[PATCH] 77864 Fix noexcept conditions for map/set default constructors

François Dumont frs.dumont@gmail.com
Tue Oct 25 19:55:00 GMT 2016


On 24/10/2016 13:03, Jonathan Wakely wrote:
> On 12/10/16 22:36 +0200, François Dumont wrote:
>> On 10/10/2016 23:01, Tim Song wrote:
>>> Trying again...with a few edits.
>>>
>>>> On Mon, Oct 10, 2016 at 3:24 PM, François Dumont 
>>>> <frs.dumont@gmail.com>
>>>> wrote:
>>>>
>>>> @@ -602,24 +612,32 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>>>>          struct _Rb_tree_impl : public _Node_allocator
>>>>          {
>>>>    _Key_compare _M_key_compare;
>>>> -  _Rb_tree_node_base _M_header;
>>>> +  _Rb_header_node _M_header;
>>>> +#if __cplusplus < 201103L
>>>>    size_type _M_node_count; // Keeps track of size of tree.
>>>> +#else
>>>> +  size_type _M_node_count = 0; // Keeps track of size of tree.
>>>> +#endif
>>>>
>>>> +#if __cplusplus < 201103L
>>>>    _Rb_tree_impl()
>>>> -  : _Node_allocator(), _M_key_compare(), _M_header(),
>>>> -    _M_node_count(0)
>>>> -  { _M_initialize(); }
>>>> +  : _M_node_count(0)
>>>> +  { }
>>>> +#else
>>>> +  _Rb_tree_impl() = default;
>>>> +#endif
>>>
>>> The default constructor of the associative containers is required to
>>> value-initialize the comparator (see their synopses in
>>> [map/set/multimap/multiset.overview]).
>> I don't have latest Standard version so can't see the exact word but 
>> I find quite annoying that the Standard doesn't allow this simple 
>> implementation.
>>
>> I don't know if unodered containers have same kind of requirements 
>> for equal or hash functors but if so current implementation doesn't 
>> do this value initialization.
>>
>> So here is another attempt. This time it simply allows to have 
>> noexcept condition in one place and closer to where operations are 
>> being invoked.
>>
>> Ok to commit after tests ?
>>
>> François
>>>
>>>  _Rb_tree_impl() = default; doesn't do that; it default-initializes the
>>>  comparator instead.
>>>
>>> Tim
>>>
>>
>
>> diff --git a/libstdc++-v3/include/bits/stl_map.h 
>> b/libstdc++-v3/include/bits/stl_map.h
>> index e5b2a1b..dea7d5b 100644
>> --- a/libstdc++-v3/include/bits/stl_map.h
>> +++ b/libstdc++-v3/include/bits/stl_map.h
>> @@ -167,11 +167,11 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
>>       /**
>>        *  @brief  Default constructor creates no elements.
>>        */
>> -      map()
>> -      _GLIBCXX_NOEXCEPT_IF(
>> - is_nothrow_default_constructible<allocator_type>::value
>> -      && is_nothrow_default_constructible<key_compare>::value)
>> -      : _M_t() { }
>> +#if __cplusplus < 201103L
>> +      map() : _M_t() { }
>> +#else
>> +      map() = default;
>> +#endif
>>
>>       /**
>>        *  @brief  Creates a %map with no elements.
>> diff --git a/libstdc++-v3/include/bits/stl_multimap.h 
>> b/libstdc++-v3/include/bits/stl_multimap.h
>> index d240427..7e86b76 100644
>> --- a/libstdc++-v3/include/bits/stl_multimap.h
>> +++ b/libstdc++-v3/include/bits/stl_multimap.h
>> @@ -164,11 +164,11 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
>>       /**
>>        *  @brief  Default constructor creates no elements.
>>        */
>> -      multimap()
>> -      _GLIBCXX_NOEXCEPT_IF(
>> - is_nothrow_default_constructible<allocator_type>::value
>> -      && is_nothrow_default_constructible<key_compare>::value)
>> -      : _M_t() { }
>> +#if __cplusplus < 201103L
>> +      multimap() : _M_t() { }
>> +#else
>> +      multimap() = default;
>> +#endif
>>
>>       /**
>>        *  @brief  Creates a %multimap with no elements.
>> diff --git a/libstdc++-v3/include/bits/stl_multiset.h 
>> b/libstdc++-v3/include/bits/stl_multiset.h
>> index cc068a9..7fe2fbd 100644
>> --- a/libstdc++-v3/include/bits/stl_multiset.h
>> +++ b/libstdc++-v3/include/bits/stl_multiset.h
>> @@ -144,11 +144,11 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
>>       /**
>>        *  @brief  Default constructor creates no elements.
>>        */
>> -      multiset()
>> -      _GLIBCXX_NOEXCEPT_IF(
>> - is_nothrow_default_constructible<allocator_type>::value
>> -      && is_nothrow_default_constructible<key_compare>::value)
>> -      : _M_t() { }
>> +#if __cplusplus < 201103L
>> +      multiset() : _M_t() { }
>> +#else
>> +      multiset() = default;
>> +#endif
>>
>>       /**
>>        *  @brief  Creates a %multiset with no elements.
>> diff --git a/libstdc++-v3/include/bits/stl_set.h 
>> b/libstdc++-v3/include/bits/stl_set.h
>> index 3938351..5ed9672 100644
