#Lambda函数#
简介
很多STL的泛型算法会使用一个函数作为参数：We can pass any kind of callable object(eg:仿函数,函数指针等) algorithm. An object or expression is callable if we can apply the call operator to it.
A lambda expression represents a callable unit of code. It can be thought of as an unnamed inline function. Like any function, a lambda has a return type, a parameter list, and a function body. Unlike a function, lambda may be defined inside a function.
[capture list] (parameter list) -> return type {function body}
where capture list is an list of local variables defined in the enclosing function; return type, parameter list and function body are the same as in any ordinary function. However, unlike ordinary functions, a lambda must use a trailing return(参见上述*->return type*) to specify its return type.
We can omit either or both of the parameter list and return type but must always include the capture list and function body:
auto f = [] {return 42;} //no arguments and return type
We can use variables local like:
int sz = 10;
[sz](const string& a) {return a.size() >= sz;};
Note: Capture List is used for local non-static variables only; lambdas can use local statics and variables declared outside the function directly.
[]:empty capture list;
[names]: by value
[&]: by reference
[=]:by value
[&, identifier_list]: identifier_list by values
[=, identifier_list]: identifier_list by reference
We have to capture variables by reference such as ostream(can not be copied), and When we return a lambda from a function, the lambda must not contain reference captures. You have to make sure the whatever you captured has the intended meaning each time the lambda is executed. So avoid capturing pointers or references.
mutable lambdas
A lambda can not change the value of a variable that it copied by value:
int v1 = 32;
auto f = [v1]() {return ++v1; }; //compiler error: v1 can not be changed
auto f = [v1]() mutable {return ++v1;} //ok
A lambda body contains any statement other than a return is assumed to return void, lambdas inferred to return void may not return a value;
transform(v1.begin(), v1.end(), v1.begin(), [](int i){if (i < 0) return -i; else return i;}); //error
transform(v1.begin(), v1.end(), v1.begin(), [](int i) -> int{if (i < 0) return -i; else return i;}); //ok, using trailing return
lambda类型是只有编译使用的类型，如果我们需要使用这个类型需要去创建模版或者使用std::function.