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In this noncompliant code example, a const-qualified method is called, which attempts to cache results by casting away the const-qualifier of this. Because s was declared const, the mutation of cachedCalc results cachedValue results in undefined behavior.
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|---|---|---|---|---|
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#include <iostream>
class S {
int cachedCalccachedValue;
int expensivecompute_computevalue() const; // expensive
public:
S() : cachedCalccachedValue(0) {}
// ...
int calculateget_value() const {
if (!cachedCalccachedValue) {
const_cast<S *>(this)->cachedCalc>cachedValue = expensivecompute_computevalue();
}
return cachedCalccachedValue;
}
};
void f() {
const S s;
std::cout << s.calculateget_value() << std::endl;
} |
Compliant Solution
This compliant solution uses the mutable keyword when declaring cachedCalccachedValue, which allows it to be mutated within a const context without triggering undefined behavior:
| Code Block | ||||
|---|---|---|---|---|
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#include <iostream>
class S {
mutable int cachedCalccachedValue;
int expensivecompute_computevalue() const; // expensive
public:
S() : cachedCalccachedValue(0) {}
// ...
int calculateget_value() const {
if (!cachedCalccachedValue) {
cachedCalccachedValue = expensivecompute_computevalue();
}
return cachedCalccachedValue;
}
};
void f() {
const S s;
std::cout << s.calculateget_value() << std::endl;
} |
Noncompliant Code Example
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