c++ 中的 mutable 关键字是什么意思?
mutable is for the case where an object is logically constant, but in practice needs to change.
class Employee {
public:
Employee(const std::string & name)
: _name(name), _access_count(0) { }
void set_name(const std::string & name) {
_name = name;
}
std::string get_name() const {
_access_count++;
return _name;
}
int get_access_count() const { return _access_count; }
private:
std::string _name;
mutable int _access_count;
};
class MathObject {
public:
MathObject() : pi_cached(false) { }
double pi() const {
if( ! pi_cached ) {
/* This is an insanely slow way to calculate pi. */
pi = 4;
for(long step = 3; step < 1000000000; step += 4) {
pi += ((-4.0/(double)step) + (4.0/((double)step+2)));
}
pi_cached = true;
}
return pi;
}
private:
mutable bool pi_cached;
mutable double pi;
};
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