This behavior seems to be specfic to newer versions of Clang, and is a language extension called "blocks".

The Wikipedia article on C "blocks" also provides information which supports this claim:

Blocks are a non-standard extension added by Apple Inc. to Clang's implementations of the C, C++, and Objective-C programming languages that uses a lambda expression-like syntax to create closures within these languages. Blocks are supported for programs developed for Mac OS X 10.6+ and iOS 4.0+, although third-party runtimes allow use on Mac OS X 10.5 and iOS 2.2+ and non-Apple systems.

Emphasis above is mine. On Clang's language extension page, under the "Block type" section, it gives a brief overview of what the Block type is:

Like function types, the Block type is a pair consisting of a result value type and a list of parameter types very similar to a function type. Blocks are intended to be used much like functions with the key distinction being that in addition to executable code they also contain various variable bindings to automatic (stack) or managed (heap) memory.

GCC also has something similar to blocks called lexically scoped nested functions. However, there are some key differences also note in the Wikipedia articles on C blocks:

Blocks bear a superficial resemblance to GCC's extension of C to support lexically scoped nested functions. However, GCC's nested functions, unlike blocks, must not be called after the containing scope has exited, as that would result in undefined behavior.

GCC-style nested functions also require dynamic creation of executable thunks when taking the address of the nested function. [...].

Emphasis above is mine.

• Lambda calculus

The lambda calculus is a computation model invented by Alonzo Church in the 30s. The syntax and semantics of most functional programming languages are directly or indirectly inspired by the lambda calculus.

The lambda calculus in its most basic form has two operations: Abstraction (creating an (anonymous) function) and application (apply a function). Abstraction is performed using the λ operator, giving the lambda calculus its name.

• Lambda expressions
• Lambda functions

Anonymous functions are often called "lambdas", "lambda functions" or "lambda expressions" because, as I said above, λ was the symbol to create anonymous functions in the lambda calculus (and the word lambda is used to create anonymous functions in many lisp-based languages for the same reason).

• Lambda programming

This is not a commonly used term, but I assume it means programming using anonymous functions or programming using higher-order functions.

A bit more information about lambdas in C++0x, their motivation and how they relate to function pointers (a lot of this is probably a repeat of what you already know, but I hope it helps explain the motivation of lambdas and how they differ from function pointers):

Function pointers, which already existed in C, are quite useful to e.g. pass a comparison function to a sorting function. However there are limits to their usefulness:

For example if you want to sort a vector of vectors by the ith element of each vector (where i is a run-time parameter), you can't solve this with a function pointer. A function that compares two vectors by their ith element, would need to take three arguments (i and the two vectors), but the sorting function would need a function taking two arguments. What we'd need is a way to somehow supply the argument i to the function before passing it to the sorting function, but we can't do this with plain C functions.

To solve this, C++ introduced the concept of "function objects" or "functors". A functor is basically an object which has an operator() method. Now we can define a class CompareByIthElement, which takes the argument i as a constructor argument and then takes the two vectors to be compared as arguments to the operator() method. To sort a vector of vectors by the ith element we can now create a CompareByIthElement object with i as an argument and then pass that object to the sorting function.

Since function objects are just objects and not technically functions (even though they are meant to behave like them), you can't make a function pointer point to a function object (you can of course have a pointer to a function object, but it would have a type like CompareByIthElement* and thus not be a function pointer).

Most functions in the C++ standard library which take functions as arguments are defined using templates so that they work with function pointers as well as function objects.

Now to lambdas:

Defining a whole class to compare by the ith element is a bit verbose if you're only ever going to use it once to sort a vector. Even in the case where you only need a function pointer, defining a named function is sub-optimal if it's only used once because a) it pollutes the namespace and b) the function is usually going to be very small and there isn't really a good reason to abstract the logic into its own function (other than that you can't have function pointers without defining a function).

So to fix this lambdas were introduced. Lambdas are function objects, not function pointers. If you use a lambda literal like x1, x2{bla} code is generated which basically does the following:

1. Define a class which has two member variables (x1 and x2) and an operator() with the arguments (y1 and y2) and the body bla.
2. Create an instance of the class, setting the member variables x1 and x2 to the values of the variables x1 and x2 currently in scope.

So lambdas behave like function objects, except that you can't access the class that's generated to implement a lambda in any way other than using the lambda. Consequently any function that accepts functors as arguments (basically meaning any non-C function in the standard library), will accept lambdas, but any function only accepting function pointers will not.

The problem

C++ includes useful generic functions like std::for_each and std::transform, which can be very handy. Unfortunately they can also be quite cumbersome to use, particularly if the functor you would like to apply is unique to the particular function.

#include <algorithm>
#include <vector>

namespace {
  struct f {
    void operator()(int) {
      // do something
    }
  };
}

void func(std::vector<int>& v) {
  f f;
  std::for_each(v.begin(), v.end(), f);
}

If you only use f once and in that specific place it seems overkill to be writing a whole class just to do something trivial and one off.

In C++03 you might be tempted to write something like the following, to keep the functor local:

void func2(std::vector<int>& v) {
  struct {
    void operator()(int) {
       // do something
    }
  } f;
  std::for_each(v.begin(), v.end(), f);
}

however this is not allowed, f cannot be passed to a template function in C++03.

The new solution

C++11 introduces lambdas allow you to write an inline, anonymous functor to replace the struct f. For small simple examples this can be cleaner to read (it keeps everything in one place) and potentially simpler to maintain, for example in the simplest form:

void func3(std::vector<int>& v) {
  std::for_each(v.begin(), v.end(), [](int) { /* do something here*/ });
}

Lambda functions are just syntactic sugar for anonymous functors.

Return types

In simple cases the return type of the lambda is deduced for you, e.g.:

void func4(std::vector<double>& v) {
  std::transform(v.begin(), v.end(), v.begin(),
                 [](double d) { return d < 0.00001 ? 0 : d; }
                 );
}

however when you start to write more complex lambdas you will quickly encounter cases where the return type cannot be deduced by the compiler, e.g.:

void func4(std::vector<double>& v) {
    std::transform(v.begin(), v.end(), v.begin(),
        [](double d) {
            if (d < 0.0001) {
                return 0;
            } else {
                return d;
            }
        });
}

To resolve this you are allowed to explicitly specify a return type for a lambda function, using -> T:

void func4(std::vector<double>& v) {
    std::transform(v.begin(), v.end(), v.begin(),
        [](double d) -> double {
            if (d < 0.0001) {
                return 0;
            } else {
                return d;
            }
        });
}

"Capturing" variables

So far we've not used anything other than what was passed to the lambda within it, but we can also use other variables, within the lambda. If you want to access other variables you can use the capture clause (the [] of the expression), which has so far been unused in these examples, e.g.:

void func5(std::vector<double>& v, const double& epsilon) {
    std::transform(v.begin(), v.end(), v.begin(),
        epsilon -> double {
            if (d < epsilon) {
                return 0;
            } else {
                return d;
            }
        });
}

You can capture by both reference and value, which you can specify using & and = respectively:

• [&epsilon, zeta] captures epsilon by reference and zeta by value
• [&] captures all variables used in the lambda by reference
• [=] captures all variables used in the lambda by value
• [&, epsilon] captures all variables used in the lambda by reference but captures epsilon by value
• [=, &epsilon] captures all variables used in the lambda by value but captures epsilon by reference

The generated operator() is const by default, with the implication that captures will be const when you access them by default. This has the effect that each call with the same input would produce the same result, however you can mark the lambda as mutable to request that the operator() that is produced is not const.