What did I just do?

What did I just do?

Mark Oates

Member #1,146

March 2001

Working on my project that has this:

class vec2d
{
public:
   float x;
   float y;
   vec2d(float x, float y) : x(x), y(y) {}
};

...and just on a whim I tried this:

void foo()
{
   vec2d m = {0.0, 0.0};  // <--- ??!!
   std::cout << m.x << ',' << m.y << std::endl;
}

And it worked!

I have no idea what I just did!

--
Visit CLUBCATT.com for cat shirts, cat mugs, puzzles, art and more <-- coupon code ALLEGRO4LIFE at checkout and get $3 off any order of 3 or more items!

AllegroFlare • AllegroFlare Docs • AllegroFlare GitHub

torhu

Member #2,727

September 2002

It's C++11's new "initializer list" feature. It's not useful in your case, but it also works in cases like this:

std::vector<int> v{1, 2, 3};

APPEND:
What you should have done is just this, but either one works:

vec2d m(0, 0);

The only difference is that my version might be faster, I don't know if the semantics of this feature are that it acts like a temporary object of the type being initialized, or if it just supplies arguments to the constructor on the left hand side.

---
Smokin' Guns - spaghetti western FPS action

Edgar Reynaldo

Major Reynaldo

May 2007

C has had static struct initialization for a while now.

exgui.c#SelectExpand
 186/* here it comes - the big bad ugly DIALOG array for our main dialog */
 187DIALOG the_dialog[] =
 188{
 /* (dialog proc)     (x)   (y)   (w)   (h) (fg)(bg) (key) (flags)     (d1) (d2)    (dp)                   (dp2) (dp3) */
 
 /* this element just clears the screen, therefore it should come before the others */
 { d_clear_proc,        0,   0,    0,    0,   0,  0,    0,      0,       0,   0,    NULL,                   NULL, NULL  },
 
 /* these just display text, either left aligned, centered, or right aligned */
 { d_text_proc,         0,  20,    0,    0,   0,  0,    0,      0,       0,   0,    "d_text_proc",          NULL, NULL  },
 { d_ctext_proc,      318,  20,    0,    0,   0,  0,    0,      0,       0,   0,    "d_ctext_proc",         NULL, NULL  },
 { d_rtext_proc,      636,  20,    0,    0,   0,  0,    0,      0,       0,   0,    "d_rtext_proc",         NULL, NULL  },
 
 /* lots of descriptive text elements */
 { d_text_proc,         0,   0,    0,    0,   0,  0,    0,      0,       0,   0,    "d_menu_proc->",        NULL, NULL  },
 { d_text_proc,         0,  40,    0,    0,   0,  0,    0,      0,       0,   0,    "d_button_proc->",      NULL, NULL  },
 { d_text_proc,         0,  70,    0,    0,   0,  0,    0,      0,       0,   0,    "d_check_proc->",       NULL, NULL  },
 { d_text_proc,         0, 100,    0,    0,   0,  0,    0,      0,       0,   0,    "d_radio_proc->",       NULL, NULL  },
 { d_text_proc,         0, 130,    0,    0,   0,  0,    0,      0,       0,   0,    "d_edit_proc->",        NULL, NULL  },
 { d_text_proc,         0, 150,    0,    0,   0,  0,    0,      0,       0,   0,    "d_list_proc->",        NULL, NULL  },
 { d_text_proc,         0, 200,    0,    0,   0,  0,    0,      0,       0,   0,    "d_text_list_proc->",   NULL, NULL  },
 { d_text_proc,         0, 250,    0,    0,   0,  0,    0,      0,       0,   0,    "d_textbox_proc->",     NULL, NULL  },
 { d_text_proc,         0, 300,    0,    0,   0,  0,    0,      0,       0,   0,    "d_slider_proc->",      NULL, NULL  },
 { d_text_proc,         0, 330,    0,    0,   0,  0,    0,      0,       0,   0,    "d_box_proc->",         NULL, NULL  },
 { d_text_proc,         0, 360,    0,    0,   0,  0,    0,      0,       0,   0,    "d_shadow_box_proc->",  NULL, NULL  },
 { d_text_proc,         0, 390,    0,    0,   0,  0,    0,      0,       0,   0,    "d_keyboard_proc. Press F1 to see me trigger the about box.", NULL, NULL  },
 { d_text_proc,         0, 410,    0,    0,   0,  0,    0,      0,       0,   0,    "d_clear_proc. I draw the white background.", NULL, NULL  },
 { d_text_proc,         0, 430,    0,    0,   0,  0,    0,      0,       0,   0,    "d_yield_proc. I make us play nice with the OS scheduler.", NULL, NULL  },
 { d_rtext_proc,      636,  40,    0,    0,   0,  0,    0,      0,       0,   0,    "<-d_bitmap_proc",      NULL, NULL  },
 { d_rtext_proc,      636,  80,    0,    0,   0,  0,    0,      0,       0,   0,    "<-d_icon_proc",        NULL, NULL  },
 
