An object that has volatile-qualified type may be modified in ways unknown to the implementation or have other unknown side effects. Referencing a volatile object by using a non-volatile lvalue results in is undefined behavior. The C Standard, 6.7.3 [ISO/IEC 9899:2011], states:
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In this noncompliant code example, a volatile object is accessed through a non-volatile-qualified reference, resulting in undefined behavior:
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The assignment ipp = &ip is not safe because it allows 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 *ip != 0 must be false if the object to which ip points is not volatile.
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Accessing an object with a volatile-qualified type through a reference with a non-volatile-qualified type is undefined behavior.
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