Implementing a Yearmonth class

Recently I wrote a program that was required to handle year and month data and so I wrote this class to encapsulate that handling. What I needed was a way to initialize the Yearmonth object based on the current local time, and allow a simple method of calculating future Yearmonth values based on a duration in months. This sample code illustrates how I use it:

ymtest.cpp

#include 
#include "Yearmonth.h"

int main()
{
    YM::Yearmonth ym;    // today
    std::cout << ym << '\n';
    ym += 2;         // 2 months from now
    std::cout << ym << '\n';
    ym += 14;        // test year increment
    std::cout << ym << '\n';
}

Yearmonth.h

#ifndef YEARMONTH_H
#define YEARMONTH_H
#include 

namespace YM {
class Yearmonth
{
public:
    // construct with today's year and month
    Yearmonth();
    // construct with year, month (1=Jan, 12=Dec)
    Yearmonth(unsigned ayear, unsigned amonth);
    // increment by given number of months
    Yearmonth &operator+=(const unsigned mon);
    // return year
    unsigned year() const;
    // return month (1=Jan, 12=Dec)
    unsigned month() const;
    // prints to ostream.  E.g. 2014 Dec ==> "201412"
    friend std::ostream& operator<<(std::ostream &out, const Yearmonth &ym);
private:
    unsigned myyear;
    unsigned mymonth;
};
}
#endif //YEARMONTH_H

Yearmonth.cpp

```
#include
#include "Yearmonth.h"

namespace YM {
Yearmonth::Yearmonth()
{
time_t tt;
time(&tt);
tm *t = localtime(&tt);
myyear = t->tm_year + 1900;
mymonth = t->tm_mon;
}

Yearmonth::Yearmonth(unsigned ayear, unsigned amonth)
: myyear(ayear), mymonth(amonth-1)
{
myyear += mymonth/12;
mymonth %= 12;
}

unsigned Yearmonth::year() const
{
return myyear;
}

unsigned Yearmonth::month() const
{
return mymonth+1;
}

Yearmonth &Yearmonth::operator+=(const unsigned mon)
{
mymonth += mon;
myyear += mymonth/12;
mymonth %= 12;
return *this;
}

std::ostream& operator<<(std::ostream &out, const Yearmont

I only have a few small remarks to make:

-
In your header file, don't include `: include instead which contains the forward declarations for every type in .

-
Also, you only use std::ostream in your source file, so you could simply include there.

-
Several times, you compute mymonth / 12 and mymonth % 12. If your class is designed to be used intensively, you could consider using std::div(mymonth, 12) which will compute both values at once and may therefore be slightly faster (if you really need it).

-
You may want to prefix time_t, time, tm and localtime with std::. Them coming from the C standard library doesn't prevent you to use std::.

-
Using a const unsigned parameter seems pretty useless. I don't have strong opinions on the const` on value parameters, but you could safely drop it since it adds little value.

StackExchange Code Review Q#87664, answer score: 4