C++ mutable keyword non-static class member modifier
Example
mutable modifier in this context is used to indicate that a data field of a const object may be modified without affecting the externally-visible state of the object.
If you are thinking about caching a result of expensive computation, you should probably use this keyword.
If you have a lock (for example, std::unique_lock) data field which is locked and unlocked inside a const method, this keyword is also what you could use.
You should not use this keyword to break logical const-ness of an object.
Example with caching:
class pi_calculator {
public:
double get_pi() const {
if (pi_calculated) {
return pi;
} else {
double new_pi = 0;
for (int i = 0; i < 1000000000; ++i) {
// some calculation to refine new_pi
}
// note: if pi and pi_calculated were not mutable, we would get an error from a compiler
// because in a const method we can not change a non-mutable field
pi = new_pi;
pi_calculated = true;
return pi;
}
}
private:
mutable bool pi_calculated = false;
mutable double pi = 0;
};
