c++ 11和java 8都支持lambda表达式

c++ 11也都支持lambda表达式了，看了这确实是有必要了。lambda主要用于一次性的回调代码段。

c++ lambda的语法格式：

[ capture clause ] (parameters) -> return-type  
{   
   definition of method   
} 

[捕获列表] (函数参数) mutable或exception声明 ->返回类型 {函数体};

捕获列表是一个lambda所在函数中定义的局部变量的列表，可以为空：

     [&] : 按照引用捕获所有外部变量 
      [=] : 按照值捕获所有外部变量 
      [a, &b] : a根据值捕获，b根据引用捕获

一般来说，编译器能够推断lambda表达式的返回值类型，所以-> return-type基本上都会被忽略。当然，如果表达式比较复杂（如带了各种if else），也可以明确的告诉编译器lambda表达式的返回值类型。

// C++ program to demonstrate lambda expression in C++
#include <bits/stdc++.h>   // 万能头文件，非标准c++或POSIX的一部分
using namespace std;
 
// Function to print vector
void printVector(vector<int> v)
{
    // lambda expression to print vector
    for_each(v.begin(), v.end(), [](int i)
    {
        std::cout << i << " ";
    });
    cout << endl;
}
 
int main()
{
    vector<int> v {4, 1, 3, 5, 2, 3, 1, 7};
 
    printVector(v);
 
    // below snippet find first number greater than 4
    // find_if searches for an element for which
    // function(third argument) returns true
    vector<int>:: iterator p = find_if(v.begin(), v.end(), [](int i)
    {
        return i > 4;
    });
    cout << "First number greater than 4 is : " << *p << endl;
 
 
    // function to sort vector, lambda expression is for sorting in
    // non-increasing order Compiler can make out return type as
    // bool, but shown here just for explanation
    sort(v.begin(), v.end(), [](const int& a, const int& b) -> bool
    {
        return a > b;
    });
 
    printVector(v);
 
    // function to count numbers greater than or equal to 5
    int count_5 = count_if(v.begin(), v.end(), [](int a)
    {
        return (a >= 5);
    });
    cout << "The number of elements greater than or equal to 5 is : "
         << count_5 << endl;
 
    // function for removing duplicate element (after sorting all
    // duplicate comes together)
    p = unique(v.begin(), v.end(), [](int a, int b)
    {
        return a == b;
    });
 
    // resizing vector to make size equal to total different number
    v.resize(distance(v.begin(), p));
    printVector(v);
 
    // accumulate function accumulate the container on the basis of
    // function provided as third argument
    int arr[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
    int f = accumulate(arr, arr + 10, 1, [](int i, int j)
    {
        return i * j;
    });
 
    cout << "Factorial of 10 is : " << f << endl;
 
    //     We can also access function by storing this into variable
    auto square = [](int i)
    {
        return i * i;
    };
 
    cout << "Square of 5 is : " << square(5) << endl;
}

[lightdb@lightdb-dev stdc]$ ./test_lambda
4 1 3 5 2 3 1 7
First number greater than 4 is : 5
7 5 4 3 3 2 1 1
The number of elements greater than or equal to 5 is : 2
7 5 4 3 2 1
Factorial of 10 is : 3628800
Square of 5 is : 25

不同捕获方式的示例如下。

// C++ program to demonstrate lambda expression in C++
#include <bits/stdc++.h>
using namespace std;

int main()
{
    vector<int> v1 = {3, 1, 7, 9};
    vector<int> v2 = {10, 2, 7, 16, 9};

    //  access v1 and v2 by reference
    auto pushinto = [&](int m)
    {
        v1.push_back(m);
        v2.push_back(m);
    };

    // it pushes 20 in both v1 and v2
    pushinto(20);

    // access v1 by copy
    [v1]()
    {
        for (auto p = v1.begin(); p != v1.end(); p++)
        {
            cout << *p << " ";
        }
    };

    int N = 5;

    // below snippet find first number greater than N
    // [N]  denotes,   can access only N by value
    vector<int>::iterator p = find_if(v1.begin(), v1.end(), [N](int i)
                                      { return i > N; });

    cout << "First number greater than 5 is : " << *p << endl;

    // function to count numbers greater than or equal to N
    // [=] denotes,   can access all variable
    int count_N = count_if(v1.begin(), v1.end(), [=](int a)
                           { return (a >= N); });

    cout << "The number of elements greater than or equal to 5 is : "
         << count_N << endl;
}

具体可见http://www.cprogramming.com/c++11/c++11-lambda-closures.html