Dynamic size of struct

[ose]

If a struct declared as:

struct x {int a; int b; string c;}

Since string c's content could be changed at runtime, does this mean that
"sizeof struct x" could be dynamic and changed at runtime as well? Is this a
good, valid way of using "struct"?

[Kai-Uwe Bux]

Re: Dynamic size of struct

ose wrote:

If a struct declared as:
>
struct x {int a; int b; string c;}
>
Since string c's content could be changed at runtime, does this mean that
"sizeof struct x" could be dynamic and changed at runtime as well?

No.

Is this a good, valid way of using "struct"?

Your member names are a little bit on the meaningless side. But the struct
is clearly valid and sometimes a struct like that (with better named
fields) even qualifies as good style.


Best

Kai-Uwe Bux

[Sam]

Re: Dynamic size of struct

ose writes:

If a struct declared as:
>
struct x {int a; int b; string c;}
>
Since string c's content could be changed at runtime, does this mean that
"sizeof struct x" could be dynamic and changed at runtime as well?

Of course not. Let's substitute your string with a char *:

struct x {int a; int b; char *c; }

sizeof(char *) is always the same, no matter how large the character string
the pointer points to. The pointer itself has constant size. Typically, on
32 bit machines the pointer is 4 bytes long, on 64 bit machines it's 8 bytes
long.

With your string, the concept is the same.

Is this a
good, valid way of using "struct"?

I see nothing wrong with that.

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[William Woody]

Re: Dynamic size of struct

In article
<1g7jk.269337$S V4.229394@bgtns c04-news.ops.worldn et.att.net>,
"ose" <ose29@hotmail. comwrote:

If a struct declared as:
>
struct x {int a; int b; string c;}
>
Since string c's content could be changed at runtime, does this mean that
"sizeof struct x" could be dynamic and changed at runtime as well? Is this a
good, valid way of using "struct"?

By 'string' I presume you mean the C++ standard string class.

Internally the std::string class maintains a pointer to a string; thus,
'string c' in your declaration above is akin to:

struct x { int a; int b; struct { overhead decls; char *ptr; } c };

Because you don't know the size of std::string because the overhead it
maintains is not defined, you cannot predict the size of struct x;
however, the overall size of struct x is fixed.

Internally variable length strings are maintained by the std::string
class by the equivalent of realloc() on ptr; thus, your struct points to
another chunk of memory that may bounce around and change size in the
heap as you manipulate the string c.

Hope this helps.

--
William Edward Woody - woody@alumni.ca ltech.edu
Chaos In Motion - http://www.chaosinmotion.com

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