关于C vrs C ++中的结构变量声明

Question

in C

   struct node {
          int x;
   };

   node *y; // incorrect
   struct node *y; // correct  

in C++

   struct node{
         int x;
   };

   node *y; // correct
   y = new node; // correct

Question: Why is the tag name acceptable to create a pointer in C++ not in C?

Answer 1

Because the C++ standard says so, and the C standard doesn't.

I would say that C++ made the change to make classes easier to use (remember, classes and structs are fundamentally the same thing in C++; they only differ in default visibility), and C didn't follow suit because that would break tons of existing C code.

Answer 2

The reason is not just because the standard says so, C++ actually has namespaces while the struct keyword allowed for a primitive form of namespace in C which allowed you to have a struct and a non-struct identifier with the same name. We can see this primitive form of a namespace from the C99 draft standard section 6.2.3 Name spaces of identifiers which says:

If more than one declaration of a particular identifier is visible at any point in a translation unit, the syntactic context disambiguates uses that refer to different entities. Thus, there are separate name spaces for various categories of identifiers, as follows:

and has the following bullets:

— label names (disambiguated by the syntax of the label declaration and use);

— the tags of structures, unions, and enumerations (disambiguated by following any24) of the keywords struct, union, or enum);

— the members of structures or unions; each structure or union has a separate name space for its members (disambiguated by the type of the expression used to access the member via the . or -> operator);

— all other identifiers, called ordinary identifiers (declared in ordinary declarators or as enumeration constants).

I could make up an example but POSIX gives us a great example with stat which is both a function and a struct :

int stat(const char *restrict path, struct stat *restrict buf);
    ^^^^                            ^^^^^^^^^^^

Answer 3

If you want to write the way node* y in C, just

typedef struct node {
    int x;
} node;