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This also applies to objects that behave as if they were defined with qualified types, such as an object at a memory-mapped input/output address.

Non-Compliant Code Example

In this example, a volatile object is accessed through a non-volatile-qualified reference, resulting in undefined behavior.

int main(void) {
  static volatile int **ipp;
  static int *ip;
  static volatile int i = 0;

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

  ipp = &ip; /* constraint violation */
  *ipp = &i; /* valid */
  if (*ip != 0) { /* valid */
     ...
  }
}

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 it is not possible that i != 0 if i is not volatile.

Implementation Specific Details

This example compiles without warning on Microsoft Visual C++ .NET (2003) and on MS Visual Studio 2005. Version 3.2.2 of the gcc compiler generates a warning but compiles.

Compliant Solution

In this compliant solution, ip is declared as volatile.

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Priority: P6 Level: L2

Accessing a volatile object through a non-volatile reference results in undefined behavior.

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