Trying to understand lvalues and rvalues

[Manik Jindal]

Hi

I was trying to understand glvalues, rvalues, lvalues, xvalues and prvalues in c++11. But I figured that I do not have a basic understanding of what lvalues and rvalues meant before c++11 (i.e. in c++03 and even before that).

Also, I tried, but I could not get my hands on a c++03 standard.

Caution: long post.

What I have understood until now (or so I believe), please correct me wherever I am wrong:

• C/C++03 ([Ln1][Ln2])

• "An object is a manipulable region of storage; an lvalue is an expression referring to an object..."
  
• we can have non modifiable lvalues too. (const qualified lvalues are non modifiable) [an expression is modifiable, if through it we can modify the object it is referring to]
• "Conceptually, an rvalue is just a value; it doesn't refer to an object. In practice, it's not that an rvalue can't refer to an object. It's just that an rvalue doesn't necessarily refer to an object."
• until now: 
• modifiable lvalue - addressable + assignable
• non modifiable lvalue - addressable not assignable
• rvalue - neither addressable not assignable
• this only applies to rvalues of non-class types [Q1][Q2]
• I understand that one can modify rvalues of class type by calling their member functions.
• Example: http://ideone.com/GtaVpS
• and if you return a const rvalue of class-type you would not be able to modify it. because rvalues of class-type retain their cv qualifiers.
• Some where in the standard it must have been mentioned that non-class rvalues always are cv-unqualified, because it gets quoted a lot.
• Hence [Q3]

Questions:

[Q1] What are class-types and non-class types?

[Q2] What's so different in rvalues of class type when compared to non-class types.

[Q3] How does one modify a non-class type rvalue? If one cannot why did the need arose in the standard to specify that non-class rvalues are always cv-unqualified.

[Ln1] http://www.embedded.com/electronics-blogs/programming-pointers/4023341/Lvalues-and-Rvalues

[Ln2] http://www.embedded.com/electronics-blogs/programming-pointers/4023799/Non-modifiable-Lvalues

[Nicol Bolas]

On Sunday, January 24, 2016 at 1:05:28 PM UTC-5, Manik Jindal wrote:

Hi

I was trying to understand glvalues, rvalues, lvalues, xvalues and prvalues in c++11. But I figured that I do not have a basic understanding of what lvalues and rvalues meant before c++11 (i.e. in c++03 and even before that).

I just did a Google search for "glvalues, rvalues, lvalues, xvalues and prvalues". The first two links produced comprehensive information about these concepts.

[Columbo]

On Sunday, January 24, 2016 at 6:05:28 PM UTC, Manik Jindal wrote:

"An object is a manipulable region of storage; an lvalue is an expression referring to an object..."

[intro.object] was amended by P0137. An object occupies a region of storage.  

[Manik Jindal]

Thanks Nicol. I think I messed up when I started the post by saying that I was trying to understand glvalues, rvalues, lvalues .... TBH, I was. I have seen both the links you mentioned but was unable to understand many aspects in these. I think this is because I do not have a firm grasp on lvalues and rvalues, what they meant in C++03. And hence failed to see the points people were making and where they are coming from. I hope I have cleared up a bit about where am I coming from.

[Manik Jindal]

Thanks Columbo. 

[Richard Smith]

You may also be interested in
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0135r0.html
(approved by EWG at Kona) which touches on some of this.

> [Ln1]
> http://www.embedded.com/electronics-blogs/programming-pointers/4023341/Lvalues-and-Rvalues
> [Ln2]
> http://www.embedded.com/electronics-blogs/programming-pointers/4023799/Non-modifiable-Lvalues
>

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