Skip to main content

Crazy Brackets - [](){}();

What does this cryptic bracket sequence mean? What programming language is it? Is it valid syntax? If there is even a weak chance of this syntax being valid, then what does it mean?

Alright, alright, alright.....It is C++. That would calm most people; with all their love (pun) for C++. Specifically, it is C++0x. Amongst many other features that we have been waiting for, C++0x gives us the power of lambdas.

The formal definition of a lambda in C++0x is as follows:-

[capture_mode] (parameters) mutable throw() -> return_type {
   body
} 

So a lambda may capture one or more variables in scope by value or by reference, or it may capture none. Specifying return_type is not necessary if the type can be inferred or is void.

For instance, a std::for_each's functor based code could be inlined with a lambda as follows:-

std::for_each(v.begin(), v.end(), [](int x) {
   cout << x << std::endl;
}); 

A lambda definition could be assigned to a variable and then used or invoked later.

auto lessthan = [](int left, int right) {
 return left < right;
}; 

In the above code, lessthan represents a function that takes two int parameters, and returns a bool. And it can be invoked as lessthan(2, 3), which returns true. The cute thing about a lambda is that it can invoked directly right after its definition. The following code defines a lambda (which takes two ints and returns a bool) and invokes it right away.

[](int left, int right) {
   return left < right;
} (2, 3);

Coming back to our initial question...............you should have guessed it by now. The bracket sequence - [](){}(); - is nothing but a definition followed by a call (right away) to a lambda taking no arguments and returning nothing.

To end with a quote, C++ code is like calligraphy. In other words, it is beautiful to those who understand it, while cryptic to others.

P.S: Pardon me if calligraphy is inappropriate in this context.

Post a Comment

Popular posts from this blog

out, ref and InvokeMember !!!

When I was working on the .NET reflection extravaganza thing that I explained in my previous column, i learnt one another interesting thing, that is about the Type.InvokeMember. How will pass out or ref parameters for the method invoked using Type.InvokeMember ? If you are going to invoke a method with the prototypeint DoSomething(string someString, int someInt);then you would use InvokeMember like this:-object obj = someType.InvokeMember("DoSomething",
BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance,
null,
this,
new object[] {"Largest Integer", 1});or use some variables in the new object[] {...}. But what do you with the args if DoSomething takes out or ref parameters ?int DoSomething(out string someString, ref int someInt);Something like this will not work string someText = string.Empty;
int someInt = 0;
object obj = someType.InvokeMember("DoSomething",
BindingFlags.Public | BindingFlags.NonPublic …

Passing CComPtr By Value !!!

This is about a killer bug identified by our chief software engineer in our software. What was devised for ease of use and write smart code ended up in this killer defect due to improper perception. Ok, let us go!CComPtr is a template class in ATL designed to wrap the discrete functionality of COM object management - AddRef and Release. Technically it is a smart pointer for a COM object.void SomeMethod() { CComPtr siPtr; HRESULT hr = siPtr.CoCreateInstance(CLSID_SomeComponent); siPtr->MethodOne(20, L"Hello"); }Without CComPtr, the code wouldn't be as elegant as above. The code would be spilled with AddRef and Release. Besides, writing code to Release after use under any circumstance is either hard or ugly. CComPtr automatically takes care of releasing in its destructor just like std::auto_ptr. As a C++ programmer, we must be able to appreciate the inevitability of the destructor and its immense use in writing smart code. However there is a difference between …

jqGrid: Handling array data !!!

This post is primarily a personal reference. I also consider this a tribute to Oleg, who was fundamental in improving my understanding of the jqGrid internals - the way it handles source data types, which if I may say led him in discovering a bug in jqGrid.

If you are working with local array data as the source for jqGrid, meaning you will get the data from the server but want the jqGrid not to talk to the server anymore, and want to have custom handling of the edit functionality/form and delete functionality, it is not going to be straightforward - you need to have a decent understanding of how jqGrid works, and you should be aware of the bug Oleg pointed in our discussion. I repeat this is all about using jqGrid to manage array data locally, no posting to server when you edit or delete, which is where the bug is.

$('#grid').jqGrid('navGrid', '#pager', { recreateForm: true, add: false, search: false, refresh: false, …