Question

For my home work I need to write String object in C++.

One of the methods is

void concatenate(String *s)

But when I check my object with Valgrind, there is a memory leak in my code. Here is method:

// add s's str to this _str
void String::concatenate(String *s)
{
  char * conc;
  int conc_size, i, j;

  conc_size = _len + s->_len;

  conc = new char[conc_size];  // line 39

  for (i = 0; i < _len; i++)
    conc[i] = _str[i];

  for (j = 0; i < conc_size || j < s->_len; i++, j++)
    conc[i] = s->_str[j];

  _str = conc;   // i'm assuming the problem is here
  _len = conc_size;
}

Here is Valgrind message:

==4706== 3 bytes in 1 blocks are definitely lost in loss record 2 of 9
==4706==    at 0x100024679: malloc (vg_replace_malloc.c:266)
==4706==    by 0x10007BF04: operator new(unsigned long) (in /usr/lib/libstdc++.6.0.9.dylib)
==4706==    by 0x10007BF96: operator new[](unsigned long) (in /usr/lib/libstdc++.6.0.9.dylib)
==4706==    by 0x100000E7E: String::concatenate(String*) (String.cpp:39)
==4706==    by 0x100001479: main (main.cpp:27)

Works with delete [] _str; and changed constructors:

// the empty string.
String::String()
{
    _len = 1;

    _str = new char[_len];
    assert(_str);
    _str[0] = '\0';
}

// store s string in this
String::String(char *s)
{
    _len = 0;

    int i = 0;
    while(s[i] != '\0')
    {
        _len++;
        i++;
    }

    _str = new char[_len];
    assert(_str);

    for (i = 0; s[i] != '\0' && i < _len; i++)
        _str[i] = s[i];

}

Answer 1

You’re not dealocating the previous contents of your member _str. I’m assuming that’s also a char*. So when you’re telling _str to point elsewhere, the memory it previously pointed to will be unacessible.

delete[] _str;
_str = conc;