 /* a menu bar - note how it auto-calculates its dimension if they are not given */
 { d_menu_proc,       160,   0,    0,    0,   0,  0,    0,      0,       0,   0,    the_menu,               NULL, NULL  },
 
 /* some more GUI elements, all of which require you to specify their dimensions */
 { d_button_proc,     160,  40,  160,   20,   0,  0,  't',      0,       0,   0,    "&Toggle Me!",          NULL, NULL  },
 { d_check_proc,      160,  70,  160,   20,   0,  0,  'c',      0,       0,   0,    "&Check Me!",           NULL, NULL  },
 { d_radio_proc,      160, 100,  160,   19,   0,  0,  's',      0,       0,   0,    "&Select Me!",          NULL, NULL  },
 { d_radio_proc,      320, 100,  160,   19,   0,  0,  'o',      0,       0,   0,    "&Or Me!",              NULL, NULL  },
 { d_edit_proc,       160, 130,  160,    8,   0,  0,    0,      0,     LEN,   0,    the_string,             NULL, NULL  },
 { d_list_proc,       160, 150,  160,   44,   0,  0,    0,      0,       0,   0,    (void *)listbox_getter, sel,  NULL  },
 { d_text_list_proc,  160, 200,  160,   44,   0,  0,    0,      0,       0,   0,    (void *)listbox_getter, NULL, NULL  },
 { d_textbox_proc,    160, 250,  160,   48,   0,  0,    0,      0,       0,   0,    (void *)the_text,       NULL, NULL  },
 { d_slider_proc,     160, 300,  160,   12,   0,  0,    0,      0,     100,   0,    NULL,                   NULL, NULL  },
 { d_box_proc,        160, 330,  160,   20,   0,  0,    0,      0,       0,   0,    NULL,                   NULL, NULL  },
 { d_shadow_box_proc, 160, 360,  160,   20,   0,  0,    0,      0,       0,   0,    NULL,                   NULL, NULL  },
 
 /* note how we don't fill in the dp field yet, because we first need to load the bitmap */
 { d_bitmap_proc,     480,  40,   30,   30,   0,  0,    0,      0,       0,   0,    NULL,                   NULL, NULL  },
 { d_icon_proc,       480,  80,   30,   30,   0,  0,    0,      0,       0,   0,    NULL,                   NULL, NULL  },
 
 /* the quit and info buttons use our customized dialog procedure, using dp3 as callback */
 { my_button_proc,      0, 450,  160,   20,   0,  0,  'q', D_EXIT,       0,   0,    "&Quit",                NULL, (void *)quit  },
 { my_button_proc,    400, 150,  160,   20,   0,  0,  'i', D_EXIT,       0,   0,    "&Info",                NULL, (void *)info1 },
 { my_button_proc,    400, 200,  160,   20,   0,  0,  'n', D_EXIT,       0,   0,    "I&nfo",                NULL, (void *)info2 },
 { my_button_proc,    400, 300,  160,   20,   0,  0,  'f', D_EXIT,       0,   0,    "In&fo",                NULL, (void *)info3 },
 242
 /* the next two elements don't draw anything */
 { d_keyboard_proc,     0,   0,    0,    0,   0,  0,    0,      0,  KEY_F1,   0,    (void *)about,          NULL, NULL  },
 { d_yield_proc,        0,   0,    0,    0,   0,  0,    0,      0,       0,   0,    NULL,                   NULL, NULL  },
 { NULL,                0,   0,    0,    0,   0,  0,    0,      0,       0,   0,    NULL,                   NULL, NULL  }
 247};

Look familiar now?

Eagle and Allegro 5 binaries | Older Allegro 4 and 5 binaries | Allegro 5 compile guide
King Piccolo will make it so Then I could just flip it with some sweet, sweet matrix math. In either case, I’ll ride this wagon until the wheels fall off. Thanks to those that keep it running

bamccaig

Member #7,536

July 2006

^ I'm inclined to blame it on C. What if you make x and y private/protected? Does it compile then?

