Improve SingletonPtr const correctness

[c1728p9]

Make all the functions of SingletonPtr const for improved const
correctness.

[c1728p9]

/morph test

[c1728p9]

@mbed-bot: TEST

HOST_OSES=ALL
BUILD_TOOLCHAINS=GCC_ARM
TARGETS=K64F,NRF51_DK,NRF51_MICROBIT,NUCLEO_F411RE

[mbed-bot]

[Build 840]
SUCCESS: Building succeeded and tests were run! Be sure to check the test results

[pan-]

I think you have to provide non const overloads as well and makes the return type match the constness:

const T* get() const;
T* get();
const T* operator->() const;
T* operator->();

So users which use a const SingletonPtr<>& will only have access to its const member (data and functions) and user which use a non const version will have access to everything.

[mbed-bot]

Result: SUCCESS

Your command has finished executing! Here's what you wrote!

/morph test

Output

mbed Build Number: 714

All builds and test passed!

[geky]

Out of curiousity, what's the benefit of this over SingletonPtr<const Class>?

[pan-]

@geky I think people will usually write const SingletonPtr<Class> instead.
The form you write might be used in generic code where at some point SingletonPtr template is instantiated with T = const Class. But in that case, it is never handwritten.

[c1728p9]

@pan- I tested locally with the overrides you mention:

const T* get() const;
T* get();
const T* operator->() const;
T* operator->();

There are still compiler errors when I do this and have an object of type const SingletonPtr<Class>. The test I ran was:

const SingletonPtr<std::string> str;

which gave the error "uninitialized const 'str' [-fpermissive]"

Making the class const in the template parameter worked as expected, and get() and ->() operate with the correct constness. I'm closing this PR since it doesn't any value. If you have any suggestions to get const SingletonPtr<Class> working feel free to post here and I'll reopen the PR.

[geky]

I think having a const-correct SingletonPtr is marginally useful, it is what a user would first expect to do. But not much of a priority.