Перегрузка функций

#ifndef _STRING2_H_
#define _STRING2_H_

#include <iostream>
#include <cstring>
#include <cctype>

class String
{
public:
    explicit String(const char* c = "");
    String(const String& s);
    ~String();
    const String& operator =(const String& s);
    const String& operator +(const String& s);
    void to_lower();
    void show() const;
    size_t size() const;
private:
    char* line;
    size_t len;
};

#endif  // _STRING2_H_

#include "String2.h"

String::String(const char* c)
{
    len = strlen(c);
    line = new char[len + 1];
    strcpy(line, c);
}
String::String(const String& s)
{
    len = s.len;
    line = new char[len + 1];
    strcpy(line, s.line);
}
String::~String()
{
    delete [] line;
}
const String& String::operator =(const String& s)
{
    if (this == &s) return *this;
    delete [] line;
    len = strlen(s.line);
    line = new char[len + 1];
    strcpy(line, s.line);
    return *this;
}
const String& String::operator +(const String& s)
{
    size_t len_sum = len + s.len;
    char* temp = new char[len + 1];
    strcpy(temp, line);
    delete [] line;
    line = new char[len_sum + 1];
    strcpy(line, temp);
    strcat(line, s.line);
    delete [] temp;
    len = strlen(line);
    return *this;
}
void String::to_lower()
{
    for (size_t i = 0; i < len; ++i)
    {
        line = tolower(line);
    }
}
void String::show() const
{
    std::cout << line << std::endl;
}
size_t String::size() const
{
    return len;
}

#include <iostream>
#include "String2.h"

int main()
{
    String A("If I fell in love with you ");
    String B(A);
    A.show();
    B.show();
    String C("Could you promise to be true");
    C.show();
    String D;
    D = B + C;
    D.show();
    D.to_lower();
    D.show();
    return 0;
}