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Adds forwarding make_optional helpers, resolves #30 #31

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akrzemi1 merged 1 commit into from
May 17, 2017
Merged

Adds forwarding make_optional helpers, resolves #30 #31

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21 changes: 21 additions & 0 deletions include/boost/optional/optional.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1276,6 +1276,25 @@ class optional<T&&>

namespace boost {

#ifndef BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES

template<class T>
inline
optional<BOOST_DEDUCED_TYPENAME boost::decay<T>::type> make_optional ( T && v )
{
return optional<BOOST_DEDUCED_TYPENAME boost::decay<T>::type>(boost::forward<T>(v));
}

// Returns optional<T>(cond,v)
template<class T>
inline
optional<BOOST_DEDUCED_TYPENAME boost::decay<T>::type> make_optional ( bool cond, T && v )
{
return optional<BOOST_DEDUCED_TYPENAME boost::decay<T>::type>(cond,boost::forward<T>(v));
}

#else
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@stevenr3 stevenr3 Oct 21, 2020

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Was it intended to remove the l-value methods with this #else?
I'm getting a compile error trying to make an optional from a variable, e.g. float x = 1; make_optional<float>(x);
(without -DBOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES)

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@akrzemi1 akrzemi1 Oct 27, 2020

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Function make_optional is not intended to be used like this, with template arguments specified explicitly. The only purpose of function make_optional is to spare you the trouble of specifying the type of optional value. If you want to specify it yourself, you have a simpler version:

float x = 1; boost::optional<float>(x);

Macro BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES is not intended to be defined by users. The library defines it when it detects old compilers without rvalue reference support.

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@stevenr3 stevenr3 Oct 27, 2020

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Sorry I missed some detail… I was attempting to use boost::make_optional<T>(bool, T) to simplify the following:

float value;
bool valid = GetValidAndValue(&value);

// return valid ? boost::optional<float>(value) : boost::none;
return boost::make_optional<float>(valid, value);

True it is not a huge simplification - but should that be possible?

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@akrzemi1 akrzemi1 Oct 27, 2020

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Again, same response. If you can afford spelling out the type of optional value use:

return boost::optional<float>(valid, value);

If you want to the type of optional value to be deduced use:

return boost::make_optional(valid, value);

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@akrzemi1 akrzemi1 Oct 27, 2020

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IOW, make_optional is only used for type deduction.

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@stevenr3 stevenr3 Oct 27, 2020

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Ah ok, got it... thanks!


// Returns optional<T>(v)
template<class T>
inline
Expand All @@ -1292,6 +1311,8 @@ optional<T> make_optional ( bool cond, T const& v )
return optional<T>(cond,v);
}

#endif // BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES

// Returns a reference to the value if this is initialized, otherwise, the behaviour is UNDEFINED.
// No-throw
template<class T>
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