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Pramod
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 Posted: Thu Jul 15, 2004 11:51 pm Post subject: Creating a data type dynamically |
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I need to create a data type whose size is determined at runtime. A
part of the program looks like this:
class CMI {
size_t objSize;
public:
union {
char data[objSize];
CMI * next ;
};
CMI(size_t s):objSize(s) {}
void func() { ... }
};
and this class will be used like this:
class AUI {
static CMI auicmi ;
public:
void foo() { auicmi.func() }
};
CMI AUI::auicmi(sizeof(AUI));
Is there any way to achieve this?
"char data[objSize]" is wrong since objSize is not a compiler
constant.
Can templates help?
Thanks
-Pramod
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Ulrich Eckhardt
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| Posted: Fri Jul 16, 2004 11:14 am Post subject: Re: Creating a data type dynamically |
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Pramod wrote:
| Quote: | I need to create a data type whose size is determined at runtime.
|
Impossible.
| Quote: | A part of the program looks like this:
class CMI {
size_t objSize;
public:
union {
char data[objSize];
CMI * next ;
};
CMI(size_t s):objSize(s) {}
void func() { ... }
};
|
Now for the workarounds:
1. you could make it a template, taking the objSize as one argument, i.e.
making different types for different sizes.
2a. you could create external storage with new/malloc, or, better, use
std::vector or something like that.
2b. You could even allocate that storage so that the fixed parts of the
object are on the same slice of memory as the non-fixed ones.
A short sketch of what I mean[1]:
CreateCMI(size_t size){
void* ptr = ::new(sizeof (CMI) + size); //TODO: padding/alignment
new (ptr)CMI(size);// placement new
return ptr;
}
char* CMI::get_storage(){
return static_cast<char*>(this+1);
}
3. you could change your design. Without knowing what you are trying to
achieve it is impossible to tell though.
Uli
[1] A GCC extension allows things like
struct header
{
size_t size;
size_t used;
char array[0];
};
This struct is then allocated with malloc() or similar tools and the array
gets the overallocated storage. Are there any pros/cons to that technique?
--
Questions ?
see C++-FAQ Lite: http://parashift.com/c++-faq-lite/ first !
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Michiel Salters
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| Posted: Fri Jul 16, 2004 1:59 pm Post subject: Re: Creating a data type dynamically |
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[email]pramod_iitg (AT) yahoo (DOT) com[/email] (Pramod) wrote in message news:<581b1e22.0407150426.b7e7292 (AT) posting (DOT) google.com>...
| Quote: | I need to create a data type whose size is determined at runtime. A
part of the program looks like this:
class CMI {
size_t objSize;
public:
union {
char data[objSize];
CMI * next ;
};
CMI(size_t s):objSize(s) {}
void func() { ... }
};
and this class will be used like this:
class AUI {
static CMI auicmi ;
public:
void foo() { auicmi.func() }
};
CMI AUI::auicmi(sizeof(AUI));
Is there any way to achieve this?
"char data[objSize]" is wrong since objSize is not a compiler
constant.
|
There is quite a lot wrong with such a design. There are much saner
implementations, which would do what you want. But basically, yes,
templates will work: (ignoring xs)
class CMI_base {
virtual void func() = 0;
};
class CMI_link : public CMI_base {
void func() { ; }
CMI_base* next;
};
template< typename TheObjectToStore >
class CMI_data : public CMI_base {
TheObjectToStore obj;
void func() { ; }
};
class AUI {
static std::auto_ptr<CMI_base> auicmi ;
public:
void foo() { auicmi->func(); }
};
std::auto_ptr<CMI_base> AUI::auicmi(new CMI_data<AUI>);
Regards,
Michiel Salters
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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