Mutable Counter in Const

Question

Problem: Mutable Counter in Const. Solve this hands-on electronics task on EWskills.

Accepted Answer

#include 
using namespace std;

class AccessTracker {
public:
    AccessTracker() : count(0) {}

    // const function allowed to update internal bookkeeping
    void logAccess() const {
        count++;
    }

    int getCount() const {
        return count;
    }

private:
    mutable int count;  // allows modification inside const functions
};

int main() {
    int n;
    cin >> n;

    const AccessTracker tracker;

    for (int i = 0; i < n; i++) {
        tracker.logAccess();
    }

    cout << tracker.getCount();
    return 0;
}

Explanation & Logic Summary:

A const member function promises not to modify the object’s logical external state, such as hardware registers or outputs.
Updating internal counters does not break this promise because they do not change externally observable behavior.
C++ provides a dedicated mechanism for this purpose: marking member variables as mutable.
This preserves const-correctness while allowing safe internal bookkeeping.

Firmware Relevance & Real Embedded Context:

Many embedded drivers update internal counters or cached values inside const functions.

Example:

class TempSensor {
public:
    int readTemperature() const {
        accessCount++;  // allowed: internal bookkeeping
        return i2cReadRegister(TEMP_REG);
    }
private:
    mutable uint32_t accessCount;
};

Common real-world uses:

Tracking how many times a sensor register was read
Counting interrupts or events
Maintaining cached sensor values
Updating timestamps inside const driver APIs
Profiling and diagnostics
Usage tracking for power/performance tuning

Keeping the driver object const is common to prevent accidental modification of hardware settings by application code.

71. Mutable Counter in Const