--
I mean the best with what I say. It doesn't always sound that way.

SiegeLord

Member #7,827

October 2006

The best bit is that you can do this:

std::vector<vec2d> vec;
vec.push_back({1, 2});

Even C can't do this. This is one of my favorite C++11 features.

"For in much wisdom is much grief: and he that increases knowledge increases sorrow."-Ecclesiastes 1:18
[SiegeLord's Abode][Codes]:[DAllegro5]:[RustAllegro]

GullRaDriel

Member #3,861

September 2003

What ? I think I could pull it easily using a variadic macro.

"Code is like shit - it only smells if it is not yours"
Allegro Wiki, full of examples and articles !!

bamccaig

Member #7,536

July 2006

bamccaig said:

^ I'm inclined to blame it on C. What if you make x and y private/protected? Does it compile then?

--
I mean the best with what I say. It doesn't always sound that way.

Audric

Member #907

January 2001

I tested, in C you can pass a ~~"literal struct"~~ as a function argument, as long as you cast it :

#SelectExpand
  1typedef struct {
  2  int x;
  3  int y;
  4} elem;
  5
  6void dostuff(elem e)
  7{
  8  printf("%d %d\n", e.x, e.y);
  9}
 10
 11int main()
 12{
 13  elem e12 = {1, 2};
 14  dostuff(e12);
 15  dostuff((elem){3,4});
 16  return 0;
 17}

edit: actually it's called a compound literal
It seems to be C99, as well as a GCC-ism in C90
http://stackoverflow.com/questions/10743804/c90-compound-literals

relpatseht

Member #5,034

September 2004

In C you can do this:

typedef struct vec2d
{
    float x, y;
} vec2d;

vec2d foo = { y: 2.5f, x: 1.5f };
printf("%f %f", foo.x, foo.y); // 1.5 2.5

Sorely missed in C++.

torhu

Member #2,727

September 2002

bamccaig said:

^ I'm inclined to blame it on C. What if you make x and y private/protected? Does it compile then?

I tried with GCC 4.8.1, it doesn't matter if they are private or not. Use the second public: instead to make x and y private.

class vec2d
{
public:
   float x;
   float y;
//public:
   vec2d(float x, float y) : x(x), y(y) {}
};


int main()
{
   vec2d m = {0.0, 0.0};
}

In both cases, this happens:

D:\prog\test>g++ -o test -std=gnu++03 test.cpp
test.cpp: In function 'int main()':
test.cpp:15:23: error: in C++98 'm' must be initialized by constructor, not by '{...}'
    vec2d m = {0.0, 0.0};
                       ^

D:\prog\test>g++ -o test -std=gnu++11 test.cpp

---
Smokin' Guns - spaghetti western FPS action

bamccaig

Member #7,536

July 2006

Ummm, in your example they're both public still. Am I missing something?

--
I mean the best with what I say. It doesn't always sound that way.

torhu

Member #2,727

September 2002

Private is the default for classes

---
Smokin' Guns - spaghetti western FPS action

bamccaig

Member #7,536

July 2006

torhu said:

Private is the default for classes

Yeah, and the very first line of your class body is "public:".

--
I mean the best with what I say. It doesn't always sound that way.

torhu

Member #2,727

September 2002

torhu said:

Use the second public: instead to make x and y private.

:-*

But why not just try it yourself?

---
Smokin' Guns - spaghetti western FPS action

bamccaig

Member #7,536

July 2006

Oh. Why the Hell not just write this:

class vec2d
{
public:
   vec2d(float x, float y) : x(x), y(y) {}
//private:
   float x;
   float y;
};

int main()
{
   vec2d m = {0.0, 0.0};
}

It's much more obvious what you're trying to do then.

But yes, now I see what you're saying.

Append:

bamccaig said:

For reference, no it does not. So definitely it's just initializing members in order, C-style. Which in hindsight explains why torhu said the constructor was likely faster. I think that's assuming it's guaranteed to run anyway, though since the only defined constructor requires arguments I'd be inclined to guess that it doesn't... For reference, no it does not. So definitely it's just initializing members in order, C-style. Which in hindsight explains why torhu said the constructor was likely faster. I think that's assuming it's guaranteed to run anyway, though since the only defined constructor requires arguments I'd be inclined to guess that it doesn't...

--
I mean the best with what I say. It doesn't always sound that way.

torhu

Member #2,727

September 2002

Because in reality I just moved public: up and down with editor commands when testing

But anyway, C++11 is nice.

---
Smokin' Guns - spaghetti western FPS action