C Programming Language :: const c structure question


	const c structure question Hi all, I have this struct typedef: typedef struct { int * test; }testst; and have this declaration: const testst abc; What is constant? The pointer (like in: int * const test) or the integer (like in const int * test) or both? Thanx, Milux "nospam" <""miluxkloosterman\"@(nospam)home.nl"> wrote in message news:f446g0$qm5$1@netlx020.civ.utwente.nl... > Hi all, > I have this struct typedef: > typedef struct > { > int * test; > }testst; > and have this declaration: > const testst abc; > What is constant? The object named 'abc'. > The pointer (like in: int * const test) or the integer (like in const int > * test) or both? The entire object is const. Which means that each individual member of the struct is const. Your struct contains only one member, a pointer (type 'int'), there is no type 'int' member. So it's not a question of 'one of or both', since that presumes two members. The pointer 'test' is const. What it points to (if anything) is not const. A pointer to a const int is declared: const int p; or int const p; What specifically are you trying to do? Write code for a particular task, or understand something written by someone else? -Mike In article <f446g0$qm@netlx020.civ.utwente.nl>, nospam <""miluxkloosterman\"@(nospam)home.nl"> wrote: >Hi all, >I have this struct typedef: >typedef struct >{ > int test; >}testst; For the purposes of exploring constness, the struct is unnecessary obfuscation. The question would apply just as well to typedef int testp; >and have this declaration: >const testst abc; const testp abcde; >What is constant? The pointer (like in: int const test) or the integer >(like in const int * test) or both? abcde is a const pointer to non-const int. abc.test is also a const pointer to non-const int. -- Alan Curry pac@world.std.com - Hide quoted text - > ... > The object named 'abc'. > ... > The entire object is const. Which means that each > individual member of the struct is const. Your struct > contains only one member, a pointer (type 'int'), there > is no type 'int' member. So it's not a question of 'one > of or both', since that presumes two members. The pointer > 'test' is const. What it points to (if anything) is not > const. A pointer to a const int is declared: > const int p; > or > int const p; > What specifically are you trying to do? Write > code for a particular task, or understand something > written by someone else? I was exploring this because the structure contains variables declared volatile. Thus volatile int const test; But because the programm was not working correctly, I thought it had to do with the object declared const. But I understand you explanation so this part was correct, thanx. - Hide quoted text - > -Mike nospam wrote: > I have this struct typedef: > typedef struct > { > int * test; > } testst; > and have this declaration: > const testst abc; > What is constant? The pointer (like in: int * const test) or the > integer (like in const int * test) or both? Nothing. You failed to initialize abc. This is the only place you can set its value. F'ups set. -- <http://www.cs.auckland.ac.nz/~pgut001/pubs/vista_cost.txt> <http://www.securityfocus.com/columnists/423> <http://www.aaxnet.com/editor/edit043.html> <http://kadaitcha.cx/vista/dogsbreakfast/index.html> cbfalconer at maineline dot net -- Posted via a free Usenet account from http://www.teranews.com nospam wrote: > I have this struct typedef: > typedef struct > { > int * test; > } testst; > and have this declaration: > const testst abc; > What is constant? The pointer (like in: int * const test) or the > integer (like in const int * test) or both? Nothing. You failed to initialize abc. This is the only place you can set its value. -- <http://www.cs.auckland.ac.nz/~pgut001/pubs/vista_cost.txt> <http://www.securityfocus.com/columnists/423> <http://www.aaxnet.com/editor/edit043.html> <http://kadaitcha.cx/vista/dogsbreakfast/index.html> cbfalconer at maineline dot net -- Posted via a free Usenet account from http://www.teranews.com nospam wrote: > I have this struct typedef: > typedef struct > { > int * test; > } testst; > and have this declaration: > const testst abc; > What is constant? The pointer (like in: int * const test) or the > integer (like in const int * test) or both? Nothing. You failed to initialize abc. This is the only place you can set its value. F'ups set. -- <http://www.cs.auckland.ac.nz/~pgut001/pubs/vista_cost.txt> <http://www.securityfocus.com/columnists/423> <http://www.aaxnet.com/editor/edit043.html> <http://kadaitcha.cx/vista/dogsbreakfast/index.html> cbfalconer at maineline dot net -- Posted via a free Usenet account from http://www.teranews.com [restored attributions] Milux wrote: > Mike Wahler wrote: >> What specifically are you trying to do? Write >> code for a particular task, or understand something >> written by someone else? > I was exploring this because the structure contains variables declared > volatile. Thus > volatile int * const test; > But because the programm was not working correctly, I thought it had > to do with the object declared const. But I understand you explanation > so this part was correct, thanx. While exploring this, you should be aware that 'const' is actually very badly named. The effect of const-qualifying something is only to mark it as read-only. It does not make it constant/unchangeable. For example, on machines that allow direct access to (memory mapped) hardware, you could have something like this const volatile int * const current_time = (const volatile int*)0x1234; to gain access to a hardware timer. Through the variable current_time, you are allowed to read the current value of the timer, but you can not modify it from within the program. Obviously, the value you read will differ over time. >> -Mike Bart v Ingen Schenau -- a.c.l.l.c-c++ FAQ: http://www.comeaucomputing.com/learn/faq c.l.c FAQ: http://www.eskimo.com/~scs/C-faq/top.html c.l.c++ FAQ: http://www.parashift.com/c++-faq-lite/