Can anybody explain this wired C++ code piece?

Question

#define WIRED\
  []() {\
    return new MyClass; \
  }()

MyClass* a = WIRED;

I don't understand the first []() and the last () in the MACRO. Can any expert explain this? Thanks.

The macro `WIRED` defines a lambda, which returns a pointer to `MyClass`. This is just a ugly way to say `MyClass* a = new MyClass();` — Rakib, Jul 02 '14 at 05:49
I will ask, where in this world did you get this weird piece of code? — Mark Garcia, Jul 02 '14 at 05:51
Downvotes are harsh here. It's certainly an odd construct, but a clear token-by-token explanation would be of benefit to many people. — ClickRick, Jul 02 '14 at 05:54
I asked this stupid question. I know its functionality. But I don't know this C++ grammer `[]()` and the last `()` — Huazhong Ning, Jul 02 '14 at 05:55
@user3796257 See [this](http://stackoverflow.com/q/7627098/1870232) — P0W, Jul 02 '14 at 05:56

Shoe · Answer 1 · 2014-07-02T06:29:36.940

4

It's a lambda returning a dynamically allocate MyClass that is created and executed in place.

Specifically:

[](){...}

is the lambda; and the remaining () is the invocation of the lambda.

It's a useless lambda AFAIK and you can simply write:

MyClass* a = new MyClass;

or better:

std::unique_ptr<MyClass> ptr(new MyClass);

edited Jul 02 '14 at 06:29

answered Jul 02 '14 at 05:50

Shoe

Of course, macros are ugly, and this could be done just as well with an inline function. It may be useless as-is, but I'd guess it was introduced to enable centralised changes to the code. Maybe it's in an `#ifdef` block which the OP didn't show - perhaps a different definition of `WIRED` is supplied when a specific macro (e.g. `DEBUG_ALLOCATIONS`) is defined. – Angew is no longer proud of SO Jul 02 '14 at 06:44

score 2 · Answer 2 · answered Jul 02 '14 at 05:55

You may need to understand lambda function from C++11.

I believe you can understand this:

MyClass* CreateMyClass() { return new MyClass; }
...

std::function<MyClass* ()> creator_function = CreateMyClass;
MyClass* a = creator_function();

Then, lambda function seems like:

std::function<MyClass* ()> creator_function = []() { return new MyClass; };
MyClass* a = creator_function();

Finally, this would also work:

MyClass* a = []() { return new MyClass; }();

2 Answers2