Lambda Expressions in C++
About Lambdas
Function Objects vs. Lambdas
When you write code, you probably use function pointers and function objects to solve problems and perform calculations, especially when you use STL algorithms. Function pointers and function objects have advantages and disadvantages—for example, function pointers have minimal syntactic overhead but do not retain state within a scope, and function objects can maintain state but require the syntactic overhead of a class definition.
A lambda combines the benefits of function pointers and function objects and avoids their disadvantages. Like a function objects, a lambda is flexible and can maintain state, but unlike a function object, its compact syntax doesn't require a class definition. By using lambdas, you can write code that's less cumbersome and less prone to errors than the code for an equivalent function object.
The following examples compare the use of a lambda to the use of a function object. The first example uses a lambda to print to the console whether each element in a vector object is even or odd. The second example uses a function object to accomplish the same task.
当你写代码时，你可能会用到函数指针，或函数对象来解决问题或者进行演算，尤其当你用STL algorithms时。函数指针和函数对象（或许可理解为仿函数？）有优缺点——例如，函数指针有最小的句法开销（？）但不能在作用域中保持状态（？），而函数对象能够保持状态却需要一个类的定义（相较而言较大的句法开销）
Example 1: Using a Lambda
// even_lambda.cpp
// compile with: cl /EHsc /nologo /W4 /MTd
#include <algorithm>
#include <iostream>
#include <vector>
using namespace std; int main()
{
// Create a vector object that contains 10 elements.
vector<int> v;
for (int i = 0; i < 10; ++i) {
v.push_back(i);
} // Count the number of even numbers in the vector by
// using the for_each function and a lambda.
int evenCount = 0;
for_each(v.begin(), v.end(), [&evenCount] (int n) {
cout << n; if (n % 2 == 0) {
cout << " is even " << endl;
++evenCount;
} else {
cout << " is odd " << endl;
}
}); // Print the count of even numbers to the console.
cout << "There are " << evenCount
<< " even numbers in the vector." << endl;
}
Output
0 is even
1 is odd
2 is even
3 is odd
4 is even
5 is odd
6 is even
7 is odd
8 is even
9 is odd
There are 5 even numbers in the vector.
Comments
Example 2: Using a Function Object
Sometimes a lambda would be too unwieldy to extend much further than the previous example. The next example uses a function object instead of a lambda, together with the for_each function, to produce the same results as Example 1. Both examples store the count of even numbers in a vector object. To maintain the state of the operation, the FunctorClass class stores the m_evenCount variable by reference as a member variable. To perform the operation, FunctorClass implements the function-call operator, operator(). The Visual C++ compiler generates code that is comparable in size and performance to the lambda code in Example 1. For a basic problem like the one in this article, the simpler lambda design is probably better than the function-object design. However, if you think that the functionality might require significant expansion in the future, then use a function object design so that code maintenance will be easier.
For more information about the operator(), see Function Call (C++). For more information about the for_eachfunction, see for_each.
有时候lambda表达式过于繁杂（扩充功能）。下一个例子采用了函数对象，同样使用for_each。都是数vector元素中偶数个数，为保持操作状态，FunctorClass类保存了m_evenCount变量作为一个成员变量的引用。为实现功能，FunctorClass重载（implement？）了function-call操作符，operator()。VC++编译器产生了与之相当大小和表现得lambda表达式代码（例一）。在这个基本问题中，简易的lambda表达式设计可能比函数对象更好。然而，若你认为functionality（函数性？）可能需要一个明确的扩展，那么就用函数对象写代码，那会简单些。
// even_functor.cpp
// compile with: /EHsc
#include <algorithm>
#include <iostream>
#include <vector>
using namespace std; class FunctorClass
{
public:
// The required constructor for this example.
explicit FunctorClass(int& evenCount)
: m_evenCount(evenCount)
{
} // The function-call operator prints whether the number is
// even or odd. If the number is even, this method updates
// the counter.
void operator()(int n) const
{
cout << n; if (n % 2 == 0) {
cout << " is even " << endl;
++m_evenCount;
} else {
cout << " is odd " << endl;
}
} private:
// Default assignment operator to silence warning C4512.
FunctorClass& operator=(const FunctorClass&); int& m_evenCount; // the number of even variables in the vector.
}; int main()
{
// Create a vector object that contains 10 elements.
vector<int> v;
for (int i = 0; i < 10; ++i) {
v.push_back(i);
} // Count the number of even numbers in the vector by
// using the for_each function and a function object.
int evenCount = 0;
for_each(v.begin(), v.end(), FunctorClass(evenCount)); // Print the count of even numbers to the console.
cout << "There are " << evenCount
<< " even numbers in the vector." << endl;
}