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Wrong comment in [intro.execution]/12's example #1650

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jabelloc opened this issue Jun 28, 2017 · 6 comments
Closed

Wrong comment in [intro.execution]/12's example #1650

jabelloc opened this issue Jun 28, 2017 · 6 comments

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@jabelloc
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[intro.execution]/12:

A full-expression is

...

[ Example:

    struct S {
          S(int i): I(i) { } // full-expression is initialization of I
          int& v() { return I; }
         ~S() noexcept(false) { }
    private:
         int I;
    };
    S s1(1); // full-expression is call of S::S(int)
    void f() {
          S s2 = 2;      // full-expression is call of S::S(int)
          if (S(3).v())        // full-expression includes lvalue-to-rvalue and
                               // int to bool conversions, performed before
                               // temporary is deleted at end of full-expression
          { }
          bool b = noexcept(S());       // exception specification of destructor of S
                                        // considered for noexcept
                                         // full-expression is destruction of s2 at end of block
    }
    struct B {
          B(S = S(0));
    };
    B b[2] = { B(), B() };     // full-expression is the entire initialization
                               // including the destruction of temporaries

—end example ]

The comment

// full-expression includes lvalue-to-rvalue and

above, should be replaced by

// full-expression includes the temporary-materialization and

Note that the prvalue S(3) is subjected to a temporary-materialzation convertion to an xvalue. The dot(.) class member operator requires a glvalue as a postfix expression. See [expr.ref]/2.
@cpplearner
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cpplearner commented Jun 29, 2017 •
edited

AFAIK S(3).v() in isolation is an lvalue of type int. It is then converted to an rvalue, and then converted to bool.

The comment probably does not contain what you expect it to say, but I personally don't think it is factually wrong.

Just my two cents.

@jabelloc
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It is then converted to an rvalue, and then converted to bool.

This is not correct. There is no lvalue-to-rvalue conversion preceding the int to bool conversion. Nevertheless, the temporary materialization conversion is required for the prvalue postfix expression S(3) at the left of the dot operator. [expr.ref]/2 clearly says that such expression must be a glvalue.

@cpplearner
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cpplearner commented Jun 30, 2017 •
edited

As far as I know, the standard conversion sequence from "lvalue of type int" to "rvalue of type bool" consists of an lvalue-to-rvalue conversion ([conv.lval]) and a boolean conversion ([conv.bool]). The comment "int to bool conversion" is just a somewhat more detailed description of the latter (the boolean conversion).

@jabelloc
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You are right. The boolean conversion expects a prvalue and so, an lvalue-to-rvalue conversion is required before that. But again, the temporary materialization conversion is also required because of [expr.ref]/2. Thanks for the feedback.

@zygoloid
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The comment is correct as written. The point of the example is to demonstrate that certain constructs occur before the temporary is destroyed; saying that the creation of the temporary is one of those things seems unnecessary to me.

@RazvanAM
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RazvanAM commented Aug 9, 2017 •
edited

Why does this paragraph state that for this construct: "S s1(1)" "full-expression is call of S::S(int)"? Shouldn't the full expression be the entire construct itself (because it is an init-declarator)? It would make sense if the comment said: "full-expression contains call of S::S(int)". Am I not understanding something correctly?

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@zygoloid @cpplearner @jabelloc @RazvanAM