lambda

lambda basic syntax and an example.

[ captures ] (parameters) -> returnTypesDeclaration { ..; }

int sum = 0, divisor = 3;
vector<int> vc { 1, 2, 3, 4, 5, 10, 15, 20, 25, 35, 45 };
for_each(vc.begin(), vc.end(), [divisor, &sum] (int y){
      if (y % divisor == 0){
         cout << y << endl;
         sum += y;
      }
  });
  cout << sum << endl;

This auto-declaration defines a closure type named factorial that you can call later instead of typing the entire lambda expression (a closure type is in fact a compiler-generated function class):

auto factorial = [](int i, int j) {return i * j;};

int arr{1,2,3,4,5,6,7,8,9,10,11,12};
long res = std::accumulate(arr, arr+12, 1, factorial);
cout<<"12!="<<res<<endl; // 479001600

Under the final C++11 spec, if you have a lambda with an empty capture specification, then it can be treated like a regular function and assigned to a function pointer. Here’s an example of using a function pointer with a capture-less lambda:

typedef int (*func)();
func f = [] () -> int { return 2; };
f();

A delegate. When you call a normal function, all you need is the function itself. When you call a method on an object, you need two things: the function and the object itself. It’s the difference between func() and obj.method(). Starting with some code that again expects a function as an argument, into which we’ll pass a delegate. lambda is like a glue to combine for_each and class MessageSizeStor().

class MessageSizeStore{
    public:
    MessageSizeStore () : _max_size( 0 ) {}
    void getMaxMessage (const std::string& message ) {...}
    int getSize (){...}
private:
    int _max_size;
};

MessageSizeStore size_store;
for_each(s.begin(),s.end(),
        [&] (const std::string& message) {
               size_store.checkMessage( message );
               });

Why lambda? 1) It can be inline, 2) It can access all the automatic varaible

int count3 = 0;
std::count_if(v.begin(), v.end(),
[&count3](int x){count3 += (x%3 == 0)} );
//how many number can be divided by 3.

1.3.3.1 lambda