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The C++ Standard, [dcl.type.cv], paragraph 4 [ISO/IEC 14882-2014], states the following:

Except that any class member declared mutable can be modified, any attempt to modify a const object during its lifetime results in undefined behavior.

Similarly, paragraph 6 states the following:

What constitutes an access to an object that has volatile-qualified type is implementation-defined. If an attempt is made to refer to an object defined with a volatile-qualified type through the use of a glvalue with a non-volatile-qualified type, the program behavior is undefined.

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In this noncompliant code example, the function g() is passed a const int &, which is then cast to an int & and modified. Because the referenced value referenced to was previously declared as const, the assignment operation results in undefined behavior.

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In this noncompliant code example, a const-qualified method is called , which that attempts to cache results by casting away the const-qualifier of this. Because s was declared const, the mutation of cachedValue results in undefined behavior.

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This compliant solution uses the mutable keyword when declaring cachedValue, which allows it cachedValue to be mutated within a const context without triggering undefined behavior:.

#include <iostream>
 
class S {
  mutable int cachedValue;
  
  int compute_value() const;  // expensive
public:
  S() : cachedValue(0) {}
  
  // ...  
  int get_value() const {
    if (!cachedValue) {
      cachedValue = compute_value();  
    }        
    return cachedValue;
  }
};

void f() {
  const S s;
  std::cout << s.get_value() << std::endl;
}

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This compliant solution assumes that the volatility of s is required, and so g() is modified to accept a volatile S &:.

#include <iostream>

struct S {
  int i;
  
  S(int i) : i(i) {}
};

void g(volatile S &s) {
  std::cout << s.i << std::endl;
}

void f() {
  volatile S s(12);
  g(s);
}

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EXP55-CPP-EX1: An exception to this rule is allowed when it is necessary to cast away const when invoking a legacy API that does not accept a const argument, provided the function does not attempt to modify the referenced variable. It However, it is always preferable to modify the API to be const-correct when possible, however. For example, the following code casts away the const qualification of INVFNAME in the call to the audit_log() function.

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If the object is declared as being constant, it may reside in write-protected memory at runtime. Attempting to modify such an object may lead to abnormal program termination or a denial-of-service attack. If an object is declared as being volatile, the compiler can make no assumptions regarding access of that object. Casting away the volatility of an object can result in reads or writes to the object being reordered , or elided entirely, resulting in abnormal program execution.

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