Question

I have a struct with a std::map of pointers inside it. I’m trying to do the following:

template <class T>
struct Foo
{
    std::map<std::string, T*> f;
    T& operator[](std::string s)
    {
        return *f[s];
    }
}

and then use it like this:

Foo<Bar> f;
f["key"] = new Bar();

but the way it’s written, it crashes the program. I also tried like this:

T* operator[](std::string s)
{
    return f[s];
}

but it doesnt compile. It says "lvalue required as left operand of assignment" on the f["key"] = new Bar() line.

I expected it to be easy since I’m trying to return a pointer and I’m storing a pointer. What is wrong with my code?

Answer 1

The correct way of doing this is:

T*& operator[](std::string s)
{
    return f[s];
}

and call it like f["key"] = new Bar().

EDIT: You should start passing non-basic types by const reference where you can:

T*& operator[](const std::string& s)