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INTERVIEW QUESTIONS LANGUAGES C++ DETAILS
Question :
mutable" Keyword - What is "mutable"?


Category C++ Interview Questions
Rating (3.8) By 12 users
Added on 7/16/2015
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Answers:

Answer1.
"mutable" is a C++ keyword. When we declare const, none of its data members can change. When we want one of its members to change, we declare it as mutable.

Answer2.
A "mutable" keyword is useful when we want to force a "logical const" data member to have its value modified. A logical const can happen when we declare a data member as non-const, but we have a const member function attempting to modify that data member. For example:

class Dummy {
public:
bool isValid() const;
private:
mutable int size_ = 0;
mutable bool validStatus_ = FALSE;
// logical const issue resolved
};

bool Dummy::isValid() const
// data members become bitwise const
{
if (size > 10) {
validStatus_ = TRUE; // fine to assign
size = 0; // fine to assign
}
}


Answer3.
"mutable" keyword in C++ is used to specify that the member may be updated or modified even if it is member of constant object. Example:

class Animal {
private:
string name;
string food;
mutable int age;
public:
void set_age(int a);
};

void main() {
const Animal Tiger(’Fulffy’,'antelope’,1);
Tiger.set_age(2);
// the age can be changed since its mutable
}



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