How to insert class objects to map correctly without pointers?

Question

this is my code:

LoggedUser.h

#pragma once
#include <string>

using std::string;

class LoggedUser {
private:
    string m_username;
public:
    LoggedUser(string username);
    string getUsername() const;
    bool operator==(LoggedUser loggedUser) const;
};

Room.h

#pragma once
#include <LoggedUser.h>
#include <string>
#include <vector>

using std::vector;
using std::string;

typedef struct RoomData {
    unsigned int id = 0;
    string name;
    unsigned int maxPlayers = 4;
    const unsigned int timePerPlay = 10;
    bool isActive = false;

    RoomData& operator=(const RoomData& roomData) {
        this->id = roomData.id;
        this->isActive = roomData.isActive;
        this->maxPlayers = roomData.maxPlayers;
        this->name = roomData.name;
        return *this;
    }
} RoomData;

class Room {
private:
    RoomData m_metadata;
    vector<LoggedUser> m_users;
public:
    Room(const Room& room);
    Room(RoomData roomData, LoggedUser loggedUser);
    int getUserIndex(const LoggedUser& loggedUser);
    void addUser(LoggedUser loggedUser);
    void removeUser(LoggedUser loggedUser);
    vector<LoggedUser> getAllUsers();

    unsigned int getId();
    string getName();
    unsigned int getMaxPlayers();
    bool getIsActive();
};

RoomManager.h

#pragma once
#include <map>
#include "Room.h"

class RoomManager {
private:
    std::map<unsigned int, Room> m_rooms;
public:
    void createRoom(LoggedUser loggedUser, RoomData roomData);
}

RoomManager.cpp

#include "RoomManager.h"

void RoomManager::createRoom(LoggedUser loggedUser, RoomData roomData)
{
    m_rooms[roomData.id] = Room(roomData, loggedUser);
}

The problem is in the file RoomManager.cpp in the function createRoom i'm trying to insert to the map<unsigned int, Room>, a room by his id, but i get this error "C2512 'Room::Room': no appropriate default constructor available"

now, i know i can fix this by inserting a pointer of Room instead of the object itself, but i want to know if there is a way to do it without pointers, just so i know what was the problem in my code. Thanks for everyone who helping.

Why not to use constructors for `Room` and others to handle implicit stuff and to use `createRoom` for explicit stuff? — rawrex, Jun 11 '21 at 08:20
@churill but i don't get it, look at the Room class, why do i need default constructor also, can you explain? — MrDakik, Jun 11 '21 at 08:24
if the key is not present in the dictionary operator[] adds it with a default value of the member object. That's why it requires Room to be default constructrable, however, if you use insert instead that requirement is relaxed (Room must be copy constructable). @rveerd answer is the correct one, see my comments below for a working example and a possible explanation of your error — Alessandro Teruzzi, Jun 11 '21 at 09:58

rawrex · Accepted Answer · 2021-06-11T10:41:18.333

2

In the m_rooms[roomData.id] = Room(roomData, loggedUser); statement you use the std::map::operator[], which...

...returns a reference to the value that is mapped to a key equivalent to (requested) key, performing an insertion if such key does not already exist.

Thus, we may use the operator only if the mapped_type has a default constructor. Since to insert a "fallback" a value into the map (if we access a key that is not in there yet) it will need to default construct one on the fly.

But since you did provide non-default constructors for Room, the compiler does not synthesizes the default one for you. Thus, the Room is not DefaultInsertable.

So we can handle this by simply introducing a defaulted default constructor:

class Room {
    // Other private members as before
public:
    // Other public members as before
    Room() = default;
};

Lesson learned: std::map::operator[] require mapped_type to be DefaultInsertable. For more, refer to the documentation.

edited Jun 11 '21 at 10:41

answered Jun 11 '21 at 08:27

rawrex

4,044
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ok.., that's fixed the problem, but why is that?, is the map needs a default constructor to sort the elements or something? and what the constructor = defualt, means? what if i create an object like that? – MrDakik Jun 11 '21 at 08:30
i just did a default constructor for the class RoomManager but still i get the error, so is the problem in the map itself that must has default constructors? – MrDakik Jun 11 '21 at 08:51
1

@TiZ_Crocodile You need a default constructor in `Room`, as in the answer. – rawrex Jun 11 '21 at 08:52
@TiZ_Crocodile `RoomManager` already has synthesized default constructor, as long as you don't introduce any non-default constructor yourself. – rawrex Jun 11 '21 at 08:54
The lesson learned is wrong: #include struct Foo { double x; Foo(double v) : x(v){}; }; int main() { std::map m; m.insert(std::make_pair(1,Foo(12.3))); } – Alessandro Teruzzi Jun 11 '21 at 09:43
@AlessandroTeruzzi already looking into the question! – rawrex Jun 11 '21 at 09:52
@AlessandroTeruzzi yeah, it is the `[]` that requires that. Just derived that from the docs, while the other answer literally says the same! – rawrex Jun 11 '21 at 10:16
@AlessandroTeruzzi thanks for pointing out! Edited in the correct answer. The research was priceless! – rawrex Jun 11 '21 at 10:31

score 2 · Answer 2 · answered Jun 11 '21 at 08:46

2

You are using operator[] to insert an item into the std::map. This operation requires that the inserted item is DefaultConstructible 1.

The Room class is not DefaultConstructibel, as explained in the other answer by @rawrex.

If you cannot or do not want to add a default constructor, you can also use the insert() function to insert an item. This function does not have the above requirement.

m_rooms.insert(std::make_pair(roomData.id, Room(roomData, loggedUser)));

answered Jun 11 '21 at 08:46

rveerd

3,620
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@churill not with template, this should be the accepted answer – Alessandro Teruzzi Jun 11 '21 at 09:42
@TiZ_Crocodile are you using an old visual studio compiler? I had problem with std::pair requiring the default constructor in C++03 https://stackoverflow.com/questions/51282596/vs2008-sp1-no-appropriate-default-constructor-available-when-pushing-a-pair-int – Alessandro Teruzzi Jun 11 '21 at 09:50

How to insert class objects to map correctly without pointers?

2 Answers2