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function arguments and returning, how to set my code out!

Dec 27, 2012 at 1:06am
SuperStinger (84)
Hi all!

I have a struct that i want to return from a function, but i think it is a bit inefficient to return a really big struct, so i want to return its pointer.

Can i create the struct variable inside the function and then return its pointer, or would the pointer be dangling and left leading to nothing.

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//Struct to hold a bmp of a font and information about it, offsets always start from 32 ( space in ascii )
typedef struct
{
	//This is the actual bitmap of ascii characters that we will blit from
	SDL_Surface* bmp;

	//The array that holds all the symbols, the array size is 94 to go from ascii space ( 32 ) to the tidle ( 126 )
	struct symbol[94];
} font;

font* loadFont ( char* fileName )
{
	font returnStruct;

	//Blah blah load struct stuff, struct.blah = 1

	//--Am i doing this right?--
	return &returnStruct;
}


How would i approach this?

Also, would i be able to modify a function argument, is there anything wrong with that?

So if created the struct variable outside of the function ( in main ) and then passed it as an argument, which was a pointer, then i modify that?

Many thanks, SuperStinger
Dec 27, 2012 at 1:38am
closed account (zb0S216C)
Never return the address of automatic storage. The safer option is to make a static pointer within the function and return that instead.

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Font *LoadFont( char *FileName_ )
{
    static Font *Font_;
    // Load the font...
    return( Font_ );
}

This has it's drawbacks, but does ensure that "Font_" remains valid until the program ends. With each call to the function, the address pointed-to by "Font_" can change. This is the safest way of returning the address of storage that's declared within a function's scope. Note that static storage declared within a function scope remains even if the function is not currently being executed.

Wazzak
Last edited on Dec 27, 2012 at 1:41am
Dec 27, 2012 at 1:38am
closed account (1yR4jE8b)
Your loadFont method should probably be a member function of the font struct.

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//Struct to hold a bmp of a font and information about it, offsets always start from 32 ( space in ascii )
typedef struct
{
	//This is the actual bitmap of ascii characters that we will blit from
	SDL_Surface* bmp;

	//The array that holds all the symbols, the array size is 94 to go from ascii space ( 32 ) to the tidle ( 126 )
	struct symbol[94];

    void loadFont ( char* fileName )
    {
	//Blah blah load struct stuff, struct.blah = 1
    }
} font;
Dec 27, 2012 at 1:49am
SuperStinger (84)
thanks for your responses

@darkestfright: from the loadFont function that i would put in the typedef struct, how would i access the variables inside the struct ( i.e bmp and symbol )

EDIT: ive tried various things by putting the function inside the typedef, but it gives me errors, i dont know how to do this, can someone show me please? many thanks

EDIT2: dont worry, im writing in pure C, not Python, so it doesnt like that, ill just make functions outside of the typedef, thanks for your help
Last edited on Dec 27, 2012 at 2:27am
Dec 27, 2012 at 3:25am
JLBorges (13770)
> but i think it is a bit inefficient to return a really big struct,
> so i want to return its pointer.

No, it is not. Every current mainstream compiler implemts NRVO.
http://en.wikipedia.org/wiki/Return_value_optimization
http://cpp-next.com/archive/2009/08/want-speed-pass-by-value/
Dec 27, 2012 at 4:29am
closed account (1yR4jE8b)
dont worry, im writing in pure C, not Python, so it doesnt like that


You should be more clear about that in the future then.
Dec 27, 2012 at 7:15am
kev82 (323)
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int loadFont ( char* fileName, font *f )
{
	//Blah blah load struct stuff, f->blah = 1

	return 1; //if loading succeeded 
}


font f;
loadFont("font name", &f); //should check for failure here
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