Adding a structure into a std::map

Question

I have seen this question (What is the preferred/idiomatic way to insert into a map?) and can populate my map like this:

SPECIAL_EVENT_S sEvent{};

sEvent.datEvent = datEvent;
sEvent.strEvent = strDescription;
sEvent.strLocation = strLocation;
sEvent.iForeignLanguageGroupMenuID = dwForeignLanguageGroup;
sEvent.eSRREventType = static_cast<EventTypeSRR>(dwEventTypeSRR);
sEvent.eMWBEventType = static_cast<EventTypeMWB>(dwEventTypeSMR);

// Duration
sEvent.datEventStartTime = datEventStartTime;
sEvent.datEventFinishTime = datEventFinishTime;
sEvent.bEventAllDay = bEventAllDay;

// Reminder
sEvent.bSetReminder = bSetReminder;
sEvent.iReminderUnitType = iReminderUnitType;
sEvent.iReminderInterval = iReminderInterval;

// Videoconference events
sEvent.iImageWidthPercent = wImageWidthPercent;
sEvent.strImagePath = strImagePath;
sEvent.strTextBeforeImage = strTextBeforeImage;
sEvent.strTextAfterImage = strTextAfterImage;
sEvent.eType = eType;

m_mapSpecialEvents.insert(
    {
        datEvent.Format(_T("%Y-%m-%d")),
        sEvent
    }
);

Is it possible to insert the structure without having to create an actual SPECIAL_EVENT_S variable first? Can this be embedded into the insert function call?

SPECIAL_EVENT_S structure definition:

using SPECIAL_EVENT_S = struct tagSpecialEvent 
{
    COleDateTime    datEvent;
    CString         strEvent;
    CString         strLocation;

    // Foreign Language Group
    EventTypeSRR    eSRREventType{};
    EventTypeMWB    eMWBEventType{};
    int             iForeignLanguageGroupMenuID{};

    // Duration
    COleDateTime    datEventStartTime;
    COleDateTime    datEventFinishTime;
    BOOL            bEventAllDay{};

    // Reminder
    BOOL            bSetReminder{};
    int             iReminderUnitType{};
    int             iReminderInterval{};

    // Videoconference events
    int             iImageWidthPercent{};
    CString         strImagePath;
    CString         strTextBeforeImage;
    CString         strTextAfterImage;
    CChristianLifeMinistryDefines::VideoConferenceEventType eType{};
};

To reproduce

MFC Dialog app

Dialog header:

#include <map>

using SPECIAL_EVENT_S = struct tagSpecialEvent
{
    CString         strEvent;
};

using SpecialEventDataMap = std::map<CString, SPECIAL_EVENT_S>;

Add a private member variable:

private:
    SpecialEventDataMap m_mymap;

In the dialog OnInitDialog:

m_mymap.insert(_T("1234-56-78"), { .strEvent });

Will not work.

@n.1.8e9-where's-my-sharem. I know about `emplace`. The question remains the same. Thanks. — Andrew Truckle, Nov 11 '21 at 15:08
but `insert` and `emplace` do not require you to construct and copy `SPECIAL_EVENT_S `. You can just use initializer list, copies will be elided. — pptaszni, Nov 11 '21 at 16:04
@pptaszni for some reason I get a syntax error if I try to use the { ... } with my structure and attempt to access the variablles with .. — Andrew Truckle, Nov 11 '21 at 16:24

康桓瑋 · Accepted Answer · 2021-11-11T17:19:57.360

1

You can use C++20 designated initializers:

m_mapSpecialEvents.insert(
    {
        datEvent.Format(_T("%Y-%m-%d")),
        {
          .datEvent = datEvent,
          .strEvent = strDescription,
          .strLocation = strLocation,
          // ...
          .eType = eType
        }
    }
);

edited Nov 11 '21 at 17:19

answered Nov 11 '21 at 15:20

康桓瑋

33,481
5
40
90

Thanks. It does not like it when I try this approach. I have added the definition of my structure to the question, if it helps. – Andrew Truckle Nov 11 '21 at 15:42
This should work, can you provide error messages and a minimal reproducible example? – 康桓瑋 Nov 11 '21 at 16:38
See updated question. – Andrew Truckle Nov 11 '21 at 16:54
Note. At the point the . is typed the error is "expected an expression". – Andrew Truckle Nov 11 '21 at 17:01
If you use `.strEvent`, you should initialize it with some value. [demo](https://godbolt.org/z/Gdo6T7TTd). – 康桓瑋 Nov 11 '21 at 17:03
Ah, my question was taggs c++17.0. After I assigned a value I then got this: **error C7555: use of designated initializers requires at least '/std:c++20'**. When I changed it to c++20 it compiled. But I am not sure of impact to migrate from c++17 to c++20. – Andrew Truckle Nov 11 '21 at 17:07
Yes, this is a C++20 feature. In addition, gcc and clang have already supported this feature in very early versions, so it can even be compiled in C++11. – 康桓瑋 Nov 11 '21 at 17:10
In VIsual Studio it will not compile unless I use c++20. – Andrew Truckle Nov 11 '21 at 17:11
I think the answer needs to clarify that I need to compile with c++20 in Visual Studio for this to work. – Andrew Truckle Nov 11 '21 at 17:16
Unfortunately my main project will not compile when I change it to c++20. – Andrew Truckle Nov 11 '21 at 17:18
I get hundred of errors about alot of stuff it doesn't like. Beyond the scope of this question – Andrew Truckle Nov 11 '21 at 17:20
`/std:c++20` implies `/permissive-`. you can override it with `/permissive`. Unfortunately in VS 2019 it does not work if you set the corresponding property, it work if you set `/permissive` in Additional Options in Command Line of the C/C++ – Alex Guteniev Nov 11 '21 at 18:34

score 0 · Answer 2 · answered Nov 11 '21 at 21:27

0

You can default construct it in the map, and then populate:

auto& sEvent = m_mapSpecialEvents[datEvent.Format(_T("%Y-%m-%d"))];

sEvent.datEvent = datEvent; sEvent.strEvent = strDescription; sEvent.strLocation = strLocation;

answered Nov 11 '21 at 21:27

Alex Guteniev

12,039
2
34
79

Adding a structure into a std::map

To reproduce

2 Answers2