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Marlene Miller
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 Posted: Wed Jan 19, 2005 11:55 pm Post subject: what is a temporary object? |
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I am trying to understand what a temporary object is by example.
string("a") + string("b)
Q1. Is the result of this expression a temporary object?
class C {
public:
operator int() { return 2; }
};
cout << C() << endl;
Q2. Is C() a temporary object?
Q3. A compiler might generate a temporary int object?
C f() { C c; return c; }
Q4. Is the object returned by f() a temporary object?
cout << &C() << endl;
cout << &f() << endl;
Q5. C() and f() are rvalues. & requires an lvalue. Why can we say &C() and
&f()? Is a temporary object involved?
[Except when performance is the issue, temporary objects rarely become the
concern of the programmer. TCPL 10.4.10  ]
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Alf P. Steinbach
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| Posted: Thu Jan 20, 2005 12:20 pm Post subject: Re: what is a temporary object? |
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* Marlene Miller:
| Quote: | I am trying to understand what a temporary object is by example.
string("a") + string("b)
Q1. Is the result of this expression a temporary object?
|
Yes.
| Quote: | class C {
public:
operator int() { return 2; }
};
cout << C() << endl;
Q2. Is C() a temporary object?
|
Yes, in this context.
| Quote: | Q3. A compiler might generate a temporary int object?
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Yes.
| Quote: | C f() { C c; return c; }
Q4. Is the object returned by f() a temporary object?
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Yes.
| Quote: | cout << &C() << endl;
cout << &f() << endl;
Q5. C() and f() are rvalues. & requires an lvalue. Why can we say &C() and
&f()? Is a temporary object involved?
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This is invalid code, but some compilers (e.g. Visual C++) accept it
as non-standard extension -- enable the relevant warnings, if any.
| Quote: | [Except when performance is the issue, temporary objects rarely become the
concern of the programmer. TCPL 10.4.10 ]
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Out of context that's a meaningless remark. There are two main reasons
why we generally care about temporary objects: efficiency and
correctness. And for the newbie also the issue of understanding what
goes on, e.g. understanding 'p++'. Neither efficiency nor correctness
is an issue with a simple type like 'int', but they both get more and
more relevant the larger the run-time and/or design-time cost of copying
an object is.
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
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Maciej Sobczak
Guest
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| Posted: Thu Jan 20, 2005 6:06 pm Post subject: Re: what is a temporary object? |
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Hi,
Marlene Miller wrote:
| Quote: | I am trying to understand what a temporary object is by example.
string("a") + string("b)
Q1. Is the result of this expression a temporary object?
|
Yes, because operator+ for strings returns by value.
This temporary object may be in some cases eliminated (depending on how
this operator is written and how you use its result).
| Quote: | class C {
public:
operator int() { return 2; }
};
cout << C() << endl;
Q2. Is C() a temporary object?
|
Yes.
| Quote: | Q3. A compiler might generate a temporary int object?
|
Yes.
| Quote: | C f() { C c; return c; }
Q4. Is the object returned by f() a temporary object?
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Yes. It may be in some cases eliminated.
| Quote: | cout << &C() << endl;
cout << &f() << endl;
Q5. C() and f() are rvalues.
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Yes.
| Quote: | & requires an lvalue.
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Yes.
| Quote: | Why can we say &C() and
&f()?
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We cannot.
But some compilers will accept it anyway.
| Quote: | Is a temporary object involved?
|
Yes.
| Quote: | [Except when performance is the issue, temporary objects rarely become the
concern of the programmer. TCPL 10.4.10 ]
|
Nevertheless, there are cases where temporary objects are not only of
concern, but may be also employed to perform useful work.
--
Maciej Sobczak : http://www.msobczak.com/
Programming : http://www.msobczak.com/prog/
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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s246794
Guest
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| Posted: Thu Jan 20, 2005 6:09 pm Post subject: Re: what is a temporary object? |
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Marlene Miller wrote:
| Quote: | I am trying to understand what a temporary object is by example.
|
A function can return a pointer to an object on the heap, e.g.,
int*
f(void)
{
int* i = new int;
*i = 0;
return i;
}
But if I redefine `f()' as follows:
int
f(void)
{
int i = 0;
return i;
}
then `i' is an automatic variable and the memory it occupies is
recovered by the run-time system when the stack frame representing the
call to `f()' is popped from the stack after `f()' returns. That's why
the following redefinition causes `gcc' to issue a warning:
int*
f(void)
{
int i = 0;
return &i;
}
When I ran an example, and output the value, it was correct, but this is
a very risky thing to do. Here, `f()' is returning an address, and at
some point during execution, it's likely that other data will be written
to this address. `int i' will no longer exist after the stack frame for
a given invocation of `f()' is popped from the stack.
Using the second definition, where `f()' returns an `int', I could call
`f()' like this:
int a = f();
`f()' causes a temporary object of type `int' to be created. Here, it's
used as the rvalue in the assignment to `a'. But I can also choose to
ignore the return value of `f()', e.g.,
f();
Even in the first case, the memory allocated for the temporary may very
well not be used for `a'. So what happens to the temporary? Well, if
you have a smart compiler, it will "disappear", i.e., the memory it
occupies will be recovered. I think this will happen in any compiler
you're likely to be using.
I think it's important to consider where a temporary could be.
Practically speaking, and leaving aside registers and data files,
there are only three kinds of memory accessible to a programmer:
static (in the memory allocated to a running process corresponding to
the `data' segment of the executable), automatic, i.e., on the stack,
and dynamic, i.e., on the heap. It's not going to be in static memory,
or it wouldn't be temporary. I don't know where C++ compilers allocate
memory for temporaries, but I think it would work on either the stack or
the heap. If anyone knows better, I'd be quite interested to find out.
A good way to gain an insight into the way temporaries work is to write
a compiler (or interpreter) using a compiler generator, such as yacc or
GNU Bison. I use temporaries in my parser for GNU 3DLDF. They are
allocated on the heap and deleted in the parser rules when they're no
longer needed. I always delete them unless I use them as the semantic
value of the left-hand side symbol in the rule. I do this so that I
don't have to implement garbage collection. One could argue that this
is a form of garbage collection, but I think of it as preventing any
"garbage" from piling up in the first place.
Laurence Finston
http://www.gnu.org/software/3dldf/LDF.html
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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