Thread: What is a structure tag?

The idea is that a structure definition can either a) declare a new type of structure, or b) declare instance variables of a structure, or c) both. If you only want to use the structure once, you don't need to give it a name (tag).

In addition to the example laserlight gave, in C you might see a structure without a tag in the case of nested structures... for example,

Code:

struct widget {
    int x, y;
    struct {
        void *(*clone)(struct widget *);  // function pointers to clone the widget
        void (*free)(struct widget *);  // and free the widget
    } func;
};

widget w;
w.func.free = &free_widget;

The inner structure separates its members from the outer structure without creating a structure that you can refer to by name. This is more common with unions than structures. (There are also anonymous unions that have neither a name nor instances, but that's a very special case.)

Originally Posted by MutantJohn

Yeah and it's boss for representing coordinates!

Code:

struct point
{
    // Use a union as hybrid storage


    union
    {
        struct
        {
            float x, y, z;
        };


        float p[3];
    };


    point(float a, float b, float c) : x(a), y(b), z(c) { };


    void print(void) const
    {
        printf("(%.00f, %.00f, %.00f)\n", x, y, z);
    };
};

Is this safe according to the standard?
Note: I would guess packing might need to be done exactly right or disabled.

Tim S.

Originally Posted by stahta01

Is this safe according to the standard?
Note: I would guess packing might need to be done exactly right or disabled.

Depends what you mean by safe.

Your code, as presented, only sets the values of struct members x,y, and z and only ever retrieves values of them. There is no usage of the member p. As such, the behaviour is well specified.

The behaviour would be undefined if one member of the union is set and another member is used to retrieve (e.g. setting x,y,z but retrieving p[0],p[1], and p[2]).

That said, a fair few library functions would rely on the layout aligning. But "what implementations do" is not the same as "what standards require".

Originally Posted by MutantJohn

What do you mean by layout aligning?

Look up structure padding.

Placing a struct and an array in the same union does not force the struct members to align with array elements.