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

[Jacopo Mondi]

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 ?

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)

> +	}
> +
> +	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
>