[PATCH v3 02/16] libcamera: matrix: Make most functions constexpr

[Kieran Bingham]

Quoting Stefan Klug (2025-04-03 16:49:07)
> By zero-initializing the data_ member we can make most functions
> constexpr which will come in handy in upcoming patches. Note that this
> is due to C++17. In C++20 we will be able to leave data_ uninitialized
> for constexpr.  The Matrix(std::array) version of the constructor can
> not be constexpr because std::copy only became constexpr in C++20.
> 
> Signed-off-by: Stefan Klug <stefan.klug at ideasonboard.com>
> Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>

Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>

> 
> ---
> 
> Changes in v2:
> - Added this patch
> 
> Changes in v3:
> - Added comment on data_ initializer
> ---
>  include/libcamera/internal/matrix.h | 20 ++++++++++++--------
>  1 file changed, 12 insertions(+), 8 deletions(-)
> 
> diff --git a/include/libcamera/internal/matrix.h b/include/libcamera/internal/matrix.h
> index b9c3d41ef855..512c1162c3bc 100644
> --- a/include/libcamera/internal/matrix.h
> +++ b/include/libcamera/internal/matrix.h
> @@ -25,9 +25,8 @@ class Matrix
>         static_assert(std::is_arithmetic_v<T>, "Matrix type must be arithmetic");
>  
>  public:
> -       Matrix()
> +       constexpr Matrix()
>         {
> -               data_.fill(static_cast<T>(0));
>         }
>  
>         Matrix(const std::array<T, Rows * Cols> &data)
> @@ -35,7 +34,7 @@ public:
>                 std::copy(data.begin(), data.end(), data_.begin());
>         }
>  
> -       static Matrix identity()
> +       static constexpr Matrix identity()
>         {
>                 Matrix ret;
>                 for (size_t i = 0; i < std::min(Rows, Cols); i++)
> @@ -63,14 +62,14 @@ public:
>                 return out.str();
>         }
>  
> -       Span<const T, Rows * Cols> data() const { return data_; }
> +       constexpr Span<const T, Rows * Cols> data() const { return data_; }
>  
> -       Span<const T, Cols> operator[](size_t i) const
> +       constexpr Span<const T, Cols> operator[](size_t i) const
>         {
>                 return Span<const T, Cols>{ &data_.data()[i * Cols], Cols };
>         }
>  
> -       Span<T, Cols> operator[](size_t i)
> +       constexpr Span<T, Cols> operator[](size_t i)
>         {
>                 return Span<T, Cols>{ &data_.data()[i * Cols], Cols };
>         }
> @@ -88,7 +87,12 @@ public:
>         }
>  
>  private:
> -       std::array<T, Rows * Cols> data_;
> +       /*
> +        * \todo The initializer is only necessary for the constructor to be
> +        * constexpr in C++17. Remove the initializer as soon as we are on
> +        * C++20.
> +        */
> +       std::array<T, Rows * Cols> data_ = {};
>  };
>  
>  #ifndef __DOXYGEN__
> @@ -141,7 +145,7 @@ constexpr Matrix<T, R1, C2> operator*(const Matrix<T, R1, C1> &m1, const Matrix<
>  }
>  
>  template<typename T, unsigned int Rows, unsigned int Cols>
> -Matrix<T, Rows, Cols> operator+(const Matrix<T, Rows, Cols> &m1, const Matrix<T, Rows, Cols> &m2)
> +constexpr Matrix<T, Rows, Cols> operator+(const Matrix<T, Rows, Cols> &m1, const Matrix<T, Rows, Cols> &m2)
>  {
>         Matrix<T, Rows, Cols> result;
>  
> -- 
> 2.43.0
>