Optimize memory usage of wifi scan for REALTEK_RTW8195AM

[Archcady]

To prevent new and delete frequently, use placement new to create instance at local array.

[0xc0170]

cc @pan-

[0xc0170]

Looks good to me. One question, with this allocation during scanning. I see ap_num to be set to 0, means it always starts 0 and allocates new wifi objects all over again. Not needed to destroy the allocated objects, no clean-up?

[Archcady]

The allocations here create new WiFiAccessPoint instance at address of ap_details, and ap_detail actually is passed in as first paramerter res in RTWInterface::scan(WiFiAccessPoint *res, unsigned count). So there's no need to delete ap_details here.

[pan-]

@Archcady Thanks for this precision, that change the fix needed a bit: It is not a chunk of uninitialized memory which is passed to the function then used. scan_handler->ap_details is actually an array of initialized WiFiAccessPoint instances.

In such case, placement new is not the right approach, instead you can just use the copy assignment operator:

scan_handler->ap_details[scan_handler->ap_num] = WiFiAccessPoint(ap);

Just to give a bit more of context, usage of one or another depends on the type of memory and the type of instance to initialize:

• If the memory is not initialized and therefore untyped: use placement new and release it with an explicit call to the destructor. Beware of alignment constraints when allocating/reserving the memory block: new (memory_block) TypeToConstruct(...)

• If an instance has to be replaced by another one and the copy assignment operator is accessible. Use the copy assignment operator: old_instance = TypeToConstruct(...).

• If an instance has to be replaced by another one and the copy assignment operator is not accessible. Destroy explicitly the existing instance then use placement new to insert the new one:

// explicit destructor call on old_instance reference
old_instance.~TypeToConstruct(); 
// construct a new instance at old_instance place
new (&old_instance) TypeToConstruct(...)

[Archcady]

@pan- Thank you very much! I tested both approaches and they all pass the test. Despite that, I think using copy assignment is better.

[theotherjimmy]

/morph test

[studavekar]

/morph test

[mbed-bot]

Result: FAILURE

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/morph test

Output

mbed Build Number: 1008

Test failed!

[mbed-bot]

Result: SUCCESS

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/morph test

Output

mbed Build Number: 1014

All builds and test passed!