly captures the variable n by value
    // and implicitly captures the variable m by reference:
    int m = 0;
    int n = 0;
    [&, n](int a) mutable { m = ++n + a; }(4);

    // Because the variable n is captured by value, its value remains 0 after the call to the lambda expression.
    // The mutable specification allows n to be modified within the lambda.
    std::cout << m << std::endl << n << std::endl;

    return 0;
}

//////////////////////////////////////////////
// reference: https://msdn.microsoft.com/zh-cn/library/dd293608.aspx
template <typename C>
void print(const std::string& s, const C& c)
{
    std::cout << s;

    for (const auto& e : c) {
        std::cout << e << " ";
    }

    std::cout << std::endl;
}

void fillVector(std::vector<int>& v)
{
    // A local static variable.
    static int nextValue = 1;

    // The lambda expression that appears in the following call to
    // the generate function modifies and uses the local static 
    // variable nextValue.
    generate(v.begin(), v.end(), [] { return nextValue++; });
    //WARNING: this is not thread-safe and is shown for illustration only
}

int test_lambda4()
{
    // The number of elements in the vector.
    const int elementCount = 9;

    // Create a vector object with each element set to 1.
    std::vector<int> v(elementCount, 1);

    // These variables hold the previous two elements of the vector.
    int x = 1;
    int y = 1;

    // Sets each element in the vector to the sum of the 
    // previous two elements.
    generate_n(v.begin() + 2,
        elementCount - 2,
        [=]() mutable throw() -> int { // lambda is the 3rd parameter
        // Generate current value.
        int n = x + y;
        // Update previous two values.
        x = y;
        y = n;
        return n;
    });
    print("vector v after call to generate_n() with lambda: ", v);

    // Print the local variables x and y.
    // The values of x and y hold their initial values because 
    // they are captured by value.
    std::cout << "x: " << x << " y: " << y << std::endl;

    // Fill the vector with a sequence of numbers
    fillVector(v);
    print("vector v after 1st call to fillVector(): ", v);
    // Fill the vector with the next sequence of numbers
    fillVector(v);
    print("vector v after 2nd call to fillVector(): ", v);

    return 0;
}

/////////////////////////////////////////////////
// reference: http://blogorama.nerdworks.in/somenotesonc11lambdafunctions/

template<typename T>
std::function<T()> makeAccumulator(T& val, T by) {
    return [=, &val]() {
        return (val += by);
    };
}

int test_lambda5()
{
    int val = 10;
    auto add5 = makeAccumulator(val, 5);
    std::cout << add5() << std::endl;
    std::cout << add5() << std::endl;
    std::cout << add5() << std::endl;
    std::cout << std::endl;

    val = 100;
    auto add10 = makeAccumulator(val, 10);
    std::cout << add10() << std::endl;
    std::cout << add10() << std::endl;
    std::cout << add10() << std::endl;

    return 0;
}

////////////////////////////////////////////////////////
// reference: http://blogorama.nerdworks.in/somenotesonc11lambdafunctions/
class Foo_lambda {
public:
    Foo_lambda() {
        std::cout << "Foo_lambda::Foo_lambda()" << std::endl;
    }

    Foo_lambda(const Foo_lambda& f) {
        std::cout << "Foo_lambda::Foo_lambda(const Foo_lambda&)" << std::endl;
    }

    ~Foo_lambda() {
        std::cout << "Foo_lambda~Foo_lambda()" << std::endl;
    }
};

int test_lambda6()
{
    Foo_lambda f;
    auto fn = [f]() { std::cout << "lambda" << std::endl; };
    std::cout << "Quitting." << std::endl;
    return 0;
}

template<typename Cal>
static void display(Cal cal)
{
    fprintf(stderr, "start\n");
    cal();
}

int test_lambda7()
{
    int num { 1 };
    // create callback
    auto fun = [&](){
        if (num % 5 == 0) {
            fprintf(stderr, "****** reset ******\n");
            fprintf(stderr, "num = %d\n", num);

            num = 0;
        } else {
            fprintf(stderr, "++++++ continue ++++++\n");
            fprintf(stderr, "num = %d\n", num);
        }

        num++;
    };

    for (int i = 0; i < 20; i++) {
        fprintf(stderr, "========= i = %d\n", i);
        display(fun);
    }

    return 0;
}

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