Hello,
I am trying to work with SDL and one of their functions takes a char * file as a function a parameter. However, i'm running into trouble how to actually set this variable in my constructor.

I am getting a problem where if I use a pointer to a char and set it as "test.bmp" I get an error stating "cannot implicitly convert expression (file) of type string to char*. After that I decided to try to set file to 'test.bmp' instead, and in that case I get: "Unterminated character constant" . Although I am familiar with what this error is referring to, I do not know how to add a terminator in D.

This is the function that I intend to use the filename in:
**Note the function LoadBMP is the one that REQUIRES a pointer to a char
--------------------------------------
	SDL_Surface * Load(char * file)
	{
		SDL_Surface * Temp = null;

		if((Temp = SDL_LoadBMP(file)) == null)
			return null;

		Surface = SDLDisplayFormat(Temp);

		SDL_FreeSurface(Temp);

		return Surface;
	}
----------------------------------------

This is the constructor that is giving me the error:
----------------------------------------------------------
	char * file;
	
	this()
	{
		this.filename = "test.bmp";
	}
-------------------------------------------------------------

On Friday, March 02, 2012 05:51:14 Chris Pons wrote:
> Hello,
> I am trying to work with SDL and one of their functions takes a
> char * file as a function a parameter. However, i'm running into
> trouble how to actually set this variable in my constructor.
> 
> I am getting a problem where if I use a pointer to a char and set it as "test.bmp" I get an error stating "cannot implicitly convert expression (file) of type string to char*. After that I decided to try to set file to 'test.bmp' instead, and in that case I get: "Unterminated character constant" . Although I am familiar with what this error is referring to, I do not know how to add a terminator in D.
> 
> This is the function that I intend to use the filename in: **Note the function LoadBMP is the one that REQUIRES a pointer to a char
> --------------------------------------
> 	SDL_Surface * Load(char * file)
> 	{
> 		SDL_Surface * Temp = null;
> 
> 		if((Temp = SDL_LoadBMP(file)) == null)
> 			return null;
> 
> 		Surface = SDLDisplayFormat(Temp);
> 
> 		SDL_FreeSurface(Temp);
> 
> 		return Surface;
> 	}
> ----------------------------------------
> 
> This is the constructor that is giving me the error:
> ----------------------------------------------------------
> 	char * file;
> 
> 	this()
> 	{
> 		this.filename = "test.bmp";
> 	}
> -------------------------------------------------------------

Use std.string.toStringz (or std.utf.toUTFz if immutable(char)* doesn't cut it).

Regardless, be careful of passing char* to C code. Even if the C code keeps the pointer, it won't stop the GC from collecting it. The D code needs to have reference to it of some kind (though a pointer in your struct as you appeart to be doing should be plenty as long as an instance of the struct remains in the D code - as opposed to passing it to the C code and then not having it in the D code anymore, which would be no better than just passing the char* to the C code without keeping a reference to it).

- Jonathan M Davis

Thank you for the reply. However, I've run into another problem.

I changed:

-----------

char * file;

this()
{
this.filename = "test.bmp";
}

---------------

To:

----------------

char * file

this()
{
this.filename = toStringz("test.bmp");
}

-----------------------

I am getting this error:

Error	1	Error: cannot implicitly convert expression (toStringz("test.bmp")) of type immutable(char)* to char*	D:\Documents\Projects\Test\Test\DPBall.d	10	

Instead I tried toUTFz, which I used like this:

--------------------------

	char * filename;
	
	this()
	{
		this.filename = toUTFz("test.bmp");
	}

--------------------------

I get these errors:

Error	1	Error: template std.utf.toUTFz(P,S) if (isSomeString!(S) && isPointer!(P) && isSomeChar!(typeof(*P.init)) && is(Unqual!(typeof(*P.init)) == Unqual!(ElementEncodingType!(S))) && is(immutable(Unqual!(ElementEncodingType!(S))) == ElementEncodingType!(S))) does not match any function template declaration	D:\Documents\Projects\Test\Test\DPBall.d	11	

Error	2	Error: template std.utf.toUTFz(P,S) if (isSomeString!(S) && isPointer!(P) && isSomeChar!(typeof(*P.init)) && is(Unqual!(typeof(*P.init)) == Unqual!(ElementEncodingType!(S))) && is(immutable(Unqual!(ElementEncodingType!(S))) == ElementEncodingType!(S))) cannot deduce template function from argument types !()(string)	D:\Documents\Projects\Test\Test\DPBall.d	11	


Am I using these functions incorrectly?

On Friday, March 02, 2012 06:22:41 Chris Pons wrote:
> Thank you for the reply. However, I've run into another problem.
> 
> I changed:
> 
> -----------
> 
> char * file;
> 
> this()
> {
> this.filename = "test.bmp";
> }
> 
> ---------------
> 
> To:
> 
> ----------------
> 
> char * file
> 
> this()
> {
> this.filename = toStringz("test.bmp");
> }
> 
> -----------------------
> 
> I am getting this error:
> 
> Error	1	Error: cannot implicitly convert expression
> (toStringz("test.bmp")) of type immutable(char)* to
> char*	D:\Documents\Projects\Test\Test\DPBall.d	10
> 
> Instead I tried toUTFz, which I used like this:
> 
> --------------------------
> 
> 	char * filename;
> 
> 	this()
> 	{
> 		this.filename = toUTFz("test.bmp");
> 	}
> 
> --------------------------
> 
> I get these errors:
> 
> Error	1	Error: template std.utf.toUTFz(P,S) if (isSomeString!(S)
> && isPointer!(P) && isSomeChar!(typeof(*P.init)) &&
> is(Unqual!(typeof(*P.init)) == Unqual!(ElementEncodingType!(S)))
> && is(immutable(Unqual!(ElementEncodingType!(S))) ==
> ElementEncodingType!(S))) does not match any function template
> declaration	D:\Documents\Projects\Test\Test\DPBall.d	11
> 
> Error	2	Error: template std.utf.toUTFz(P,S) if (isSomeString!(S)
> && isPointer!(P) && isSomeChar!(typeof(*P.init)) &&
> is(Unqual!(typeof(*P.init)) == Unqual!(ElementEncodingType!(S)))
> && is(immutable(Unqual!(ElementEncodingType!(S))) ==
> ElementEncodingType!(S))) cannot deduce template function from
> argument types
> !()(string)	D:\Documents\Projects\Test\Test\DPBall.d	11
> 
> 
> Am I using these functions incorrectly?

toStringz returns an immutable(char)*, so you can assign it to a const(char)* or an immutable(char)* but not a char*. If you want to assign to a char*, then you need to use toUTFz, which you are most definitely using incorrectly. The documentation gives several examples on how to use it correctly, but if what you want is a char*, then you'd do

this.filename = toUTFz!(char*)("test.bmp");

toUTFZ is a templated function which requires that you give it the type that you want to convert to. You were trying to call it without giving the type.

- Jonathan M Davis

Ok, got it all sorted. Thank you for the guidance.

SDL_LoadBMP is declared as:
SDL_Surface *SDL_LoadBMP(const char *file);
http://www.libsdl.org/cgi/docwiki.cgi/SDL_LoadBMP

So you don't need a mutable char*. I'd recommend using Derelict since it already has all these prototypes declared properly.