Class with no name | 易学教程

问题

I read this about class in the C++ standard document:

A class is a type. Its name becomes a class-name (9.1) within its scope.
class-name: identifier template-id

I found this grammar for an identifier in the C++ Standard:

 2.10 Identifiers
 identifier: nondigit
 identifier nondigit
 identifier digit

 nondigit: one of universal-character-name 
 _ a b c d e f g h i j k l m n o p q r s t u  v w x y z A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 
 digit: one of 0 1 2 3 4 5 6 7 8 9

Now I tried doing this:

class
{
public:
  int i;
};

and it compiles fine without any name.

Can anyone give me an explanation regarding this? Isn't it a violation of the grammar specified for an identifier?

Nawaz had asked a follow up question regarding the standard compliance of the code I had given. Those interested can check it out here.

回答1:

The grammar goes

class-specifier:
    class-head { member-specification_opt }

class-head:
    class-key attribute-specifier-seq_opt class-head-name class-virt-specifier-seq_opt base-clause_opt
    class-key attribute-specifier-seq_opt base-clause_opt

class-key:
    class
    struct
    union

In your case, the second production of class-head is used -- no class-name is involved.

回答2:

The identifier is omitted entirely, so the question of correct grammar for the identifier is moot. Nothing in the description says the identifier must be present. Anonymous classes are probably allowed for consistency with C struct rules, which allows constructs such as:

typedef struct { int i; } Foo;

struct { int x, y; } points[] = { {1, 2}, {3, 4} };

I don't think I've ever seen this done for a class.

回答3:

class {public: int i;}

is quite useless, but you can specyfy a class without a name and then create instances of this class. Knowing that you can use the following:

class {public: int i;} a,b,c;
a.i = 5;
b.i = 4;
c.i = 3;
cout<<a.i<<" "<<b.i<<" "<<c.i;

You can use this inside a function too (as an anonimous class), so knowing that you can use something like this:

void x()
{
    class {public: int i;} a,b,c;
    a.i = 5;
    b.i = 4;
    c.i = 3;
    cout<<a.i<<" "<<b.i<<" "<<c.i;
}

int main() {
    x();
}

回答4:

The code class { int i; }; is fully-standard conformant.You've quoted the irrelevant reference from the Standard which has nothing to do with anonymous class.

来源：https://stackoverflow.com/questions/13134845/class-with-no-name

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