Remove substrings from a C-style string "in place" in C++ code - c++

Question

I had an interview task to remove substring from a current string without using string functions or additional memory...I tried just with strlen but didn't find the way to change it in place...
#include <stdio.h>
#include <string.h>
#include <iostream>
using namespace std;
int main()
{
char * str = "this that there";
char * substr = "th";
removeSubstr(str,substr);
cout<<str; //should be now "is at ere"
return 0;
}
void removeSubstr(char * str, const char * substr){
}

Answer 1

Since you remove characters from the original string, the string is shrinking, so you do not need additional space. You just have to copy characters from a higher index (source) to a lower index (destination).
If the source index points to a position that starts with the searched substring, you must skip it.
The skip function simply compares the beginning of the source string against the substring, and returns the source (if it does not start with the substring) or the source plus the length of substring (if it starts with the substring).
Demo
#include <iostream>
char* skip_if( char* s, const char* ss )
{
char* p = s;
const char* pp = ss;
while( *p == *pp && *p )
p++, pp++;
return *pp ? s : p;
}
void remove( char* s, const char* ss )
{
char *ps = s; // source
char *pd = s; // destination
while( *ps )
{
ps = skip_if( ps, ss );
*pd++ = *ps++;
}
*pd = 0;
}
int main()
{
using namespace std;
char str[] = "this that there this that there";
const char* substr = "th";
remove( str, substr );
cout << str;
return 0;
}

Answer 2

The string functions exist for a reason. If you are not to use them, can you create them from scratch?
If so, here is my solution to the problem. Very educational to write custom string functions with the const requirements.
As mentioned in the comments, this won't work unless the string passed in can be modified, so I made it a non-constant string.
Let me know if this meets the interview challenge:
#include <iostream>
bool myStrnCmp(char *str1, const char *str2, int len) {
char *cptr1 = (char *) str1;
char *cptr2 = (char *) str2;
for (int i = 0; i < len; i++) {
if (*(cptr1 + i) != *(cptr2 + i))
return false;
}
return true;
}
int mystrlen(const char* str) {
int i = 0;
while(*(str + i) != '\0')
i++;
return i;
}
int findSubStr(char *str, const char *substr) {
int position = 0;
int len = mystrlen(substr);
while(*(str + position) != '\0') {
for (int i = 0; i < len; i++) {
if (myStrnCmp(str + position + i, substr, len))
return position + i;
}
++position;
}
return -1;
}
void myStrCpy(char *str, const char *substr) {
memmove(str, substr, mystrlen(substr) + 1);
}
void removeSubstr(char *str, const char *substr) {
int position = findSubStr(str, substr);
while(position >= 0) {
myStrCpy(str + position, str+position+mystrlen(substr));
position = findSubStr(str, substr);
}
}
int main() {
char str[]{"this that there"};
char* substr = "th";
removeSubstr(str,substr);
std::cout<<str; //should be now "is at ere"
return 0;
}