Iterate through a char pointer from a struct - c

Question

So I'm trying to combine two strings together but I'm getting str is a read only value on the last line of the second while loop. Is their anyone can I do this without changing the function header?
Also String is a struct I created that has a *char called str.
String * String_Combine(String * tar, const String * src) {
//end of string
while (* tar->str != '\0') {
tar->str++;
}
//int i = 0;
//copy string
while (*source->str != '\0') {
*tar->str = *src->str;
*tar->str++;
*src->str++;
// i++;
}
return tar;
}

Answer 1

Copy the pointer before modifying it. I guess modifying tar->str may also be harmful because it will destroy the information that where the string starts.
String * String_Combine(String * tar, const String * src) {
char * tar_str = tar->str, * src_str = src->str;
//end of string
while (* tar_str != '\0') {
tar_str++;
}
//int i = 0;
//copy string
while (*src_str != '\0') { /* assuming that "source" is a typo and it should be "src" */
*tar_str = *src_str; /* with hope that there is enough writable buffer allocated */
tar_str++;
src_str++;
// i++;
}
//terminate string
*tar_str = '\0';
return tar;
}

Answer 2

Two things:
Make sure you allocate enough memory for tar->strto hold both tar->str and src->str
Store a pointer tar/src->str locally and iterate thought them so you wouldn't loose the pointer to original str;
I wrote a test case for you to understand it easily ;)
#include <iostream>
#include <string.h>
struct String {
char* str;
};
using namespace std;
String * String_Combine(String * tar, const String * src) {
// take a local copy of strs
char* tar_str = tar->str;
char* src_str = src->str;
//end of string
while (* tar_str != '\0') {
tar_str++;
}
//int i = 0;
//copy src string to tar string
while (*src_str != '\0') {
*tar_str = *src_str;
*tar_str++;
*src_str++;
// i++;
}
return tar;
}
int main()
{
String* src = (String*) malloc(sizeof(String));
src->str = new char[20];
strcpy(src->str, "World!");
String* tar = (String*) malloc(sizeof(String));
tar->str = new char[20];
strcpy(tar->str, "Hello ");
String* result = String_Combine(tar,src);
cout << result->str << endl;
return 0;
}