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Fix issue 4184: The spec should warn about using copy constructors. #4185

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Mar 21, 2025
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Fix issue 4184: The spec should warn about using copy constructors. #4185

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53 changes: 45 additions & 8 deletions spec/struct.dd
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Original file line number Diff line number Diff line change
Expand Up @@ -1122,6 +1122,29 @@ $(H3 $(LNAME2 field-init, Field initialization inside a constructor))

$(H2 $(LEGACY_LNAME2 StructCopyConstructor, struct-copy-constructor, Struct Copy Constructors))

$(PANEL
$(P $(RED Warning:) The plan is for
$(RELATIVE_LINK2 struct-copy-constructor, copy constructors) to replace
$(RELATIVE_LINK2 struct-postblit, postblit constructors), leaving
postblit constructors only in legacy code. However, because the
compiler hooks in druntime which deal with dynamic arrays and
associative arrays have not yet all been updated to support copy
constructors properly
($(HTTPS github.com/dlang/dmd/issues/20970, issue #20970)), any type
that might be used in a dynamic array or associative array should use a
postblit constructor rather than a copy constructor. The copy
constructor will not be called in all the cases where it should be for
the elements of a dynamic array or the keys or values in an associative
array. Postblit constructors do not have this problem.)

$(P For backward compatibility reasons, a `struct` that explicitly
defines both a copy constructor and a postblit will only use the
postblit for implicit copying. However, if the postblit is disabled,
the copy constructor will be used. If a struct defines a copy
constructor (user-defined or generated) and has fields that define
postblits, a deprecation will be issued, informing that the postblit
will have priority over the copy constructor.))

$(P Copy constructors are used to initialize a `struct` instance from
another instance of the same type. A `struct` that defines a copy constructor
is not $(RELATIVE_LINK2 POD, POD).)
Expand Down Expand Up @@ -1644,14 +1667,28 @@ $(GNAME Postblit):
$(D this $(LPAREN) this $(RPAREN)) $(GLINK2 function, MemberFunctionAttributes)$(OPT) $(GLINK2 function, MissingFunctionBody)
)

$(P $(RED Warning): The postblit is considered legacy and is not recommended for new code.
Code should use $(RELATIVE_LINK2 struct-copy-constructor, copy constructors)
defined in the previous section. For backward compatibility reasons, a `struct` that
explicitly defines both a copy constructor and a postblit will only use the postblit
for implicit copying. However, if the postblit is disabled, the copy constructor will
be used. If a struct defines a copy constructor (user-defined or generated) and has
fields that define postblits, a deprecation will be issued, informing that the postblit
will have priority over the copy constructor.)
$(PANEL
$(P $(RED Warning:) The plan is for
$(RELATIVE_LINK2 struct-copy-constructor, copy constructors) to replace
$(RELATIVE_LINK2 struct-postblit, postblit constructors), leaving
postblit constructors only in legacy code. However, because the
compiler hooks in druntime which deal with dynamic arrays and
associative arrays have not yet all been updated to support copy
constructors properly
($(HTTPS github.com/dlang/dmd/issues/20970, issue #20970)), any type
that might be used in a dynamic array or associative array should use a
postblit constructor rather than a copy constructor. The copy
constructor will not be called in all the cases where it should be for
the elements of a dynamic array or the keys or values in an associative
array. Postblit constructors do not have this problem.)

$(P For backward compatibility reasons, a `struct` that explicitly
defines both a copy constructor and a postblit will only use the
postblit for implicit copying. However, if the postblit is disabled,
the copy constructor will be used. If a struct defines a copy
constructor (user-defined or generated) and has fields that define
postblits, a deprecation will be issued, informing that the postblit
will have priority over the copy constructor.))

$(P $(I Copy construction) is defined as initializing
a struct instance from another instance of the same type.
Expand Down