Why use a typedef class {} Name ?
I learnt this in IBM C++ doc, no hint to use here.
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7Legacy from C. Don't use. – Tadeusz Kopec for Ukraine Mar 19 '13 at 14:05
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4I personally don't think this question deserves downvotes. I also advice against using that but it is a fair question. Maybe explanation **why** would do better here. – Ivaylo Strandjev Mar 19 '13 at 14:07
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@IvayloStrandjev - agreed, I was just writing the same. That's really ugly and unnecessary and I'm kinda surprised, that it's used in IBM's doc. – Kiril Kirov Mar 19 '13 at 14:11
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@TadeuszKopec: The `typedef` is very convenient short hand for complex types, such as function pointers and pointers to special arrays. – Thomas Matthews Mar 19 '13 at 15:28
2 Answers
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This is a hangover from the 'C' language.
In C, if you have
struct Pt { int x; int y; };
then to declare a variable of this struct, you need to do
struct Pt p;
The typedef helped you avoid this in C
typedef struct { int x; int y; } Pt;
Now you can do
Pt p;
in C.
In C++, this was never necessary because
class Pt { int x; int y; };
allowed you to do
Pt p;
It provides no notational benefits in C++ as it does in C. OTOH, it leads to restrictions because this syntax does not provide any mechanism for construction, or destruction.
i.e. you cannot use the name typedef name in the constructor or destructor.
typedef class { int x; int y; } Pt;
You cannot have a constructor called Pt, nor a destructor. So in essence, most of the time, you shouldn't do this in C++.
user93353
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Thanks! what about benefice in C ? the one described by Steve Jessop only ? – kiriloff Mar 19 '13 at 14:08
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It still says "does not provide any mechanism for inheritance" – Jonathan Wakely Mar 19 '13 at 14:38
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That certainly not an issue because class and struct declarator syntax do alow type identificator after respective keyword – Swift - Friday Pie Nov 26 '16 at 08:37
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3
This answer assumes that there's some interesting content in the class, not just {}.
In C++, you can have a function with the same name as a class (for compatibility with C), but you pretty much never want to.
You can't have a function with the same name as a typedef, so doing this protects you against ill-disciplined name choices. Pretty much nobody bothers, and even if you're going to bother you'd probably write it:
class Name {};
typedef Name Name; // reserve the name
If the code you're referring to really is as written (I can't see it by following your link), then it's rather like class Name {}; (which is a peculiar thing to write, why would you call an empty class Name?), but modified for the above consideration.
Steve Jessop
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I had no idea about this special treatment of typedef names. Grood to know! Also, I find it surprising and possibly disturbing that `typedef A A;` is allowed. – Agentlien Mar 19 '13 at 14:27
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After playing around a bit with it, I can see that most compilers seem to implement it as described, but I can't find the relevant section in the standard. Do you think you could help with a reference? – Agentlien Mar 19 '13 at 14:32
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@Agentlien: it's special treatment of class names rather than of typedefs. The relevant bit in C++11 is 3.3.10/2 ([basic.scope.hiding]). – Steve Jessop Mar 19 '13 at 15:00
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So, if I understand this correctly (having read said section), the behavior stems from the fact that *only* `enum`s and `class`es can be hidden by function/data member/variable/enumerator declarations in the same scope. So, the `typedef` introduces a new name which *cannot* be hidden by a function (nor a variable, data member or enumerator). – Agentlien Mar 19 '13 at 15:26