C++ Why didn't the default copy work? [duplicate] - c++

Question

This question already has answers here:
Why does the C++ map type argument require an empty constructor when using []?
(6 answers)
Closed 5 years ago.
I've done a lot of Googling and can't seem to figure out what's going on. I'm teaching myself C++ (I'm more familiar with Java).
I have Item Class objects that are being stored in an Inventory Class map, not as pointers. I want to retrieve one of the items from the Inventory in a function, assign it to a temp variable while I delete it from the Inventory map, and then return the object itself so something else can use it. When I originally tried using the code within my function it was returning the error (followed by the stack trace of c++ library stuff):
no matching constructor for initialization of 'Item'
::new ((void*)__p) _Tp();
I tried creating a copy constructor, but to no avail. Eventually, it worked by including an empty constructor ( Item(); ) in my header file and defining it in my cpp file ( Item::Item() {} ).
I would just like to understand why this was necessary so I can recognize it in the future to know what I'm doing.
EDIT: Upon further inspection of the error stack trace, it turned out the actual problem with with the Inventory::addItem function. When assigning an object to a map using operator[], the map first instantiates the value type to the key using the default constructor before making the assignment. No default constructor was available, so the error was returned.
It was fixed by changing the line to map.insert({key, value})
Here are the important parts of the two class files:
//item.h
#include <string>
using namespace std;
class Item {
private:
string name;
int type;
int levelReq;
public:
Item(string name, int type, int levelReq);
Item();
string getName() {return name;}
int getType() {return type;}
friend ostream &operator<<(ostream &out, const Item &item);
};
---------------------------------------------------------------
//item.cpp
#include <string>
#include "item.h"
using namespace std;
Item::Item(string n, int t, int l) : name(n), type(t), levelReq(l) {}
Item::Item() {}
ostream &operator<<(ostream &out, const Item &item) {
return out << item.name;
}
---------------------------------------------------------------
//inventory.h
#include <map>
#include "item.h"
class Inventory {
private:
map <int, Item> inventory;
int size;
bool full;
int nextFree;
void findNextFree();
public:
Inventory();
bool isFull() {return full;}
void addItem(Item item);
Item getItem(int slot);
void showInv();
};
---------------------------------------------------------------
//inventory.cpp
#include <iostream>
#include <string>
#include "inventory.h"
#include "item.h"
using namespace std;
Inventory::Inventory() {
full = false;
nextFree = 1;
size = 28;
}
void Inventory::addItem(Item item) {
if (!full) {
inventory[nextFree] = item;
findNextFree();
}
else {
cout << "Your inventory is full (Inv::addItem)";
}
}
Item Inventory::getItem(int slot) {
Item item = inventory.at(slot);
inventory.erase(slot);
full = false;
if (nextFree > slot) {
nextFree = slot;
}
return item;
}
void Inventory::findNextFree() {
nextFree++;
if (nextFree == size + 1) {
full = true;
}
else if (inventory.count(nextFree)) {
findNextFree();
}
}

Answer 1

I think the issue rose because you declared a constructor for your item class.
C++ will automatically generate the necessary constructors if you don't provide any custom constructors.
The necessary constructors are the default, copy and move constructors.
The moment you provide one, the default constructors won't be generated and you have this issue. This principle will also apply to structs.
Check the reference to see for yourself:
http://en.cppreference.com/w/cpp/language/default_constructor
http://en.cppreference.com/w/cpp/language/copy_constructor
http://en.cppreference.com/w/cpp/language/move_constructor
Hope this answers your question.