[libcamera-devel] [PATCH v5 1/9] libcamera: base: utils: Provide defopt to simplify std::optional::value_or() usage

[Florian Sylvestre]

Hi Laurent,

On Thu, 28 Jul 2022 at 12:30, Laurent Pinchart via libcamera-devel
<libcamera-devel at lists.libcamera.org> wrote:
>
> Hi Jacopo,
>
> On Thu, Jul 28, 2022 at 08:47:05AM +0200, Jacopo Mondi wrote:
> > Hi Laurent
> >
> > On Thu, Jul 28, 2022 at 01:21:41AM +0300, Laurent Pinchart via libcamera-devel wrote:
> > > The std::optional<T>::value_or(U &&default_value) function returns the
> > > contained value if available, or \a default_value if the std::optional
> >
> > Are the \a in the commit message intended ?
>
> Oops :-) I'll drop it.
>
> > > has no value. If the desired default value is a default-constructed T,
> > > the obvious option is to call std::optional<T>::value_or(T{}). This
> > > approach has two drawbacks:
> > >
> > > - The \a default_value T{} is constructed even if the std::optional
> > >   instance has a value, which impacts efficiency.
> > > - The T{} default constructor needs to be spelled out explicitly in the
> > >   value_or() call, leading to long lines if the type is complex.
> > >
> > > Introduce a defopt variable that solves these issues by providing a
> > > value that can be passed to std::optional<T>::value_or() and get
> > > implicitly converted to a default-constructed T.
> > >
> > > Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> > > ---
> > >  include/libcamera/base/utils.h | 14 +++++++++
> > >  src/libcamera/base/utils.cpp   | 21 +++++++++++++
> > >  test/utils.cpp                 | 54 ++++++++++++++++++++++++++++++++++
> > >  3 files changed, 89 insertions(+)
> > >
> > > diff --git a/include/libcamera/base/utils.h b/include/libcamera/base/utils.h
> > > index cfff05836de7..889bb4a2270e 100644
> > > --- a/include/libcamera/base/utils.h
> > > +++ b/include/libcamera/base/utils.h
> > > @@ -361,6 +361,20 @@ decltype(auto) abs_diff(const T &a, const T &b)
> > >             return a - b;
> > >  }
> > >
> > > +namespace details {
> > > +
> > > +struct defopt_t {
> > > +   template<typename T>
> > > +   operator T() const
> > > +   {
> > > +           return T{};
> > > +   }
> > > +};
> > > +
> > > +} /* namespace details */
> > > +
> > > +constexpr details::defopt_t defopt;
> > > +
> >
> > Clever! this indeed explains how it works:
> >
> > template< class U >
> > constexpr T value_or( U&& default_value ) const&;
> >
> > Equivalent to bool(*this) ? **this : static_cast<T>(std::forward<U>(default_value))
> >
> >
> > Reviewed-by: Jacopo Mondi <jacopo at jmondi.org>
> >
> > >  } /* namespace utils */
> > >
> > >  #ifndef __DOXYGEN__
> > > diff --git a/src/libcamera/base/utils.cpp b/src/libcamera/base/utils.cpp
> > > index 6a307940448e..9cd6cb197243 100644
> > > --- a/src/libcamera/base/utils.cpp
> > > +++ b/src/libcamera/base/utils.cpp
> > > @@ -463,6 +463,27 @@ std::string toAscii(const std::string &str)
> > >   * \a b
> > >   */
> > >
> > > +/**
> > > + * \var defopt
> > > + * \brief Constant used with std::optional::value_or() to create a
> > > + * default-constructed object
> > > + *
> > > + * The std::optional<T>::value_or(U &&default_value) function returns the
> > > + * contained value if available, or \a default_value if the std::optional has no
> > > + * value. If the desired default value is a default-constructed T, the obvious
> > > + * option is to call std::optional<T>::value_or(T{}). This approach has two
> > > + * drawbacks:
> > > + *
> > > + * * The \a default_value T{} is constructed even if the std::optional instance
> > > + *   has a value, which impacts efficiency.
> > > + * * The T{} default constructor needs to be spelled out explicitly in the
> > > + *   value_or() call, leading to long lines if the type is complex.
> > > + *
> > > + * The defopt variable solves these issues by providing a value that can be
> > > + * passed to std::optional<T>::value_or() and get implicitly converted to a
> > > + * default-constructed T.
> > > + */
> > > +
> > >  } /* namespace utils */
> > >
> > >  #ifndef __DOXYGEN__
> > > diff --git a/test/utils.cpp b/test/utils.cpp
> > > index d65467b5102c..129807a63ec6 100644
> > > --- a/test/utils.cpp
> > > +++ b/test/utils.cpp
> > > @@ -7,6 +7,7 @@
> > >
> > >  #include <iostream>
> > >  #include <map>
> > > +#include <optional>
> > >  #include <sstream>
> > >  #include <string>
> > >  #include <vector>
> > > @@ -169,6 +170,55 @@ protected:
> > >             return TestPass;
> > >     }
> > >
> > > +   int testDefopt()
> > > +   {
> > > +           static bool defaultConstructed = false;
> > > +
> > > +           struct ValueType {
> > > +                   ValueType()
> > > +                           : value_(-1)
> > > +                   {
> > > +                           defaultConstructed = true;
> > > +                   }
> > > +
> > > +                   ValueType(int value)
> > > +                           : value_(value)
> > > +                   {
> > > +                   }
> > > +
> > > +                   int value_;
> > > +           };
> > > +
> > > +           /*
> > > +            * Test that utils::defopt doesn't cause default-construction
> > > +            * of a ValueType instance when value_or(utils::defopt) is
> > > +            * called on a std::optional that has a value.
> > > +            */
> > > +           std::optional<ValueType> opt = ValueType(0);
> > > +           ValueType value = opt.value_or(utils::defopt);
> > > +
> > > +           if (defaultConstructed || value.value_ != 0) {
> > > +                   std::cerr << "utils::defopt didn't prevent default construction"
> > > +                             << std::endl;
> > > +                   return TestFail;
> > > +           }
> > > +
> > > +           /*
> > > +            * Then test that the ValueType is correctly default-constructed
> > > +            * when the std::optional has no value.
> > > +            */
> > > +           opt = std::nullopt;
> > > +           value = opt.value_or(utils::defopt);
> > > +
> > > +           if (!defaultConstructed || value.value_ != -1) {
> > > +                   std::cerr << "utils::defopt didn't cause default construction"
> > > +                             << std::endl;
> > > +                   return TestFail;
> > > +           }
> > > +
> > > +           return TestPass;
> > > +   }
> > > +
> > >     int run()
> > >     {
> > >             /* utils::hex() test. */
> > > @@ -281,6 +331,10 @@ protected:
> > >             if (testDuration() != TestPass)
> > >                     return TestFail;
> > >
> > > +           /* utils::defopt test. */
> > > +           if (testDefopt() != TestPass)
> > > +                   return TestFail;
> > > +
> > >             return TestPass;
> > >     }
> > >  };
>
> --
> Regards,
>
> Laurent Pinchart

Reviewed-by: Florian Sylvestre <fsylvestre at baylibre.com>

-- 
Florian Sylvestre