C++ how to modify a struct using a call from another function

Question

I'm trying to understand how to write a function that can modify a struct. I want to add a day to my struct Date. However, the addOneDay function below doesn't work.

My goal is to addOneDay to my birthday, using a function. How do I manipulate the data in the struct to get addOneDay to work?

#include <iostream>

    using namespace std;

    struct Date
    {
        int year;
        int month;
        int day;
    };


    Date addOneDay(const Date& date);

    Date addOneDay(const Date& date)
    {
        Date rdate = date.day+1; /* <- this doesn't work */
        return rdate;
    };

void assignValues(Date& myBirthday)
{
    myBirthday.day = 27;
    myBirthday.month = 1;
    myBirthday.year = 1962;
}

main()
{
    Date x;
    assignValues(x);

    cout << x.month << "/" << x.day << "/" << x.year << endl;

    //addOneDay(x)
};

Can you post an error message? It saves people from having to compile it to see the error for themselves. — Paul Rooney
– Paul Rooney, Commented Mar 1, 2015 at 23:57
Sure, thing. The compiler says: No viable conversion from 'int' to 'Date' — Tim Elhajj
– Tim Elhajj, Commented Mar 2, 2015 at 0:29

Paul Rooney · Accepted Answer · 2015-03-02 00:36:17Z

3

This line is wrong. As you found out.

Date rdate = date.day+1; /* <- this doesn't work */

Because you are assigning an integer value to a date object and as it stands the compiler doesn't know how to do that conversion.

You need to assign the value back to the day field of Date like

date.day += 1;

This fails because date is const.

If you want the input parameter as const then you must return a new Date object. The one you passed in is const and cannot be altered.

Date addOneDay(const Date& date)
{ 
    Date d(date);
    d.day = date.day + 1;
    return d;
};

This inccurs the cost of a few temporary objects.

If you removed the const restriction, you could use the same date like so.

void addOneDay(Date& date)
{
    date.day += 1;       
};

edited Mar 2, 2015 at 0:36

answered Mar 2, 2015 at 0:15

Paul Rooney

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4 Comments

Tim Elhajj Over a year ago

Yes, I wanted to do it by returning a value.

Paul Rooney Over a year ago

then return a reference to your original date from addOneDay.

πάντα ῥεῖ Over a year ago

@PaulRooney "hen return a reference" Note that this won't work for the const Date& version again.

Paul Rooney Over a year ago

Absolutely. I was only suggesting that for the non const

πάντα ῥεῖ · Accepted Answer · 2015-03-02 00:53:06Z

3

"How do I manipulate the data in the struct to get addOneDay to work?"

It doesn't work, because you pass a const reference parameter, which actually means the parameter cannot be changed. The solution is to actually use the return value

Date addOneDay(const Date& date);

Date addOneDay(const Date& date) {
    Date rdate(date);
    rdate.day = rdate.day + 1;
    return rdate;
};

edited Mar 2, 2015 at 0:53

answered Mar 1, 2015 at 23:59

πάντα ῥεῖ

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3 Comments

Tim Elhajj Over a year ago

I couldn't get either of these to work in Xcode. It wants "date.day =+ 1".

πάντα ῥεῖ Over a year ago

@TimElhajj "It wants date.day =+ 1." Uhm, what? Are you sure we're talking about c++ and not objective-c or something? Also why did you accept the other answer then, which proposes exactly the same?

Tim Elhajj Over a year ago

I see now that I was mistaken. My apologies. When I attempted to implement this yesterday, I failed, but it was my fault in implementation, not anything you wrote.

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