如何将结构体直接嵌入insert调用插入std::map无需提前声明变量

我已查阅过相关问题，目前我通过如下方式填充我的map：

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
    }
);

我能否不先创建实际的SPECIAL_EVENT_S变量，直接将该结构体嵌入insert函数调用中完成插入操作？

SPECIAL_EVENT_S结构体定义如下：

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{};
};

复现步骤 #

MFC对话框应用
对话框头文件代码：

#include <map>

using SPECIAL_EVENT_S = struct tagSpecialEvent
{
    CString         strEvent;
};

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

添加私有成员变量：

private:
    SpecialEventDataMap m_mymap;

在对话框的OnInitDialog方法中添加如下代码：

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

上述代码无法正常运行。

解答 #

你写的代码报错有两个核心原因：

1. std::map::insert没有接收键、值两个单独参数的重载，你需要把键值对包装成std::pair或者用花括号包裹的{键, 值}形式传参
2. 你用的C++版本不支持你写的这种不完整的指定初始化语法，指定初始化需要给成员显式赋值，正确写法是.strEvent = 你要赋值的内容

你要的直接嵌入结构体插入的写法完全可以实现，适配你场景的代码示例如下：

m_mapSpecialEvents.insert(
    {
        datEvent.Format(_T("%Y-%m-%d")),
        {
            datEvent,
            strDescription,
            strLocation,
            static_cast<EventTypeSRR>(dwEventTypeSRR),
            static_cast<EventTypeMWB>(dwEventTypeSMR),
            dwForeignLanguageGroup,
            datEventStartTime,
            datEventFinishTime,
            bEventAllDay,
            bSetReminder,
            iReminderUnitType,
            iReminderInterval,
            wImageWidthPercent,
            strImagePath,
            strTextBeforeImage,
            strTextAfterImage,
            eType
        }
    }
);

如果你用的是C++20及以上版本，还可以用指定初始化让代码可读性更高，不需要严格按结构体成员声明顺序赋值：

m_mapSpecialEvents.insert(
    {
        datEvent.Format(_T("%Y-%m-%d")),
        {
            .datEvent = datEvent,
            .strEvent = strDescription,
            .strLocation = strLocation,
            .iForeignLanguageGroupMenuID = dwForeignLanguageGroup,
            .eSRREventType = static_cast<EventTypeSRR>(dwEventTypeSRR),
            .eMWBEventType = static_cast<EventTypeMWB>(dwEventTypeSMR),
            .datEventStartTime = datEventStartTime,
            .datEventFinishTime = datEventFinishTime,
            .bEventAllDay = bEventAllDay,
            .bSetReminder = bSetReminder,
            .iReminderUnitType = iReminderUnitType,
            .iReminderInterval = iReminderInterval,
            .iImageWidthPercent = wImageWidthPercent,
            .strImagePath = strImagePath,
            .strTextBeforeImage = strTextBeforeImage,
            .strTextAfterImage = strTextAfterImage,
            .eType = eType
        }
    }
);

你复现用的最小示例修改后可以正常运行的写法如下：

m_mymap.insert({ _T("1234-56-78"), { .strEvent = _T("测试事件内容") } });

如果你的编译器不支持C++20的指定初始化，就按成员声明顺序赋值即可：

m_mymap.insert({ _T("1234-56-78"), { _T("测试事件内容") } });

内容的提问来源于stack exchange，提问作者Andrew Truckle