const char * question

HarryBoy · 02-02-2010, 04:15 AM

Hi, its been a while since I have worked with C strings.

I have a function that needs to return a const char*. The char will be 5 charachers long (4 + NULL).

Code:

const char* getname( void )
{
   const char *name = (const char *)malloc(5);//(4+NULL)

   int newfilenumber = 5;

   //Format the string so it ahs leading 0's e.g. '0005'
    sprintf(filename,"%04d\n",newfilenumber);

    return name;

}

But I get the error: invalid conversion from ‘const char*’ to ‘char*’
on the sprintf line.

What am I doing wrong??

Thanks

Aquarius_Girl · 02-02-2010, 04:27 AM

Code of OP:

Code:

sprintf(filename,"%04d\n",newfilenumber);

Where is this variable 'filename' declared ?

HarryBoy · 02-02-2010, 04:46 AM

Sorry a typo on my part.

the first line should be
[CODE]
const char *filename = (const char *)malloc(5);//(4+NULL)/[CODE]

carbonfiber · 02-02-2010, 04:52 AM

Do you understand what const means in const char *filename? Do you know the difference between it and char *const filename? From `man sprintf` on my system:

Code:

int sprintf(char *str, const char *format, ...);

HarryBoy · 02-02-2010, 04:55 AM

not really

verdeboy2k · 02-02-2010, 06:03 AM

Basically, it cant be a constant if you expect sprintf to modify it, cast it afterwards.

This should work:

Code:

const char* getname( void )
{
   char *filename = (char *)malloc(5);//(4+NULL)

   int newfilenumber = 5;

   //Format the string so it ahs leading 0's e.g. '0005'
    sprintf(filename,"%04d\n",newfilenumber);

    return (const char *)filename;

}

ForzaItalia2006 · 02-02-2010, 07:20 AM

Quote:

Originally Posted by carbonfiber

Do you understand what const means in const char *filename? Do you know the difference between it and char *const filename?

The const keyword means "constant" or in other words "read-only".

Though, if you define

Code:

const char *str;

Pointer to a constant/read-only character (or char array), though the memory where the pointer points to can't be modifed (at least NOT through this type)

Code:

char *str;

Pointer to a character (or char array), memory can be modified

Code:

char * const str;

Constant pointer to a character (or char array), meaning that the memory where the pointer points to can be modified, but the pointer itself can't be modified

And finally,

Code:

const char * const str;

Constant pointer to a constant/read-only character (or char array)

Hope that helps to understand the differences,

- Andi -