constant behaviour in C

Question

Please look at this simple code..

#include <iostream>
using namespace std;

void fun(const int *p) {
    int *q = (int *)p;
    int tmp = 6;
    memcpy(q, &tmp, 4);
}

int main() {
    int foo = 5;
    const int *p = &foo;
    cout << foo << endl;
    fun(p);
    cout << foo << endl;
    return 0;
}

result: 5 6

I don't think foo should not changeable.

Answer 1

Well since you told the compiler to ignore the const, you're getting
undefined behaviour.

Turn the warnings up on your compiler and you'll see it complain:

[pherring@thinkpad-pjh tmp]$ cat const.cpp
#include <iostream>
#include <cstring>

using namespace std;

void fun(const int *p) {
        int *q = (int *)p;
        int tmp = 6;
        memcpy(q, &tmp, 4);
}

int main() {
        int foo = 5;
        const int *p = &foo;
        cout << foo << endl;
        fun(p);
        cout << foo << endl;
        return 0;
}
[pherring@thinkpad-pjh tmp]$ g++ const.cpp -o const -g -O -Wall -Weffc++
-pedantic  -pedantic-errors -Wextra  -Wall -Waggregate-return -Wcast-align
-Wcast-qual  -Wchar-subscripts  -Wcomment -Wconversion
-Wdisabled-optimization -Werror -Wfloat-equal  -Wformat  -Wformat=2
-Wformat-nonliteral -Wformat-security  -Wformat-y2k -Wimplicit  -Wimport
-Winit-self  -Winline -Winvalid-pch   -Wunsafe-loop-optimizations
-Wlong-long -Wmissing-braces -Wmissing-field-initializers
-Wmissing-format-attribute   -Wmissing-include-dirs -Wmissing-noreturn
-Wpacked  -Wpadded -Wparentheses  -Wpointer-arith -Wredundant-decls
-Wreturn-type -Wsequence-point  -Wshadow -Wsign-compare  -Wstack-protector
-Wstrict-aliasing -Wstrict-aliasing=2 -Wswitch  -Wswitch-default
-Wswitch-enum -Wtrigraphs  -Wuninitialized -Wunknown-pragmas
-Wunreachable-code -Wunused -Wunused-function  -Wunused-label
-Wunused-parameter -Wunused-value  -Wunused-variable  -Wvariadic-macros
-Wvolatile-register-var  -Wwrite-strings
cc1plus: warnings being treated as errors
const.cpp: In function ‘void fun(const int*)’:
const.cpp:7:18: error: cast from type ‘const int*’ to type ‘int*’ casts
away qualifiers
[pherring@thinkpad-pjh tmp]$

Answer 2

Declaring *const int ** is equals to *int const ** i.e pointer to constant integer. Attribute of constant integer is one can not change its content and will give you compilation error if you do so.

But we can change the content of constant integer using pointer to integer. By looking at the below code, p is not pointing to constant integer, will not give you compile time error and hence will allow to change the content of foo.

Hope this helps.