返回结构而不初始化它

Question

I have a very simple color struct like this

struct {
    unsigned char red;
    unsigned char green;
    unsigned char blue;
    unsigned char alpha;
} RGBA;

Now in some function, I would like to return a structure like this.

But I can't figure out how to do that. All I can think of is creating a local structure, filling each field separately, and then returning it.

RGBA localStructure;
localStructure.red = r;
localStructure.green = g;
localStructure.blue = b;
localStructure.alpha = a;
return localStructure;

Where I really would like to do something like this

return RGBA(r,g,b,a);

is there a way to achieve this in C/C++?

Answer 1

First, in your first snippet, RGBA is an object whose type is an unnamed struct I don't think that's what you intended. You probably meant either:

typedef struct { ... } RGBA; /* what a lot of C programmers do */

or

struct RGBA { ... }; /* probably what you meant */

---

Anyway, to answer your question, give it a constructor!

struct RGBA {
    RGBA(unsigned char r, unsigned char g, unsigned char b, unsigned char a) : red(r), green(g), blue(b), alpha(a) {
    }

    unsigned char red;
    unsigned char green;
    unsigned char blue;
    unsigned char alpha;
};

Then you can write:

return RGBA(r,g,b,a);

Answer 2

In C you can initialize structures with this shortcut :

RGBA x = { r, g, b, a };

So a quick way to do what you want could be this one:

return (RGBA) { r, g, b, a };

Answer 3

In C++ a struct can have a constructor.

struct RGBA{
    unsigned char red;
    unsigned char green;
    unsigned char blue;
    unsigned char alpha;

    RGBA( unsigned char _red, unsigned char _green, unsigned char _blue, unsigned char _alpha ) : red( _red ), green( _green ), blue( _blue ), alpha( _alpha )
    {}

};

Answer 4

struct { ... } RGBA defines an object RGBA of an unnamed struct. What you probably mean is typedef struct { ... } RGBA; . Then you can do:

RGBA fun() {
    RGBA obj = { 0xff, 0xff, 0xff, 0 };
    return obj;
}

GCC supports compound C99 literals in C90 mode and in C++ , so than you can do:

RGBA fun() {
    return ((RGBA){ 0xff, 0xff, 0xff, 0 });
}

Answer 5

Yes, declare a constructor.

struct RGBA { 

   RGBA(unsigned char r, unsigned char g, unsigned b, unsigned char a = 0xff)
     : red(r)
     , green(g)
     , blue(b)
     , alpha(a)
   {
   }

   ...
};

Then use it like this:

 RGBA t = RGBA(0xff,0xff,0);

or this

 return RGBA(0,0,0,0xff);

Note: constructors are an essential C++ feature. You should definitely read up on object orientation in C++. It will make your life much happier.

Answer 6

Supposing your RGBA struct is unable to be changed, eg 3rd party or needs to be in C, and your wish to simplify your return in C++, then you can define an initialisor type to do the work:

struct RGBAInitialisor
{
    RGBAInitialisor(unsigned char r, unsigned char g, unsigned char b, unsigned char a) {
        impl_.alpha = a;
        impl_.blue = b;
        impl_.green = g;
        impl_.red = r;
    }
    // This converts back to the RGBA on the fly.
    operator RGBA ()
    {
        return impl_;
    }
    RGBA impl_;

};

RGBA f()
{
    return RGBAInitialisor(1,2,3,4);
}

Answer 7

您可以创建一个类而不是一个结构（或保持结构），并为它提供一个带有4个参数的构造函数。

Answer 8

A C++ struct is really a class, so you can add a constructor to it, and then provide code to initialize the fields in the constructor.

AC struct has a size, so if you want to initialize all of the fields, you can generally

RGBA localStructure;
memset(&localStructure, 0, sizeof(localStructure));