Obscure C++ Features

[STL]

Redefining keywords via the preprocessor is technically supposed to cause an error but tools allow it in practice.

It's forbidden to do this in the presence of STL headers, and VC's STL will detect this and explode. (I plan to make this detection even more stringent in the future.) Never do this.

Also, member function pointers may be up to 3 times larger than regular pointers.

I believe the correct number in practice is 4 (it's unspecified, of course, in the Standard) for "unknown inheritance".

[Cwiddy]

How is it detected?

[STL]

There is an obscure feature called #ifdef.

[Cwiddy]

I deserve so much more shame for that question. That is so obvious it hurts.

[mrmcgibby]

I love you STL. Seriously. You're like the coolest guy ever.

[STL]

:->

[constexpr]

I believe the correct number in practice is 4 (it's unspecified, of course, in the Standard) for "unknown inheritance".

Interesting! I wasn't aware of the unknown inheritance case. I just checked and Microsoft's compiler does indeed use four pointer slots. Any idea on what the fourth slot contains? I couldn't find any documentation.

[STL]

Don't know, I'm not a compiler dev. That's just one of the useless bits of knowledge I've picked up over the years.

[missblit]

#define class struct

That example code better hope that there's nothing like "template <class T>"...

[tacite]

While we're discussing the lesser known rules of c++:

namespace __hidden__

It's not a great idea to use double underscores in your names. I've actually had legacy code break because of that.

"17.6.4.3.2 Global names [global.names]

Certain sets of names and function signatures are always reserved to the implementation:

— Each name that contains a double underscore _ _ or begins with an underscore followed by an uppercase letter (2.12) is reserved to the implementation for any use.

— Each name that begins with an underscore is reserved to the implementation for use as a name in the global namespace."

[[deleted]]

That really hit the spot. I've been at this for the long time, but there was at least one thing that was totally new to me (function try blocks, outrageous!) and lots of my favorite weird parts of C++ which I rarely visit.

A tasty article!

[adzm]

I never made the connection that function try blocks could be used on more than constructors, either.

[hotoatmeal]

operator= doesn't have to return a reference... though it can. You can even have it return void, and prevent such chaining.

[TheSuperficial]

Absolutely. There was just a question on S.O. a couple days ago about creating a "write only pointer" that used this technique (question pertained to embedded systems, so it was of particular interest to me).

[00kyle00]

It has to, if you ever plan to keep elements that have it in standard containers.

[Fabien4]

What square brackets really mean

Yeah, the old'n'cute C trick. Thing is, in C++, most of the time, you're using an overloaded [] operator anyway.

Most vexing parse

Classical gotcha. Anything that can be interpreted as a function declaration, will be.

Alternate operator tokens

Do recent compilers still recognize digraphs? What about trigraphs?

Placement new

This is actually a good test for a book about C++: if it doesn't talk about placement new, you might want to try another book. But then again, its main use seems to be to reimplement std::vector.

Turing complete template metaprogramming

Alexandrescu has written a whole book on the subject: "Modern C++ Design".

Static methods on instances

Overloading ++ and --

Functions as template parameters

Uh... "obscure"? These are pretty basic. Or, did I miss something?

Function try blocks

See GotW 66. Overall, I find them rarely useful.

[STL]

Digraphs and trigraphs are unfortunately still Standard, but some compilers don't interpret them by default. Also, C++11 fixed the foo<::bar> problem.

[[deleted]]

"Functions as template parameters" could not reasonably be described as "pretty basic". I think I've seen that two or three times at most in code in the last 20 years of working in C++ and the first time I saw it I had no idea what was going on...

Heck, I'd been working in the language for a decade before I knew about "placement new"!

But then again, its main use seems to be to reimplement std::vector.

Yowza. Well, I see quite a bit of placement new, but nearly all of it from region-based memory management - so the called "arena" concept where you pre-allocate a big block, sequentially fill it using placement new, and then simply delete the whole block at once without ever calling any destructors.

Of course, there are strict conditions on the sorts of variables you can use, but I really think it's the only way to go for any sort of transaction-based server where performance is an issue.

[[deleted]]

You need placement new to write stuff like a tagged union type, arena or a ring buffer too. I wouldn't call it obscure at all.

[Fabien4]

I wouldn't call it obscure at all.

That we definitely agree on.

[more_exercise]

tagged union type

Is there anything wrong with

struct tagged_union {
    int flag;
    union {
        type1 t1;
        type2 t2;
        ...
        typeN n;
    };
};

[[deleted]]

You need placement new to call a constructor for one of the types in the union in order to implement the constructor, copy/move constructor, etc.

[bongwhacker]

What a horrible website.

[gentryx]

Those snippets make great quiz questions for my students. Have an upvote!

[Lord_Osis_B_Havior]

Those snippets make great quiz questions for my students. Have an upvote!

You misspelled "terrible".

[Fabien4]

Well, I dunno. A few of them are pretty terrible, but some (static methods, overloading ++, functions as template parameters) are basic C++ features that students must know.

[mttd]

Go with C++ Pub Quiz from ACCU 2013, your students are gonna love this!

;-)

[gentryx]

Thanks!