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static volatile int **ipp;
static int *ip;
static volatile int i = 0;

printf("i = %d.\n", i);

ipp = &ip; /* producesProduces warnings in modern compilers */
ipp = (int**) &ip; /* constraintConstraint violation, also produces warnings */
*ipp = &i; /* validValid */
if (*ip != 0) { /* validValid */
  /* ... */
}

The assignment ipp = &ip is unsafe because it would allow the valid code that follows to reference the value of the volatile object i through the non-volatile-qualified reference ip. In this example, the compiler may optimize out the entire if block because i != 0 must be false if i is not volatile.

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This example compiles without warning on Microsoft Visual Studio 2012 when compiled in C mode (/TC) , but causes errors when compiled in C++ mode (/TP).

Version GCC 4.8.1 of GCC generates a warning but compiles successfully.

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Casting away volatile allows access to an object through a nonvolatile reference . This and can result in undefined and perhaps unintended program behavior.

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Bibliography

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