Appending to a char array that has free space

Question

struct FF{

   void append( char *str ){

         const int strLength = strlen( str );
    const int totalLength = strLength + size;
    char *tmp;
    if( freeSpace < strLength ){
        //not enough free space available for the str to append
        //allocate the total str length including the current size + free space
        tmp = new char[totalLength + 10];
        freeSpace = 10;

        //cpy old string and new string to tmp
        for( int i = 0; i < size; i++ ){
            tmp[i] = strVal[i];
        }
        for( int i = size; i < totalLength; i++ ){
            tmp[i] = str[i];
        }

        delete[] strVal;
        strVal = new char[totalLength+10];
        size = totalLength;
        strcpy( tmp, strVal );

    }else{
        for( int i = size; i <= totalLength; i++ ){
            strVal[i] = str[i];
        }

        freeSpace -= strLength;
        size += strLength;

    }


   }
   char *strVal;      
  unsigned int size;
  unsigned int freeSpace;

};


int main(){

FF a;
a.strVal = new char[10];
a.freeSpace = 10;
a.size = 0;

a.append( "str" ); // should have 7 bytes left
a.append( "len" ); // should have 4 bytes left

std::cout << a.strVal << std::endl; //prints str instead of strlen

return 0;
}

I want the strVal to have free space so I don't have to allocate space everytime I append something to it. However, the first append works just fine. But when I append it another time, it does not work. So at the end only str will be printed out.

I didnt read the script much, but if you use char* for strings, and you are using c++, consider using std::string — Creris
– Creris, Commented Nov 4, 2014 at 12:06
@Creris I know that std::string exists, this is for educational purpose only. — Davlog
– Davlog, Commented Nov 4, 2014 at 12:07
why not educate with std::string, it does automatic allocation, and you can just call myString.reserve(myString.size() + 10); — Creris
– Creris, Commented Nov 4, 2014 at 12:09
@Creris Do I know how that works ? No, so this won't help me understand how to implement a string with free space. — Davlog
– Davlog, Commented Nov 4, 2014 at 12:12
strcpy( tmp, strVal );... which is dest and which is source? — doctorlove
– doctorlove, Commented Nov 4, 2014 at 12:15

sachse · Accepted Answer · 2014-11-04 12:19:55Z

2

The problem is, that the else clause has to be changed to:

...
}else{
    for( int i = 0; i <= strLength; i++ ){
        strVal[i+size] = str[i];
    }

    freeSpace -= strLength;
    size += strLength;

}

answered Nov 4, 2014 at 12:19

sachse

311 bronze badge

Sign up to request clarification or add additional context in comments.

1 Comment

WhozCraig Over a year ago

As long as you're changing that else code (which is far from the only problem), may as well simply use std::copy(str, str+strLength+1, strVal+size)

Rene · Accepted Answer · 2014-11-04 12:35:26Z

1

You have a problem in the second for loop:
... for( int i = size; i < totalLength; i++ ){ tmp[i] = str[i]; }

Index i is okay for tmp, but not for str:
... for( int i = size; i < totalLength; i++ ){ tmp[i] = str[i<b> - size</b>]; }

Afterwards, you don't need to allocate a new buffer for strVal and copy from tmp (again): Simply assign tmp to strVal.
That get's rid of the strcpy(), where the order of parameters is wrong as already indicated by @doctorlove.

Finally, you have a memory leak: You allocate a char array for tmp but never release it. If you change the handling as described above, you get rid of that too.

answered Nov 4, 2014 at 12:35

Rene

2,4861 gold badge16 silver badges18 bronze badges

Collectives™ on Stack Overflow

Appending to a char array that has free space

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related