Strings, pointers and memory

BrianWren · April 24th, 2009, 04:14 PM

I am trying to use strings of characters where I do not know the lenght before hand; I need dynamic-length strings.

I don't understand why this fails:

Code:

Code                               My understanding
char *rep_nm = "ThisIsRepName"; // Allocates & fills memory. rep_nm
                                // holds the addr. of 1st char.
                                // String is \0 terminated.
char *msg = "";                 // Allocate & fills mem for the
                                // terminating \0.
msg = strcat(msg, "Test\n");    // Determines size of the 2 args,
                                // allocates new mem large enough
                                // for str1 & str2 & a \0.
                                // Should rtrn ptr to that addr.
msg = strcat(msg, rep_nm);

msg = strcat(msg, "Test\n") raises an access error. I presume I am running off the end of legitimate storage space, but I don't understand why.

BrianWren · April 28th, 2009, 10:51 AM

I found out (elsewhere) that strcpy and strcat do not allocate memory.

In C++, to do the actions I was trying to do, space for the strings should be allocated with new, and the memory thus obtained freed up with free when youâre done with it.

If it is to be dynamic, the required space would need to be ascertained in some way.

But also in C++, using the variable type string will eliminate many of the issues involved.

Finally, if you create a string with string myVar; to pass the value to functions that require a poiter to char, use myVar.c_str(). For instance, where you would use:

Code:

  char* myVar = "C:\\Temp\\MyFile.txt";
  FILE *fp    = {0};
 
  fp = fopen(fName.c_str(), "r");

you would use:

Code:

  string myVar;
  myVar    = "C:\\Temp\\MyFile.txt";
  FILE *fp = {0};
 
  fp = fopen(fName.c_str(), "r");