Перегрузка бинарного -, ошибка: undefined reference to operator -(.)

class tDoubleString
{
public:
    char* str;
    size_t number_symvols;
 
public:
    tDoubleString();//стандартный конструктор
    friend tDoubleString operator -(tDoubleString& left,tDoubleString& right);
    ~tDoubleString();
 
};
 
tDoubleString::tDoubleString()
{
    str =new char[256];
    str[0] ='\0';
    number_symvols =0;
}
 
tDoubleString operator -(const tDoubleString& left, const tDoubleString& right)
{//ну вот что с ним не так?
tDoubleString temp;
    double d = atof(left.str) -atof(right.str);
    sprintf(temp.str, "%f",d);
    temp.number_symvols =strlen(temp.str);
return temp;
}
 
tDoubleString::~tDoubleString()
{
    if(str)
        delete str;
}

int main()
{
    tDoubleString test1,test2,test3;
    test3 =test2-test1;
    cout << "Hello world!" << endl;
    return 0;
}

tDoubleString& operator =(const tDoubleString& right);

tDoubleString& tDoubleString:: operator =(/*неявно: tDoubleString* this */const tDoubleString& right)
{
    if(this ==&right)
        return *this;
    strcpy(this->str, right.str);
    this->number_symvols =right.number_symvols;
return *this;
}

friend tDoubleString operator-(const tDoubleString& left, const tDoubleString& right);

class tDoubleString
{
public:
    char* str;
    size_t number_symvols;
public:
tDoubleString();//стандартный конструктор
~tDoubleString();
friend tDoubleString operator -(const tDoubleString& left,const tDoubleString& right);
};
 
tDoubleString::tDoubleString()
{
    str =new char[256];
    str[0] ='\0';
    number_symvols =0;
}
 
tDoubleString::~tDoubleString()
{
    if(str)
        delete str;
}
 
tDoubleString operator -(const tDoubleString& left, const tDoubleString& right)
{
tDoubleString temp;
    double d = atof(left.str) -atof(right.str);
    sprintf(temp.str, "%f",d);
    printf("numbers: %s %s",left.str, right.str);
    printf("temp is %s",temp.str);
    temp.number_symvols =strlen(temp.str);
return temp;
}