Cellular: CellularContext must provide access to CellularDevice

[jarvte]

Description

When using NetworkInterface::get_default_instance() application gets handle
to CellularInterface which is actually CellularContext derived from CellularInterface.
Application needs also handle to CellularDevice to open other interfaces.

Pull request type

[x] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

Reviewers

@AnttiKauppila @kivaisan

Release Notes

When using NetworkInterface::get_default_instance() application gets handle
to CellularInterface which is actually CellularContext derived from CellularInterface.
Application needs also handle to CellularDevice to open other interfaces.

[ciarmcom]

@jarvte, thank you for your changes.
@AnttiKauppila @kivaisan @ARMmbed/mbed-os-ipcore @ARMmbed/mbed-os-wan @ARMmbed/mbed-os-maintainers please review.

[jarvte]

Force pushed review fixes:

• removed virtual from Cellularcontext
• removed inherited methods from AT_CellularContext

[trowbridgec]

This is a great feature to add! Whenever working with the cell infrastructure, I always find it cumbersome to work between CellularContext and CellularDevice.

[jarvte]

pushed AT-specific device getter to AT_CellularContext as @kjbracey-arm suggested.

[AriParkkila]

Should getter be virtual so that an application doesn't need to know exact type?

[kjbracey]

Should getter be virtual so that an application doesn't need to know exact type?

(I don't think I'm talking nonsense here, but it's possible...)

The application doesn't need to know the type - if it is using CellularContext as its local static type, it will non-virtually call CellularContext::get_device and get a CellularDevice *.

If it's using AT_CellularContext as its local static type, it will non-virtually call AT_CellularContext::get_device and get an AT_CellularDevice *.

So we're just using whatever static type information we have at the call-site, and maintaining that static type information in the output.

The actual dynamic type we don't know, and can't determine anyway as RTTI is turned off, and either way you're getting a pointer to the same object - the two implementations do the same thing, so it doesn't matter which one you call.

An application using CellularContext * doesn't need to polymorphically invoke AT_CellularContext::get_device to get the correct result.

It's just a bit like the overloads:

• const char *strstr(const char *str, const char *target) and
• char *strstr(char *str, const char *target).

Both do the same thing, so it doesn't really matter which you call, but the static type of the output depends on the static type of the input.

[cmonr]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 13 of 13 test jobs passed
Build number : 1
Build artifacts