>> --- a/libstdc++-v3/include/bits/stl_set.h
>> +++ b/libstdc++-v3/include/bits/stl_set.h
>> @@ -147,11 +147,11 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
>>       /**
>>        *  @brief  Default constructor creates no elements.
>>        */
>> -      set()
>> -      _GLIBCXX_NOEXCEPT_IF(
>> - is_nothrow_default_constructible<allocator_type>::value
>> -      && is_nothrow_default_constructible<key_compare>::value)
>> -      : _M_t() { }
>> +#if __cplusplus < 201103L
>> +      set() : _M_t() { }
>> +#else
>> +      set() = default;
>> +#endif
>>
>>       /**
>>        *  @brief  Creates a %set with no elements.
>> diff --git a/libstdc++-v3/include/bits/stl_tree.h 
>> b/libstdc++-v3/include/bits/stl_tree.h
>> index ee2dc70..b6a3c1e 100644
>> --- a/libstdc++-v3/include/bits/stl_tree.h
>> +++ b/libstdc++-v3/include/bits/stl_tree.h
>> @@ -137,6 +137,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>>     }
>>   };
>>
>> +  struct _Rb_header_node : public _Rb_tree_node_base
>> +  {
>> +    _Rb_header_node() _GLIBCXX_NOEXCEPT
>> +    {
>> +      _M_color = _S_red;
>> +      _M_parent = _Base_ptr();
>> +      _M_left = _M_right = this;
>> +    }
>> +  };
>> +
>>   template<typename _Val>
>>     struct _Rb_tree_node : public _Rb_tree_node_base
>>     {
>> @@ -602,24 +612,34 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>>         struct _Rb_tree_impl : public _Node_allocator
>>         {
>>       _Key_compare        _M_key_compare;
>> -      _Rb_tree_node_base     _M_header;
>> +      _Rb_header_node     _M_header;
>> +#if __cplusplus < 201103L
>>       size_type         _M_node_count; // Keeps track of size of tree.
>> +#else
>> +      size_type         _M_node_count = 0; // Keeps track of size of 
>> tree.
>> +#endif
>>
>>       _Rb_tree_impl()
>> -      : _Node_allocator(), _M_key_compare(), _M_header(),
>> -        _M_node_count(0)
>> -      { _M_initialize(); }
>> +      _GLIBCXX_NOEXCEPT_IF(
>> + is_nothrow_default_constructible<_Node_allocator>::value
>> +        && is_nothrow_default_constructible<_Key_compare>::value)
>> +      : _M_key_compare()
>> +#if __cplusplus < 201103L
>> +      , _M_node_count(0)
>> +#endif
>> +      { }
>
> I still think this part is pointless. Why use conditional compilation
> here, when we could just always set it to zero?
>
> What is the advantage of using #if here, except adding more lines of
> code?
>
>>
>>       _Rb_tree_impl(const _Key_compare& __comp, const 
>> _Node_allocator& __a)
>> -      : _Node_allocator(__a), _M_key_compare(__comp), _M_header(),
>> -        _M_node_count(0)
>> -      { _M_initialize(); }
>> +      : _Node_allocator(__a), _M_key_compare(__comp)
>> +#if __cplusplus < 201103L
>> +      , _M_node_count(0)
>> +#endif
>> +      { }
>>
>> #if __cplusplus >= 201103L
>>       _Rb_tree_impl(const _Key_compare& __comp, _Node_allocator&& __a)
>> -      : _Node_allocator(std::move(__a)), _M_key_compare(__comp),
>> -        _M_header(), _M_node_count(0)
>> -      { _M_initialize(); }
>> +      : _Node_allocator(std::move(__a)), _M_key_compare(__comp)
>> +      { }
>> #endif
>>
>>       void
>> @@ -630,16 +650,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>>         this->_M_header._M_right = &this->_M_header;
>>         this->_M_node_count = 0;
>>       }
>> -
>> -    private:
>> -      void
>> -      _M_initialize()
>> -      {
>> -        this->_M_header._M_color = _S_red;
>> -        this->_M_header._M_parent = 0;
>> -        this->_M_header._M_left = &this->_M_header;
>> -        this->_M_header._M_right = &this->_M_header;
>> -      }            };
>
> Since we can't remove the constructors that called _M_initialize() we
> don't need to get rid of _M_initialize() either. That means we don't
> need the _Rb_header_node type (so don't need to produce RTTI for a new
> type).
>
> The purpose of the patch is to allow _Rb_tree() = default and then
> set() = default, map() = default etc.
>
> That can be done by simply putting the _GLIBCXX_NOEXCEPT_IF on
> _Rb_tree_impl() and forgetting all the other changes to _Rb_tree_impl.
> Using a default member initializer for _M_node_count and removing
> _M_initialize() aren't necessary to achieve the purpose of the patch
Indeed, here is the simplified patch.

But I will come back to this base type for the next patch to generalize 
the defaulted constructors and assignment operators.

Tested under Linux x86_64, ok to commit ?

François

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