const: Difference in C and C++

[dilip.mathews]

I have executed the below programm on both C and C++ compiler and got different results

#include <stdio.h>
int main() 
{
 const int a=10;
 int *p=(int*)(&a); 
 *p = 20;
    printf("address of a=%u\t%d\n", &a,a);
    printf("address of p=%u\t%d\n", p,*p);
    return 0;
}

With C compiler I got the output
address of a=1245052 20
address of p=1245052 20

With C++ compiler I got
address of a=1245052 10
address of p=1245052 20

Can anybody tell me why the different value for *p and a even thiough both are pointing to the same memory location?

[Ancient Dragon]

I think you are encountering what is commonly called "undefined behavior" or "implemention specific". In C, const int a points to a memory location that can be modified, but in c++ variable a is not located at any memory variable but just used as an immediate value, which is similar to just hard-coding a literal value in the program. You could have rewritten the lines like this:

c++
 printf("address of a=%u\t%d\n", &a,10);

c
 printf("address of a=%u\t%d\n", &a,a);

[dilip.mathews]

Can you tell me where the variable be put when it is declared as const. Will it go into const segment.

"You can also specify sections for initialized data (data_seg), functions (code_seg), and const variables (const_seg)."

The above is an extract from msdn. If it goes into a particular segment of memory, why the value is not getting changed when modified thru a pointer.
From your expalnation what I understood is that the const variable is replaced by the value whenever it is encountered by the compiler.
i.e it is not stored at a specific location.

Now if I use C compiler why is the value getting changed or why it is behaving differently?

[WolfPack]

Actually the memory location for a does get modified in Visual C++. But this is implementation specific. It may not be so in gcc. But the catch is that Visual C++ never uses the value stored in a 's memory location. It is allowed to do so by the C++ standard. That is because if you declare a as const int , that means you are never going to change it's value. So as an optimization, the compiler can use the literal directly instead of the actual value stored in a . You can see this fact if you take a look at the assembly code for your program.

I have attached the assembly files in text format and line 7 is the relevant portion.

CPP Assembly

; Line 7
    push    10                  ; 0000000aH
    lea edx, DWORD PTR _a$[ebp]
    push    edx
    push    OFFSET FLAT:$SG611
    call    _printf
    add esp, 12                 ; 0000000cH

The code in red just pushes the literal 10 to the stack. Not the contents of a . So the contents of a will NOT be displayed.

C Assembly

; Line 7
    mov edx, DWORD PTR _a$[ebp]
    push    edx
    lea eax, DWORD PTR _a$[ebp]
    push    eax
    push    OFFSET FLAT:$SG795
    call    _printf
    add esp, 12                 ; 0000000cH

The code in red loads the contents of a to edx , and then pushes the contents in edx to the stack. So the contents in a will be displayed.

[dilip.mathews]

Your explanation was very usefull. I have one more doubt in the same context.

This assembly thing is quite new for me. In the cpp assembly line 7 states push 10.
So does that mean in Cpp the constant variable are also pushed onto the stack?????

[WolfPack]

I can't say anything about cpp. That is not specified in the standard. It only happens in Visual C++. It may be different in gcc. But in VC++, the variable is not pushed into the stack. Only the literal it holds. In this case it is 10.

[Dave Sinkula]

We had this topic no so long ago...
[thread]50101[/thread]

[WolfPack]

bah. where were you yesterday?