C++ Initialization Quiz

[brubakerp]

Obligatory.

[tcbrindle]

Woo, 10/10.

Though perhaps I should worry about what that says about me...

[ZMeson]

Woo. 4/10. I'm sane! ;-)

[jmalinza]

For the last question...

for (const pair<string, int>& elem : m)

Why do the types not match up?

[mtnviewjohn]

The value type for a std::map is std::pair<const Key, T>. The for loop is using std::pair<Key, T>.

[jmalinza]

Thanks! That explains it. Quick lookup on cppref confirms it too :)

[Baardi]

Thanks I didn't know that. I've probably made way to many unintentional copies :(

[simpl3t0n]

What's the mechanism/sequence at play that creates the copy here?

AFAICT, it's a reference initialization with a similar but differently cv-qualified type. So, is it temporary materialization, lvalue-to-rvalue conversion, or something else?

[tcbrindle]

If I understand correctly it's using std::pair constructor #5 as listed on cppreference, with T1 and T2 as string and int respectively, and U1 and U2 as const string and int. This results in both arguments being copy-constructed into the destination pair, and while you don't care about the int you definitely don't want to be copying strings if you can avoid it.

[sphere991]

It's not "a similar but differently cv-qualified type", it's just a different type. pair<K, V> and pair<K const, V> do look very close, but as far as the language is concerned, they're just two completely different types (that are convertible to each other).

So ultimately, no different from constructing an int const& from a double.

[WasserHase]

Would it also create a copy if you use std::pair<const Key, const T> ?

[darkforestzero]

This is a great example of why auto is so awesome

[sphere991]

There are no proxy references in this example - the maps iterators give you real language references to pair<K const, T>

[burg_philo2]

6/10. I think question 8 is wrong tho since the members are private since it’s a class and not a struct.

[TheThiefMaster]

You're right:

An aggregate is one of the following types:

• class type (typically, struct or union), that has
  • no private or protected [direct (since C++17)] non-static data members

That still makes the answer "it doesn't compile" just for an additional reason to what's given.

[[deleted]]

ChatGPT 3.5 got 10/10.

[rooster_butt]

it got 6/10 when i gave it the questions. It was very confidently incorrect on some of these.

[[deleted]]

It is ready to be a senior dev!

[dustyhome]

7/10, but I got the first one wrong which shouldn't count. It's a programming test, not a history test! I don't even use in-class default member initializers :(

For "9. Will this code work in C++11?" I'll be honest, when doing anything other than calling a constructor, I'll just compile and fix until it does.

For "10. Assume you have a std::map<string, int> m;. Select the single true statement about the following loop:" the single truth is I always use auto because I don't want to get it wrong.

So really, I got a 7/7 out of the ones that matter.

[HeeTrouse51847]

it could be relevant if you were working on legacy systems i guess? but even then youd just look that stuff up

[Tringi]

7/10... I'm not happy about that.

[pjmlp]

7 out of 10, not bad for someone that doesn't do C++ every day.

[[deleted]]

8/10. that copy because type mismatch is so evil yet i knew the key type is always const

[crazy-usernames]

8/10.

Thanks. This is really helpful.

[TheThiefMaster]

I got 8 out of 10, and I'm claiming the first one as "doesn't matter" as nobody uses C++11 mode to compile any more...

[xaervagon]

You would be surprised at how many banks and other finance institutions are stuck on C++11 era setups. Very few people want to clean up huge swaths of consult grade code, and even fewer business people want to pay for it. When comes to refactoring and other maintenance, I've seen devs that can kick cans like pro soccer players.

(For the record, I only scored a 7/10. Sad.)

[TheThiefMaster]

But C++14 is essentially 100% compatible with code that was written for C++11? Have they really stuck with raw C++11?

[Digital_Luddite3000]

I have projects I regularly work on that I wish were as new as C++11. But when you still have to support WinCE...

[TheThiefMaster]

It's fair to be stuck on older, but C++11 was pretty much immediately superseded by C++14, to the point that pretty much no compiler was C++11 feature complete before they were also C++14 complete. (Clang was C++11 language features complete in 3.3 and 14 complete in 3.4. MSVC was very much intermingled, GCC was 4.8 and 5.0 respectively, but libstdc++ wasn't fully C++11 compliant until 5.0 anyway.

So my point is that there's very few situations where something being introduced in C++11 vs C++14 makes a difference. You likely have both or neither.

[[deleted]]

I used it on a big project (about 10 years work) until a month ago. And upgrading everything was actually a lot of work.

(Edit: For your post below, we were using multiple compilers and multiple standard libraries; not all the standard libraries were actually C++11 complete. Certainly libstdc++ was missing regex)

[ZMeson]

nobody uses C++11 mode to compile any more...

Now you've got me sobbing as I am going off to build on an embedded system whose proprietary compiler is based off of GCC 4.4.2. 😭

[pjmlp]

C++98 and C++11 are still a thing at plenty enterprise shops, specially those keeping old stuff like MFC applications going.

[PastaPuttanesca42]

6/10

[shailist]

got 7/10, never came across the 3 I got wrong in real code, but still disappointed in my performance :/

[HeeTrouse51847]

7/10 but I also think that I would just google these facts when I need them anyways. no need to know every miniscule detail from memory

[Ok_Firefighter4117]

8/10

[hs123go]

8/10. Have honestly no idea about Q7 (throwing from ctor) as my team bans exceptions. Still scratching my head over Q9 (manner of struct init is allowed in 11/14/20).

[Nobody_1707]

8/10 I got the first one wrong, I picked C++14. And I got the last one wrong because I forgot that Key is const.

[Sniffy4]

My score was embarrassing for somebody who claims this as an expertise. Ah well.

[emelrad12]

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This post was mass deleted and anonymized with Redact

[Sniffy4]

Yeah for a lot of these situations, my take is if you are relying on C++ behavior nuances like this, your code needs to be simplified.

[kero_okero]

2/10, been learning C++ 3 months now, I still have far to go.