Template method inside a class - c++

Question

I have a class called time which has day, month and year.
I have a problem with returning the right value in my method, where it, depending on what we enter as a string "s" it should return an int value from one of those 3 fields.
So, for example, if I want to get days in my date I should call the function d["day"].
My question is, Is there something wrong with my code here? And, what should I put instead of
int operator[] (string s)
{
if (s == "day" || s == "month" || s == "year")
{
return ? ? ? ;
}
}

Answer 1

From the explanation, if I understood correctly, you need the following. You need to return appropriate member (i.e. either day or month or year) according to the string match. (Assuming that you have mDay, mMonth, and mYear as int eger members in your Date class)
int operator[] (std::string const& s)
{
if (s == "day") return mDay;
if (s == "month") return mMonth;
if (s == "year") return mYear;
// default return
return -1;
}
or alternatively using a switch statement
// provide a enum for day-month-year
enum class DateType{ day, month, year};
int operator[] (DateType type)
{
switch (type)
{
case DateType::day: return mDay;
case DateType::month: return mMonth;
case DateType::year: return mYear;
default: return -1;
}
}

Answer 2

A dimple way is to define the date as an array of three elements instead of declaring three separate data members.
In this case the operator can look the following way as it is shown in the demonstrative program below.
#include <iostream>
#include <string>
#include <iterator>
#include <algorithm>
#include <stdexcept>
class MyDate
{
private:
unsigned int date[3] = { 26, 12, 2019 };
public:
unsigned int operator []( const std::string &s ) const
{
const char *date_names[] = { "day", "month", "year" };
auto it = std::find( std::begin( date_names ), std::end( date_names ), s );
if ( it == std::end( date_names ) )
{
throw std::out_of_range( "Invalid index." );
}
else
{
return date[std::distance( std::begin( date_names ), it )];
}
}
};
int main()
{
MyDate date;
std::cout << date["day"] << '.' << date["month"] << '.' << date["year"] << '\n';
return 0;
}
The program output is
26.12.2019
Otherwise you should use either if-else statements or a switch statement within the operator.