我可以在 c++ 中键入未命名的结构/类吗？

Question

In C, I can typedef unnamed (no tag) struct:

typedef struct {
 int val;
} Foo_t;

But when I tried to do the same in c++:

typedef struct
{
    A(int a) : a_var(a) {}
    int a_var;
} A;

typedef struct : public A
{
    B(int a, int b) : A(a), b_var(b) {}
    int b_var;
} B;

B &getB()
{
    static B b(1, 2);
    return b;
}
int main() {}

output :

error: ISO C++ forbids declaration of ‘A’ with no type
error: only constructors take member initializers
error: class ‘<unnamed struct>’ does not have any field named ‘A’

I know I am using constructor A(int a) of "unnamed" struct, but right after it, it is typedef ed. So constructors are only available to know types

Answer 1

The problem for example with this typedef declaration

typedef struct
{
    A(int a) : a_var(a) {}
    int a_var;
} A;

is that within the unnamed structure there is used undeclared name A as a name of a constructor. So this declaration is invalid.

By the way the same problem exists else in C.

Consider for example a typedef declaration of a structure used to define a node of a linked list.

typedef struct
{
    int data;
    A *next;
} A;

Again the name A within the structure definition is undefined.

Even if you will write like

typedef struct A
{
    int data;
    A *next;
} A;

nevertheless the name A is still undeclared within the structure in C. You have to write in C

typedef struct A
{
    int data;
    struct A *next;
} A;

On the other hand, in C++ such a typedef declaration is valid.

Answer 2

Yes, you can, but it's not possible to make use of the type name inside the structure, as you said.

// this is valid
typedef struct{
   int x;
   void set(int n){x=n;}
} A;

// this is not
typedef struct{
    int x;
    B(int n){x = n;}
} B;

// you can only have constructor with named structs
typedef struct st_C{
    int x;
    st_C(int n){x = n;}
} C;

// this is valid
C *foo = new C(3); 

Answer 3

In C, you often have to write typdefs like this:

typedef struct {
 int val;
} Foo_t;

to avoid having to write

struct Foo_t f;

every time, which soon becomes quite tedious.

In C++, all struct s union s and enum s declarations act as though they are implicitly typedef ed. So you can declare a struct like this:

struct A
{
    A(int a) : a_var(a) {}
    int a_var;
};

However, structs in C++ are often aggregates (a simple collection of data), and you don't need the constructors. So the following is fine:

struct A
{
    int a_var;
    int b_var;
};

The constructor is implicitly defined by the compiler, and when you use value initialisation with braces:

A a{};

all the members of the struct will be zeroed out.