Add new insert_or_assign method to Values

[varunagrawal]

This PR adds a new method insert_or_assign in support of #958.

Modern databases support the upsert method which tries to perform an update and, if a key doesn't exist, defaults to an insert.

insert_or_assign is a C++17 std::map method so this method aligns us better with that.

This simplifies this common pattern:

if (values.exists(key)) {
	values.update(key, value);
} else {
	values.insert(key, value);
}

to the more concise:

values.insert_or_assign(key, value);

[dellaert]

Already exists as tryInsert ?

[varunagrawal]

tryInsert doesn't perform an update if the key already exists. upsert will perform the update.

[dellaert]

I’m not in love with this. If we modify Values, I’d prefer to align it more with stl::map, especially the c++11 and c++17 versions.

It’s probably better to do a design-review these PRs to the core data structures via an issue that proposes a change, before spending a lot of time.

If you disagree strongly, let’s discuss off-line.

[varunagrawal]

Adding some research I did here on templated operator overloading, thanks to this link.
Apparently it is possible. We can define this:

template <typename T>
T& operator[](Key k) {
...
}

but the operator call would be ugly

Values values;
Pose3 pose;
values[key] = pose; // Compiler error due to template mismatch!!
values.operator[]<Pose3>(pose);  // Works, but is more verbose and defeats the purpose.

[dellaert]

Yeah, that’s really ugly. Did you read the entire c++17 interface to standard map though? Is there another function that already exists that does what the imperative operator does?

[varunagrawal]

I did. There's insert_or_assign which is exactly what upsert is. Should we rename upsert to that?

I guess the cpp standards committee likes verbose function names.

[dellaert]

Yeah. I like shorter names as well, but insert_or_assign will create less surprise for developers.

[varunagrawal]

Cool, I'll update the method name. :)

[varunagrawal]

@dellaert updated

[dellaert]

Nice !