C/C++: casting away volatile considered harmful?

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If the variable is declared volatile then it is undefined behaviour to cast away the volatile, just as it is undefined behaviour to cast away the const from a variable declared const. See Annex J.2 of the C Standard:

The behavior is undefined in the following circumstances:

…

— An attempt is made to modify an object defined with a const-qualified type through
use of an lvalue with non-const-qualified type (6.7.3).

— An attempt is made to refer to an object defined with a volatile-qualified type through
use of an lvalue with non-volatile-qualified type (6.7.3).

If, however, you just have a volatile pointer or a volatile reference to a non-volatile variable then you can freely cast away volatile.

volatile int i=0;
int j=0;

volatile int* pvi=&i; // ok
volatile int* pvj=&j; // ok can take a volatile pointer to a non-volatile object

int* pi=const_cast<int*>(pvi); // Danger Will Robinson! casting away true volatile
int* pj=const_cast<volatile int*>(pvj); // OK
*pi=3; // undefined behaviour, non-volatile access to volatile variable
*pj=3; // OK, j is not volatile

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