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