void to_lower(const char* s)
{
	if (!s) return; //if s doesn't point to anything
	int size = 5;
	char* new_cstring = new char[size];
	for (int i = 0; i < size; new_cstring[i] = 0, ++i); //initialize array with 0
	int counter = 0;
	//checks if there are still characters in the array
	while (*s)
	{
		//if it's an UPPERCASE letter
		if((int)*s >= 65 && (int)*s <= 90)
		{
			new_cstring[counter] = ((int)*s + 32); //changes this letter to lowercase
		}
		else
		{
		new_cstring[counter] = *s;
		}
		++counter;
		++s;
		if (counter == size - 1)
		{
			//creating new array to store contents of new_cstring
			char* arr = new char[counter];
			for (int i = 0; i < counter; arr[i] = 0, ++i); //initializes arr with 0
			int i = counter;
			for (; i >= 0; arr[i] = new_cstring[i], --i); //copies from old array to new one
			new_cstring = new char[counter + 10]; //creates new bigger array
			for(int i = 0; i < (counter + 10); new_cstring[i] = 0, ++i); //initialize new_string with 0
			for (int i = 0; i < counter; new_cstring[i] = arr[i], ++i);//copies from arr to new_cstring
			size = counter + 10;
			//delete[] arr;
		}
	}
	s = new_cstring;
 
}

void to_lower(const char*& s)

char s[] = "Test";
const char* p = s;
to_lower(p);
cout << p << endl;

#include <algorithm>
#include <cctype>
#include <iostream>
#include <string>
 
int lowercase ( int c )
{
    return std::tolower ( (unsigned char)c );
}
 
int main()
{
    std::string foo ( "THIS IS A TEST" );
 
    std::cout<< foo <<'\n';
    std::transform ( foo.begin(), foo.end(), foo.begin(), lowercase );
    std::cout<< foo <<'\n';
 
    std::cin.get();
}