[PATCH v1 2/3] libcamera: controls: Implement move ctor/assignment

[Barnabás Pőcze]

Hi


2025. 04. 18. 12:33 keltezéssel, Jacopo Mondi írta:
> Hi Barnabás
> 
> On Thu, Apr 17, 2025 at 01:35:38PM +0200, Barnabás Pőcze wrote:
>> Implement a move constructor and move assignment operator for `ControlValue`.
>> The `ControlValue` type already has an "empty" state that it used when
> 
> s/it used/is used/
> 
>> creating a default constructed `ControlValue`, so have the moved-from
>> instance return to that state after move construction/assignment.
>>
>> This is useful, for example, for `std::vector` as most implementations will
>> use the copy constructor when reallocating if no nothrow move constructor
>> is available. Having a nothrow move constructor avoids the extra copies.
>> It is also useful when using temporaries of `ControlValue` with other
>> containers such as `std::optional`, and it also makes `ControlInfo`
>> "cheaply" move constructible/assignable.
>>
>> Signed-off-by: Barnabás Pőcze <barnabas.pocze at ideasonboard.com>
>> ---
>>   include/libcamera/controls.h | 28 ++++++++++++++++++++++++++++
>>   src/libcamera/controls.cpp   | 17 +++++++++++++++++
>>   2 files changed, 45 insertions(+)
>>
>> diff --git a/include/libcamera/controls.h b/include/libcamera/controls.h
>> index 1dc6ccffa..86245e7a9 100644
>> --- a/include/libcamera/controls.h
>> +++ b/include/libcamera/controls.h
>> @@ -14,6 +14,7 @@
>>   #include <stdint.h>
>>   #include <string>
>>   #include <unordered_map>
>> +#include <utility>
>>   #include <vector>
>>
>>   #include <libcamera/base/class.h>
>> @@ -165,6 +166,33 @@ public:
>>   	ControlValue(const ControlValue &other);
>>   	ControlValue &operator=(const ControlValue &other);
>>
>> +	ControlValue(ControlValue &&other) noexcept
>> +		: type_(other.type_),
>> +		  isArray_(std::exchange(other.isArray_, false)),
>> +		  numElements_(other.numElements_),
>> +		  storage_(std::exchange(other.storage_, {}))
> 
> So here, compared to the copy connstructor that does a memcpy on the
> (now renamed) storage_.external, while here we're only copying the
> pointer to the allocated memory, right ?

Yes, the pointer (or inline value) is copied (the ownership of the
allocated memory is transferred to the `this` instance), and the other
is left in an "empty" state.


> 
> Seems sane, I wonder if there's a reason why this hadn't been done
> already.
> 
>> +	{
>> +		other.type_ = ControlTypeNone;
>> +		other.numElements_ = 0;
> 
> I presume there's a reason why you haven't defined this using the
> newly introduced operator=(&&) (the same as it is done for the copy
> constructor)

That can lead to suboptimal code in my experience (especially here since
`release()` cannot be inlined), if we wanted to avoid the repetition,
then I would replace `operator=` with the following:

   ControlValue(std::move(other)).swap(*this);
   return *this;


Regards,
Barnabás Pőcze

> 
>> +	}
>> +
>> +	ControlValue &operator=(ControlValue &&other) noexcept
>> +	{
>> +		if (this != &other) {
>> +			release();
>> +
>> +			type_ = other.type_;
>> +			isArray_ = std::exchange(other.isArray_, false);
>> +			numElements_ = other.numElements_;
>> +			storage_ = std::exchange(other.storage_, {});
>> +
>> +			other.type_ = ControlTypeNone;
>> +			other.numElements_ = 0;
>> +		}
>> +
>> +		return *this;
>> +	}
>> +
>>   	ControlType type() const { return type_; }
>>   	bool isNone() const { return type_ == ControlTypeNone; }
>>   	bool isArray() const { return isArray_; }
>> diff --git a/src/libcamera/controls.cpp b/src/libcamera/controls.cpp
>> index d384e1ef7..885287e71 100644
>> --- a/src/libcamera/controls.cpp
>> +++ b/src/libcamera/controls.cpp
>> @@ -155,6 +155,23 @@ ControlValue &ControlValue::operator=(const ControlValue &other)
>>   	return *this;
>>   }
>>
>> +/**
>> + * \fn ControlValue::ControlValue(ControlValue &&other) noexcept
>> + * \brief Move constructor for ControlValue
>> + * \param[in] other The ControlValue object to move from
>> + *
>> + * Move constructs a ControlValue instance from \a other. After this opreation
> 
> s/opreation/operation
> 
>> + * \a other will be in the same state as a default constructed ControlValue instance.
> 
> Can easily fit in 80 cols
> 
> With minors fixed, and unless there are reasons I'm missing why we
> didn't add it some time ago:
> 
> Reviewed-by: Jacopo Mondi <jacopo.mondi at ideasonboard.com>
>> + */
>> +
>> +/**
>> + * \fn ControlValue &ControlValue::operator=(ControlValue &&other) noexcept
>> + * \brief Move assignment operator for ControlValue
>> + * \param[in] other The ControlValue object to move from
>> + *
>> + * \sa ControlValue::ControlValue(ControlValue &&other)
>> + */
>> +
>>   /**
>>    * \fn ControlValue::type()
>>    * \brief Retrieve the data type of the value
>> --
>> 2.49.0
>>