for(unsigned int i = 0; i < num_characters; i++)
        _value[i] = data[i];

#ifndef _String_h_
#define _String_h_
 
#include <cstdlib>
 
#ifndef NULL
#define NULL 0
#endif
 
class String {
	public:
		String();
		String(const char *init_value);
 
		unsigned int getNumCharacters();
		bool isEmpty();
 
		const char *toString();
 
		void append(const char *data);
		char charAt(unsigned int index);
 
		void toUpperCase();
		void toLowerCase();
 
		void replaceData(const char *data);
		void replaceChar(char oldChar, char newChar);
		void clear();
 
	private:
		char *_value;
		unsigned int _num_characters;
};
 
#endif

#include "string.h"
 
String::String() {
	_value = (char *)calloc(1, 1);
	_num_characters = 1;
}
 
String::String(const char *init_value) {
	unsigned int num_characters = 0;
 
	while(init_value[num_characters] != (char)NULL)
		num_characters += 1;
 
	_num_characters = num_characters;
	_value = (char *)malloc(num_characters + 1);
	_value[num_characters] = (char)0;
 
	for(unsigned int i = 0; i < num_characters; i++)
        _value[i] = init_value[i];
}
 
unsigned int String::getNumCharacters() {
	return _num_characters;
}
 
bool String::isEmpty() {
	return _num_characters == 1;
}
 
const char *String::toString() {
	char *result = (char *)malloc(_num_characters + 1);
 
    result[_num_characters + 1] = (char)NULL;
 
	for(unsigned int i = 0; i <= _num_characters; i++)
		result[i] = _value[i];
 
	return result;
}
 
void String::append(const char *data) {
	unsigned int num_data_characters = 0;
	unsigned int new_num_characters;
 
	while(data[num_data_characters] != (char)NULL)
		num_data_characters += 1;
 
	realloc(_value, _num_characters + num_data_characters + 1);
 
	_value[_num_characters + num_data_characters] = (char)NULL;
	new_num_characters = _num_characters + num_data_characters;
 
	for(unsigned int i = 0; i < num_data_characters; i++)
        _value[i] = data[i];
 
	_num_characters = new_num_characters;
}
 
char String::charAt(unsigned int index) {
	return _value[index];
}
 
void String::toUpperCase() {
	for(unsigned int i = 0; i <= _num_characters; i++) {
		if(_value[i] >= (char)97 && _value[i] <= (char)122)
			_value[i] -= (char)32;
	}
}
 
void String::toLowerCase() {
	for(unsigned int i = 0; i <= _num_characters; i++) {
		if(_value[i] >= (char)65 && _value[i] <= (char)90)
			_value[i] += (char)32;
	}
}
 
void String::replaceData(const char *data) {
    unsigned int num_characters = 0;
 
	while(data[num_characters] != (char)NULL)
		num_characters += 1;
 
    _num_characters = num_characters;
	realloc(_value, num_characters + 1);
	_value[num_characters] = (char)0;
 
	for(unsigned int i = 0; i < num_characters; i++)
        _value[i] = data[i];
}
 
void String::replaceChar(char oldChar, char newChar) {
	for(unsigned int i = 0; i <= _num_characters; i++) {
		if(_value[i] == oldChar)
			_value[i] = newChar;
	}
}
 
void String::clear() {
	realloc(_value, 1);
	_value[0] = (char)NULL;
	_num_characters = 1;
}

#include <iostream>
 
#include "string.h"
 
int main(int argc, char *argv[]) {
	String cadena = "";
 
	cadena.replaceData("Hola, soy un programador :'D");
 
	std::cout << cadena.toString();
 
	std::cin.get();
	return 0;
}