Why are tokens wrapped by parentheses not r-value expressions?

Question

Consider the following code:

#include <iostream>

struct Foo
{
  Foo() : bar( 0 ) {}

  int bar;
};


int main()
{
  Foo foo;

  ++(foo.bar);

  std::cout<< foo.bar << std::endl;

  system("pause");
  return 0;
};

Why does foo.bar evaluate to 1?

Doesn't the parentheses in (foo.bar) create an unnamed (r-value) expression which is then incremented?

No, are you confusing this with some other language where `f(x)` and `f((x))` are different? — Bo Persson, Oct 31 '12 at 08:42
@BoPersson But there are some contexts where it makes a difference. For example `decltype(x)` and `decltype((x))` can be different. — juanchopanza, Oct 31 '12 at 08:57

score 9 · Accepted Answer · answered Oct 31 '12 at 08:42

9

Because the standard explicitly states that in 3.4.2 para 6:

A parenthesized expression is a primary expression whose type and value are identical to those of the enclosed expression. The presence of parentheses does not affect whether the expression is an lvalue.

emphasis mine.

answered Oct 31 '12 at 08:42

SingerOfTheFall

29,228
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2

I was confused, because: `struct Widget{} w;` -> `decltype(w)` and `decltype((w))` are not the same. See also [link](https://skydrive.live.com/view.aspx?resid=F1B8FF18A2AEC5C5!1062), slide 33. – Martin85 Oct 31 '12 at 08:52
@user1787909: `decltype` is special. – CB Bailey Oct 31 '12 at 09:19

score 1 · Answer 2 · answered Oct 31 '12 at 08:43

1

No, parenthesis do not have any meaning besides changing the order of operations.

To create an rvalue you need to use special function std::move(x).

answered Oct 31 '12 at 08:43

Kirill Kobelev

10,252
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you may want to create a new stackoverflow question on that. – Johannes Schaub - litb Oct 31 '12 at 08:56
1

Is it so difficult to unabbreviate here? – Kirill Kobelev Oct 31 '12 at 08:57
adl = argument dependent name lookup – Martin85 Oct 31 '12 at 08:58
in an effort to improve the stackoverflow database, i have come to not answer comment questions anymore. – Johannes Schaub - litb Oct 31 '12 at 09:09

Why are tokens wrapped by parentheses not r-value expressions?

2 Answers2