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[stmt.return,class.{ctor,dtor}] Clarify no return operand #4737

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Jan 14, 2022
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[stmt.return,class.{ctor,dtor}] Clarify no return operand #4737

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[stmt.return,class.{ctor,dtor}] Clarify no return operand 
Highlight that constructors and destructors do not have a
return type and thus a return statement within a constructor
or destructor cannot have an operand.
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@jensmaurer
jensmaurer committed Jan 14, 2022
commit 516a4b38146760ba8e88c482bbb522f6ddb8f73b
16 changes: 11 additions & 5 deletions 16 source/classes.tex
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Expand Up @@ -1208,12 +1208,13 @@
most derived object\iref{intro.object} ends.

\pnum
\indextext{restriction!constructor}%
A
\tcode{return}
statement in the body of a constructor shall not specify a return value.
\indextext{constructor!address of}%
The address of a constructor shall not be taken.
\indextext{restriction!constructor}%
\begin{note}
A \tcode{return} statement in the body of a constructor
cannot specify a return value\iref{stmt.return}.
\end{note}

\pnum
A constructor shall not be a coroutine.
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the selected destructor may be deleted\iref{dcl.fct.def.delete}.

\pnum
\indextext{restriction!destructor}%
\indextext{destructor!address of}%
The address of a destructor shall not be taken.
\indextext{restriction!destructor}%
\begin{note}
A \tcode{return} statement in the body of a destructor
cannot specify a return value\iref{stmt.return}.
\end{note}
\indextext{\idxcode{const}!destructor and}%
\indextext{\idxcode{volatile}!destructor and}%
A destructor can be invoked for a
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7 changes: 5 additions & 2 deletions 7 source/statements.tex
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Expand Up @@ -811,15 +811,18 @@
\indextext{\idxcode{return}!constructor and}%
\indextext{\idxcode{return}!constructor and}%
A \tcode{return} statement with an operand of type \keyword{void} shall be used only
in a function whose return type is \cv{}~\keyword{void}.
in a function that has a \cv{}~\keyword{void} return type.
A \tcode{return} statement with any other operand shall be used only
in a function whose return type is not \cv{}~\keyword{void};
in a function that has a return type other than \cv{}~\keyword{void};
\indextext{conversion!return type}%
the \tcode{return} statement initializes the
returned reference or prvalue result object
of the (explicit or implicit) function call
by copy-initialization\iref{dcl.init} from the operand.
\begin{note}
A constructor or destructor does not have a return type.
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@xmh0511 xmh0511 Aug 7, 2021 •
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If we claim that a constructor of a class does not have a return type, consider [dcl.init.general] p15

if the function is a constructor, the call is a prvalue of the cv-unqualified version of the destination type whose result object is initialized by the constructor.

[expr.call] p14

A function call is an lvalue if the result type is an lvalue reference type or an rvalue reference to function type, an xvalue if the result type is an rvalue reference to object type, and a prvalue otherwise.

These rules will be in conflict. How could the call of a constructor be a prvalue of the cv-unqualified version of the destination type?

Similarly, if a destructor does not have a return type, how does the following rule work?

If the postfix-expression names a destructor or pseudo-destructor ([expr.prim.id.dtor]), the type of the function call expression is void;

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I'm not seeing the quoted text in [dcl.init.general] p15.

A constructor invocation is not a syntactic function call as described in [expr.call] (it's an explicit type conversion instead), thus any statements about the type of the function call expression don't apply.

We can explicitly call a destructor in a function call expression, and the rule you're quoting overrides the general rule that the type of a function call expression is the return type of the called function.

I'm not seeing any conflicts.

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I'm not seeing the quoted text in [dcl.init.general] p15.

It is moved to dcl.init#general-16.6.3 in the current draft

if the function is a constructor, the call is a prvalue of the cv-unqualified version of the destination type whose result object is initialized by the constructor. The call is used to direct-initialize, according to the rules above, the object that is the destination of the copy-initialization.

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@jensmaurer Final thoughts?

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Constructors / destructors don't have a return type in the "function declaration" sense, and that absence makes "return something" syntactically ill-formed. I think the quoted stuff doesn't change that; it just means that a constructor call (in the context of copy-initialization, not in general) has (is considered to have) a certain type and value category. That has no bearing on the syntactic constraints on constructor declarations.

I thinks this should go in.

\end{note}
\begin{note}
A \tcode{return} statement can involve
an invocation of a constructor to perform a copy or move of the operand
if it is not a prvalue or if its type differs from the return type of the function.
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