STL map: Access violatoin reading location

Question

Here is my code:

class Base
{
public:
    virtual void show() const = 0;
};

class Child : public Base
{
private:
    static const int i = 1;
public:
    virtual void show() const
    {
        cout << i;
    }
};

map<int, const Base &> myMap{
    { 0, Child() },
    { 1, Child() },
};

Base & b = Child();

int main()
{
    b.show();

    myMap.at(0).show(); // This provokes the error

    system("pause>NUL");
    return 0;
}

As you see, I'm trying to use a global(or static) data, which will call some virtual functions. When I test Base & b = Child(); and in main: b.show();, everything goes well.

But, if I use map like above, I will get an error:

0xC0000005: Access violatoin reading location 0x00000000.

I've tried to debug this code and I found that when it arrived myMap.at(0).show();, I got this:

It seems that the table of virtual functions is Unable to read...

Then I tried to use the pointer:
map<int, Base *> and {0, new Child()}.
This works.

So it seems that this error comes from temporary reference.

But I don't know why b works. b is a temporary reference too.
In my opinion, this map contains many b.
Why does b works whereas map doesn't work?

Answer 1

You have a map of references to temporaries. I'm surprised that even compiled

map<int, Base &> myMap{
    { 0, Child() },
    { 1, Child() },
};

Drop the reference and switch to unique_ptr

std::map<int, std::unique_ptr<Base>> myMap;

You can insert elements into this map using this method. This will prevent object slicing.

You do the same thing again here

Base & b = Child();

You cannot hold non-const references to temporary objects