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What is a mutable type specifier in C++?
Danil
The mutable keyword permits modification of a class member declared mutable. This is permitted even if the containing object is declared const.
Code
#include <unordered_map>
#include <iostream>
struct FactorialHelper {
// Overloaded call operator
long long operator()(int n) {
long long result = 1;
// If cache contains factorial of n, then load it, else calculate it and store in cache
if(cache.find(n) != cache.end()) {
result = cache[n];
} else {
for(int i = 1; i <= n; ++i) {
result *= i;
}
cache[n] = result;
}
return result;
}
private:
std::unordered_map<int, long long> cache;
};
int main() {
FactorialHelper factorial;
std::cout << factorial(5);
}
This code compiles without any errors and outputs the number
, as it should. 120 5! = 1 * 2 * 3 * 4 * 5 = 120
But our public API isn’t perfect, and doesn’t allow using our class in a constant context:
main.cpp
factorial.h
#include <iostream>
#include "factorial.h"
class SomeClass {
long long someFunction(int a, int b, int c) const {
return factorial((a + b) % c);
}
private:
FactorialHelper factorial;
};
int main() {
SomeClass object;
object.someFunction(2, 10, 5);
// ^ Error: candidate function not viable: 'this' argument has type 'const FactorialHelper', but method is not marked const
const FactorialHelper f;
std::cout << f(5);
// ^ Error: candidate function not viable: 'this' argument has type 'const FactorialHelper', but method is not marked const
}To fix that, we decide to mark our overloaded call operator as const:
struct FactorialHelper {
long long operator()(int n) const {
// ...
}
// ...
};
But now, we are faced with another error. Since our method is marked as const, the compiler does not allow us to change the state of the cache by adding a new value to it.
This is where the mutable keyword comes in.
Let’s change our code like this:
struct FactorialHelper {
long long operator()(int n) {
// ...
}
private:
mutable std::unordered_map<int, long long> cache;
};
Now everything works well, even in a const context.
main.cpp
factorial.h
#include <iostream>
#include "factorial.h"
struct SomeClass {
long long someFunction(int a, int b, int c) const {
return factorial((a + b) % c);
}
private:
FactorialHelper factorial;
};
int main() {
SomeClass object;
std::cout << object.someFunction(2, 10, 5) << std::endl;
const FactorialHelper f;
std::cout << f(5) << std::endl;
}
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