Why this const variable is not changing even after applying const

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Top Forums Programming Why this const variable is not changing even after applying const_cast? Post 302518096 by Loic Domaigne on Thursday 28th of April 2011 02:49:39 PM

04-28-2011

Registered User

Unless I am mistaken, modifying any object that was actually defined as const leads to an undefined behavior (which 10 at the end might indeed result).

I checked on Gnu/Linux : it seems that g++ simply replace i by the value of 10 during compilation (even tough it computes 11), as we can verify on the produced assembler code:

Code:

main:
.LFB957:
   .cfi_startproc
   .cfi_personality 0x3,__gxx_personality_v0
   pushq %rbp
   .cfi_def_cfa_offset 16
   movq  %rsp, %rbp
   .cfi_offset 6, -16
   .cfi_def_cfa_register 6
   subq  $16, %rsp
   movl  $10, -4(%rbp)
   leaq  -4(%rbp), %rax
   movl  $11, (%rax)            # here it is 11 
   movl  $10, %esi              # but 10 is used for cout
   movl  $_ZSt4cout, %edi
   call  _ZNSolsEi
   movl  $_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_, %esi
   movq  %rax, %rdi
   call  _ZNSolsEPFRSoS_E
   movl  $0, %eax
   leave
   ret
   .cfi_endproc

To verify this claim, it had the weird idea to qualify the const int i variable as volatile (why on the earth would we need to always re-read a const value from memory?). And tata: I got then 11.

Obviously, this is highly compiler/system depends. So the bottom line is: don't do that.

Cheers, Lo�c

Loic Domaigne

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gupnp_service_proxy_add_notify

GUPNP_SERVICE_PROXY_ADD_NOTIFY(3)					 1					 GUPNP_SERVICE_PROXY_ADD_NOTIFY(3)

gupnp_service_proxy_add_notify - Sets up callback for variable change notification

SYNOPSIS

       bool gupnp_service_proxy_add_notify (resource  $proxy, string  $value, int  $type, mixed  $callback, [mixed  $arg])

DESCRIPTION

	Sets up callback to be called whenever a change notification for variable is recieved.

PARAMETERS

	      o $proxy
		- A service proxy identifier.

	      o $value
		- The variable to add notification for.

	      o $type
		- The type of the variable. Type can be one of the following values:

		     o GUPNP_TYPE_BOOLEAN - Type of the variable is boolean.

		     o GUPNP_TYPE_INT - Type of the variable is integer.

		     o GUPNP_TYPE_LONG - Type of the variable is long.

		     o GUPNP_TYPE_DOUBLE - Type of the variable is double.

		     o GUPNP_TYPE_FLOAT - Type of the variable is float.

		     o GUPNP_TYPE_STRING - Type of the variable is string.

	      o $callback
		-  The	callback  to  call  when variable changes. Typically, callback function takes on three parameters. The $variable parameter
		being the first, $value parameter being the second, and the $arg is third.

	      o $arg
		- User data for $callback.

RETURN VALUES

	Returns TRUE on success or FALSE on failure.

SEE ALSO

       gupnp_service_proxy_remove_notify(3).

PHP Documentation Group 											 GUPNP_SERVICE_PROXY_ADD_NOTIFY(3)

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