Details
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Task
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Status: Closed (View Workflow)
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Major
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Resolution: Won't Do
Description
This task was canceled. See a comment below.
There is some code duplication in Item implementation.
class Item_str_func
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{
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double val_real()
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{
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int err_not_used;
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char *end_not_used;
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String *res;
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res= val_str(&str_value);
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return res ? my_strntod(res->charset(),(char*) res->ptr(),
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res->length(), &end_not_used, &err_not_used) : 0.0;
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}
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};
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class Item_func_group_concat
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{
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double val_real()
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{
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int error;
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const char *end;
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String *res;
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if (!(res= val_str(&str_value)))
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return 0.0;
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end= res->ptr() + res->length();
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return (my_strtod(res->ptr(), (char**) &end, &error));
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}
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};
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class Item_func_udf_str
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{
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double val_real()
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{
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int err_not_used;
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char *end_not_used;
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String *res;
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res= val_str(&str_value);
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return res ? my_strntod(res->charset(),(char*) res->ptr(),
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res->length(), &end_not_used, &err_not_used) : 0.0;
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}
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};
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The same type of duplications happens with all other val_xxx() methods.
The same duplication happens in deriving Item_xxx from Type_handler_xxx:
class Item_sum_sum :public Item_sum_num,
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public Type_handler_hybrid_field_type
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{
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...
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enum_field_types field_type() const
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{ return Type_handler_hybrid_field_type::field_type(); }
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enum Item_result result_type () const
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{ return Type_handler_hybrid_field_type::result_type(); }
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enum Item_result cmp_type () const
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{ return Type_handler_hybrid_field_type::cmp_type(); }
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...
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};
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class Item_sum_hybrid :public Item_sum, public Type_handler_hybrid_field_type
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{
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...
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enum Item_result result_type () const
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{ return Type_handler_hybrid_field_type::result_type(); }
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enum Item_result cmp_type () const
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{ return Type_handler_hybrid_field_type::cmp_type(); }
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enum enum_field_types field_type() const
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{ return Type_handler_hybrid_field_type::field_type(); }
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...
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};
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class Item_hybrid_func: public Item_func,
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public Type_handler_hybrid_field_type
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{
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...
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enum_field_types field_type() const
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{ return Type_handler_hybrid_field_type::field_type(); }
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enum Item_result result_type () const
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{ return Type_handler_hybrid_field_type::result_type(); }
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enum Item_result cmp_type () const
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{ return Type_handler_hybrid_field_type::cmp_type(); }
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...
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};
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Duplicate code can be avoided by using diamond-style inheritance, deriving Item from Type_handler virtually, so these methods are automatically assigned by just the class declaration.
Diamond style inheritance:
Top
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/ \
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Left Right
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\ /
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Bottom
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Deriving from Top to Left and from Top to Right is done virtually. This is important to have only one copy of "Top" in "Bottom".
In our case:
- Top is Type_handler
- Left is the data type: Type_handler_str, Type_handler_longlong, Type_handler_double, etc.
- Right is the Item type: Item_func, Item_sum, Item_cache, etc.
- Bottom is the Item type with the data type: Item_func_str, Item_sum_int, Item_cache_real, etc.
Type_handler/ \Type_handler_longlong Item_func\ /Item_int_func
First, we inherit two different branches of Type_handler virtually (notice "public virtual"):
// The data type branch (string, double, int, decimal, date, time,datetime)
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class Type_handler_str: public virtual Type_handler
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{
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// This class implements field_type(), cmp_type(), result_type()
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// as well as conversion methods val_int(), val_decimal(), val_real(), get_date().
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// Notice, it does not implement the native method val_str(). It's pure virtual here.
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};
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// The Item type branch (Item_basic_constant, Item_cache, Item_func, Item_sum, Item_field, all the rest)
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class Item: public Value_source,
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public Type_std_attributes,
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public virtual Type_handler
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{
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// This class implements the code that returns the native value.
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// It does NOT implement field_type(), cmp_type(), return_type(), they are pure virtual. here.
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// It does NOT implement non-native val_xxx(), get_date(). They are pure virtual here.
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};
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Now we join the two branches into a single object:
class Item_func_udf_str: public Item_udf_func, public Type_handler_str
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{
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// No needs to declare field_type(), result_type(), cmp_type().
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// No needs to declare val_real(), val_int(), val_decimal(), get_date().
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// They are derived automatically from Type_handler_str.
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// This class leaves only one pure virtual method val_str(),
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// and its children are responsible to implement it.
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};
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class Item_func_str: public Item_func, public Type_handler_str
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{
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// Same here.
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};
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class Item_func_group_concat: public Item_sum, public Type_handler_str
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{
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// Same here, but it does not have children and implements val_str() itself.
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};
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A similar example with Type_handler_hybrid_field_type
class Item_sum_sum :public Item_sum_num,
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public Type_handler_hybrid_field_type
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{
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// No needs to declare field_type(), result_type(), cmp_type()
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// They are automatically assigned from Type_handler_hybrid_field_type
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}
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Attachments
Issue Links
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MDEV-4912 Data type plugin API version 1
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- Closed
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