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Petr Maxa
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 Posted: Mon Nov 10, 2003 7:56 pm Post subject: linkage of cost objects |
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First of all, I would like to ask, if the following sentence is valid:
"C++ compiler is allowed not to store constant in memory, but in a case that
it is used only to initialization or array size specification, it can use
directly its value."
If the above is valid, then I have the following questions:
1) What is the linkage of const object? Internal or no linkage ?
2) Provided I have the following code:
/* const_test.cpp */
#include <cstdio>
const int cp = 10;
void foo() { printf("%d n",cp); }
int main()
{
int *pp = const_cast<int*>(&cp);
*pp = 4;
foo();
return 0;
}
is the above code correct?
Thanks,
Petr Maxa (petr dot maxa at siemens dot com)
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Jack Klein
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| Posted: Tue Nov 11, 2003 5:29 am Post subject: Re: linkage of cost objects |
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On Mon, 10 Nov 2003 19:56:30 +0000 (UTC), [email]Petr_Maxa (AT) siemens (DOT) com[/email] ("Petr
Maxa") wrote in comp.std.c++:
| Quote: | First of all, I would like to ask, if the following sentence is valid:
"C++ compiler is allowed not to store constant in memory, but in a case that
it is used only to initialization or array size specification, it can use
directly its value."
|
That is correct. If the address of a constant object is not taken,
the compiler does not need to generate the object and may merely use
the value to which it was initialized.
| Quote: | If the above is valid, then I have the following questions:
1) What is the linkage of const object? Internal or no linkage ?
|
In C++, a const object defined at file scope has internal linkage
unless defined with the extern keyword. This is a quiet change from
C, where such objects have external linkage unless defined with the
static keyword.
| Quote: | 2) Provided I have the following code:
/* const_test.cpp */
#include <cstdio
const int cp = 10;
void foo() { printf("%d n",cp); }
int main()
{
int *pp = const_cast
*pp = 4;
foo();
return 0;
}
is the above code correct?
|
No the code above is not correct. It is illegal to attempt to modify
the value of an object defines as const.
| Quote: | Thanks,
Petr Maxa (petr dot maxa at siemens dot com)
|
--
Jack Klein
Home: http://JK-Technology.Com
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Fergus Henderson
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| Posted: Wed Nov 12, 2003 4:32 am Post subject: Re: linkage of cost objects |
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[email]Petr_Maxa (AT) siemens (DOT) com[/email] ("Petr Maxa") writes:
| Quote: | First of all, I would like to ask, if the following sentence is valid:
"C++ compiler is allowed not to store constant in memory, but in a case that
it is used only to initialization or array size specification, it can use
directly its value."
|
That is correct. But constants whose address is taken and used in ways
that the compiler can't analyze may have to be stored in memory.
| Quote: | 1) What is the linkage of const object? Internal or no linkage ?
|
Internal.
| Quote: | 2) Provided I have the following code:
/* const_test.cpp */
#include <cstdio
const int cp = 10;
void foo() { printf("%d n",cp); }
int main()
{
int *pp = const_cast
*pp = 4;
foo();
return 0;
}
is the above code correct?
|
No. This has undefined behaviour, because it modifies an object that
was defined as const.
--
Fergus Henderson <fjh (AT) cs (DOT) mu.oz.au> | "I have always known that the pursuit
The University of Melbourne | of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh> | -- the last words of T. S. Garp.
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