typedef struct，但在关键字“struct”下保留新类型的命名空间

Question

How can I typedef a struct but still keep the new type namespaced under the keyword 'struct'?

example:

struct foo {
    int x; 
}; 

typedef struct foo struct bar; 


int main(void) {
    struct bar bar; 
    bar.x = 10; 

    return 0; 
}

but this doesn't work obviously. Here are the errors if anyone is curious:

main.c:5:20: error: two or more data types in declaration specifiers
 typedef struct foo struct bar;
                    ^
main.c:5:27: warning: useless storage class specifier in empty declaration
 typedef struct foo struct bar;
                           ^
main.c: In function 'main':
main.c:9:13: error: storage size of 'bar' isn't known
  struct bar bar;

Answer 1

How can I typedef a struct but still keep the new type namespaced under the keyword 'struct'? .

You cannot. A namespace is a declarative region that provides a scope to the identifiers (names of the types, function, variables etc) inside it. The concept of Namespace as it is defined within C++ , is not inherent in C .

So, if you are okay with minor changes in your requirements, instead of doing something unnatural, use a simple typedef:

Instead of this:

struct foo {
    int x; 
};  

do this:

typedef struct {
    int x; 
}foo;  

Then this will work:

typedef foo bar;

int main(void )
{   
    bar b;  
  
    b.x = 10;
    return 0;  
}

Note: Although namespaces are not inherent in in C , as they are in C++ , there are some interpretations eg: as discussed here , that argue the point.

Answer 2

How can I typedef a struct but still keep the new type namespaced under the keyword 'struct'?

So, it seems that you want to define a structure alias for another structure type.

This is not possible with typedef as it creates a single word alias. The new alias can't be consisted of multiple white space separated words.

But you can use a single name like struct_bar with struct implemented inside of the name to show that bar is a structure.

#include <stdio.h>

struct foo {
    int x; 
}; 

typedef struct foo struct_bar;


int main(void) {
    struct_bar bar;
    bar.x = 10; 

    return 0; 
}

Answer 3

You can't. This isn't how it works.

You cannot "create" a type whose name is more than one word, nor can you refer to a type alias using the keyword struct .

The purpose of writing struct , in this context, is to refer to a struct type by a name that wasn't introduced as an alias. The keyword is there to say that that's what you want to do. But it's not part of the name; it cannot be part of the name.

Fortunately, there's no reason to need or even want this.

Answer 4

C doesn't have any type of support for namespaces (at least not in the sense that C++ does).

When you create a typedef , the new type name is a single identifier, not multiple words. So struct bar can't be an alias for another type. You would have to call it bar or some other one-word name.

Answer 5

I found a solution that works for cygwin:

struct foo {
    int x; 
}; 

struct bar {
    struct foo; 
};