C++11 Lambda Expressions

The first time I saw this kind of expression, I feel so strange with it:

file_buffer<uint8_t>::open(outputFileName, std::ios::out).then([=](streambuf<uint8_t> outFile) -> pplx::task<http_response>
{
    *fileBuffer = outFile; 

    // Create an HTTP request.
    // Encode the URI query since it could contain special characters like spaces.
    http_client client(U("http://www.bing.com/"));
    return client.request(methods::GET, uri_builder(U("/search")).append_query(U("q"), searchTerm).to_string());
})

 

So what does [=] (typename pram) -> typename { } exactly mean?

It is Lambda expression in C++11. 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, lambdas may be defined inside a function. A lamba expression has the form:

[capture list] (parameter list) -> return type { function body }

There is a detailed description of Lambda Expression Syntax on MSDN and CPPReference , I will not explain the syntax of Lambda Expression, I would like to introduce my understanding and usage of Lambda Expression.

As my understanding, Lambda Expression creates an Object of an Unnamed Functor (NOT a Function).

A functor is pretty much just a class which defines the operator(). That lets you create objects which “look like” a function (Stackoverflow):

// this is a functor
struct add_x {
  add_x(int x) : x(x) {}
  int operator()(int y) { return x + y; }

private:
  int x;
};

// Now you can use it like this:
add_x add42(42); // create an instance of the functor class
int i = add42(8); // and "call" it
assert(i == 50); // and it added 42 to its argument

std::vector<int> in; // assume this contains a bunch of values)
std::vector<int> out;
// Pass a functor to std::transform, which calls the functor on every element
// in the input sequence, and stores the result to the output sequence
std::transform(in.begin(), in.end(), out.begin(), add_x(1));
assert(out[i] == in[i] + 1); // for all i

 
As we have functor so, why do you need Lambda?

I think one important feature of Lambda is, it can create Anonymous Object, it can be Run On Defined.

Java programmers must be familiar with the code when they create an anonymous class, such expression was not supported in C++.  But Lambda express in C++11 can make a similar achievement. Java Programmers can define an anonymous Thread class:

public class A {
    public static void main(String[] arg)
    {
        new Thread()
        {
            public void run() {
                System.out.println("blah");
            }
        }.start();
    }
}

 

C++ now can directly pass a Lambda express into a function call, cause it just bass an object into that function. The grammar is different from anonymous class in Java:

void fillVector(vector<int>& v)
{
    // A local static variable.
    static int nextValue = 1;

    // The lambda expression that appears in the following call to
    // the generate function modifies and uses the local static
    // variable nextValue.
    generate(v.begin(), v.end(), [] { return nextValue++; });
    //WARNING: this is not thread-safe and is shown for illustration only
}

 
Programmer can directly pass a functor object with the function body expressions into a parameter, the code will run on define.

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