Add characters to a character array c++ - c++

Question

Can someone tell me what's wrong with the following?
I'm trying to add characters to a character array. name is a pointer to a character array in the MyString class.
void MyString::add_chars(char* c)
{
if(l < strlen(c)+strlen(name))
name = resize(name, l, sizeof(c));
int i,j;
for(i=0; i<strlen(c); i++) {
name[i+l-1] = c[i];
l++;
}
}
char* MyString::resize(char* vptr, int currentsize, int extra) {
char* temp = new char[currentsize + extra];
int i;
for (i = 0; i < currentsize; i++) {
temp[i] = vptr[i];
}
vptr = temp;
return vptr;
}
And in main:
MyString g ("and");
g.add_chars("baasdf");
cout << g.get_name() << "\n";
But get_name returns "andb". How can I fix my code?
Edit:
Updated code, still same result..
void StringList::add_chars(char* c)
{
char* my_new_string = resize(name, l, sizeof(char));
if( my_new_string != NULL )
{
delete [] name;
name = my_new_string;
}
int i,j;
for(i=0; i<strlen(c); i++) {
name[i+l-1] = c[i];
l++;
}
name[l-1] = '\0';
}
char* StringList::resize(char* vptr, int currentsize, int extra) {
char* temp = new char[currentsize + extra + 1];
int i;
for (i = 0; i < currentsize; i++) {
temp[i] = vptr[i];
}
vptr = temp;
return vptr;
}

Answer 1

This line is wrong:
name = resize(name, l, sizeof(c));
You should not take the sizeof(char*), which your c variable is, but you should do sizeof(char) or just 1.
Also, make sure that you do +1 on the size to take care of the zero termination char at the end of your string.

Answer 2

How can I fix my code?
Don't fix it. Throw it away and use vector<char> or just string.
But I insist, how can I fix my code!?
OK, OK, here is how...
Get a nice debugger, for example this one.
Step carefully through the code, constantly inspecting the variables and comparing them with what you expect them to be.
When you reach the call to resize, take note of sizeof(c) (assigned to extra parameter of resize). When you realize it is not what you expected, ask yourself: what is the purpose of sizeof, and you'll understand why.
BTW, you also have a memory leak and a very poor performance due all these strlens.

Answer 3

Firstly, am I right in assuming that this is a learning exercise for you in learning "how to create your own string class"? C++ has already got a built-in string type which you should always prefer for the most part.
the sizeof operator yields the size (in bytes) of its operand, which in this case is c whose type is char* - it looks like what you're actually after is the length of a null-terminated character array (a "C" string") - you're already using strlen, so I'd suggest you simply want to use that again. (taking a null-terminator into account too)
name = resize(name, l, strlen(c) + 1);
Note, that your code looks as if it suffers from memory leaks. You're assigning a new value to your name variable without clearing up whatever existed there first.
if(l < strlen(c)+strlen(name))
{
char* my_new_string = resize(name, l, strlen(c));
if( my_new_string != NULL )
{
delete [] name;
name = my_new_string;
}
}
EDIT: As other replies have pointed out, there's still plenty wrong with the code which could be resolved using C++'s string and vector.
Here's one possible way you could implement add_chars
void MyString::add_chars(char* c)
{
if( c != NULL && name != NULL )
{
size_t newlength = strlen(c) + strlen(name) + 1;
char* newstring = new char[newlength];
if( newstring != NULL )
{
size_t namelength = strlen(name);
size_t remaining = newlength - namelength;
strncpy( newstring, name, newlength );
strncpy( &newstring[namelength] , c, remaining );
delete [] name;
name = newstring;
}
}
}