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authorLaurent Pinchart <laurent.pinchart@ideasonboard.com>2020-01-14 21:37:27 +0200
committerLaurent Pinchart <laurent.pinchart@ideasonboard.com>2020-01-16 19:29:16 +0200
commit38dd90307ab2b0d25a0a233eae04455f769153b4 (patch)
tree679bc2bb2c1ad36ea877a5d502b073d657e783e7 /Documentation
parentacf18e4265dec2991e62f7c8baecfacf1a6708b3 (diff)
libcamera: Remove std::piecewise_construct where not necessary
When inserting an element with emplace(), the element is constructed in-place with the parameters to the emplace() method being forwarded to the constructor of the element. For std::map containers, the element is an std::pair<const Key, T>. The constructors of std::pair<T1, T2> fall into three categories: (1) Default, copy and move constructors (and related versions) (2) Constructors that take lvalue or rvalue references to T1 and T2 (3) A forwarding constructor that forwards parameters to the constructors of T1 and T2 The first category isn't useful in most cases for std::map::emplace(), as the caller usually doesn't have an existing std::pair<const Key, T> for the element to be inserted. The constructor from the third category is useful to avoid constructing intermediate Key or T instances when the caller doesn't have them available. This constructor takes two std::tuple arguments that contain the arguments for the Key and T constructors, respectively. Due to template deduction rules, usage of such a constructor couldn't be deduced by the compiler automatically in all cases, so the constructor takes a first argument of type std::piecewise_construct_t that lets the caller force the usage ot the forwarding constructor (also known for this reason as the piecewise constructor). The caller uses a construct such as map.emplace(std::piecewise_construct, std::forward_as_tuple(args_for_Key, ...), std::forward_as_tuple(args_for_T, ...)); This syntax is a bit heavy, but is required to construct Key and T in-place from arguments to their non-default constructor (it is also the only std::pair non-default constructor that can be used for non-copyable non-movable types). When the caller of std::map::emplace() already has references to a Key and a T, they can be passed to the std::pair piecewise constructor, and this will create std::tuple instance to wrap the Key and T references arguments to ultimately pass them to the Key and T copy constructors. map.emplace(std::piecewise_construct, std::forward_as_tuple(Key_value), std::forward_as_tuple(T_value)); While this mechanism works, it's unnecessary complex. A constructor of std::pair that takes references to Key and T can be used without any performance penalty, as it will also call the copy constructor of Key and T. In this case we can use a simpler constructor of std::pair, and thus a simpler call of std::map::emplace. map.emplace(Key_value, T_value); We have a couple occurrences of this above misuse of piecewise construction. Simplify them, which simplifies the code and reduces the generated code size. Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se> Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
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