Does Const have an effect on the execution

The other day, a C++ trainer said to me that “const” does have meaning only in compile time (statically) and thus doesn’t have an influence in runtime… but when I test this example :

const int x = 5;
int * px = const_cast(&x);
*px = 10;
std::cout << "x = " << x <<std::endl; // x = 5 ???

x is not modified with 10 ! whereas, this example works as expected if we use pointers:

const int * x = new int(5);
int * px = const_cast(x);
*px = 10;
std::cout << "x = " << *x <<std::endl; // x = 10

So, this C++ trainer is wrong ?

It’s undefined behaviour to modify a const T
via const_cast
. This allows the compiler to optimise your code assuming it never happens. In your first code example, the compiler probably inserted a literal 5
in the code during the call to operator<<

Your second example is completely well defined, because x
is really pointing at an int
, not
a const int
. In this case, casting away the const
ness is fine.

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