How can I modify the value of a string defined in a struct?

Question

Hi,

Based on your comments, let me modified the original question...

I want to create a struct with size of 4kb (this size is a requirement so I have to meet it). The problem was that I couldn't modify the value of the string variable contained in the struct because the compiler throws a segmentation fault. Currently, if I use a pointer to string instead of a string variable, I now know how to do it (thanks to you guys), however, I read that the way that I'm using to allocate the 4kb of memory (malloc) is not the best or the appropriate. If I use the "new" keyword, it dynamically allocates enough memory for the struct and it probably uses a different value than 4kb, right? and this is what I don't want.

I still have the doubt about why I could not modify the value of a string variable (not pointer) contained in my struct (something like paginas -> dato = "test"). It probably should be a consequence of the use of malloc

Anyway, I would really appreciate your advices about how to allocate the 4kb of memory.

The original code in c++ is the following:

#define TAM 4000
#define NUMPAGS 512

struct pagina 
{
   bitset<12> direccion; 
   char operacion; 
   char permiso;
   string *dato; //I prefer to have a string variable
   int numero;
}; 

void crearPagina(pagina* pag[], int pos, int dir) 
{
 pagina * paginas = (pagina*)malloc(sizeof(char) * TAM); 
 paginas -> direccion = bitset<12> (dir);
 paginas -> operacion = 'n';
 paginas -> permiso = 'n'; 
 string **tempDato = &paginas -> dato;
 char *temp = " ";
 **tempDato = temp;
 paginas -> numero = 0;
 pag[pos] = paginas;  
}

Thanks in advance!!!

Answer 1

dato is a pointer to a string. That is, it tells you where to look for the string, but it doesn't actually contain the string.

So if you wish to assign a string to it, you must allocate the memory to hold the string yourself, and give the pointer to that memory to the struct to hold for you.

pagina->dato = new string("test");

When you are finished with the struct, you will need to remember to release the memory for the string or you will get a "memory leak" (because, again, the struct is not responsible for the memory containing the string, only for remembering where you put it):

delete pagina->dato;
pagina->data = NULL;

Answer 2

struct pagina 
{
   bitset<12> direccion; 
   char operacion; 
   char permiso;
   std::string dato;
   int numero;
   pagina(int dir)
     : direccion(dir), operation('n'), permiso('n'), dato(" "), numero()
   {
   }
};

void crearPagina(std::vector<pagina>& pag, int dir) 
{
    pag.push_back(pagina(dir));
}

Answer 3

struct pagina 
{
   pagina(int dir)
         : direccion(dir)
         , operacion('n')
         , permiso('n')
         , dato(" ")
         , numero(0) {}

   bitset<12>  direccion; 
   char        operacion; 
   char        permiso;
   std::string dato;
   int         numero;
};

void createPagina(pagina* pag[], int pos, int dir) {
    pag[pos] = new pagina(dir);
}

This is more C++ (as oppose to C) way of doing what you want. Create a constructor instead of creation procedure and use new instead of malloc. Also I would advice using std::vector<pagina> instead of raw pointer array.