How do you update/reassign a class member Chrono::timestamp for rolling elapse times

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30-08-2024

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Updating Chrono::timestamp for Rolling Elapse Times: A Deep Dive

This article explores the common challenge of updating Chrono::timestamp values within a class for tracking rolling elapsed times. We'll analyze the provided example code, understand the error, and provide solutions with additional insights.

The Problem: Modifying a const Member

The core issue arises from trying to modify m_timestamp within a const member function. The error message "passing 'const std::chrono::_V2::system_clock::time_point' ... as 'this' argument discards qualifiers" highlights this conflict.

The code snippet provided, with the const qualifier on the trackTime function, prevents modification of member variables within the function's scope.

Example Code Breakdown:

#include <iostream>
#include <chrono>
#include <cstdint>
namespace chrono = std::chrono;
namespace chrono_lit = std::chrono_literals;
using namespace chrono_lit;

class TrackTimeElapsed {
public:
    TrackTimeElapsed(uint64_t interval) :
        m_timestamp(chrono::steady_clock::now()), m_interval(interval) {}
    ~TrackTimeElapsed() {}
    void trackTime() const { // This line causes the error
        chrono::time_point<chrono::steady_clock> now = chrono::steady_clock::now();
        auto diff = chrono::duration_cast<std::chrono::seconds>(now - m_timestamp);
        if (diff.count() > m_interval) {
            std::cout << "Time elapsed" << std::endl;
            // Update timestamp to help track when next print out needs to occur.
            m_timestamp = now; // Error occurs here
        }
    }
private:
    chrono::time_point<chrono::steady_clock> m_timestamp;
    uint64_t m_interval;
};

Solutions and Considerations

Here are two approaches to address this issue:

1. Remove const Qualifier:

   The simplest solution is to remove the const qualifier from the trackTime function. This allows the function to modify the m_timestamp member.

   void trackTime() { // Remove the `const` qualifier
       chrono::time_point<chrono::steady_clock> now = chrono::steady_clock::now();
       // ... rest of the code
       m_timestamp = now; 
   }
   

   Important Note: This approach might be suitable if the trackTime function truly needs to update the m_timestamp. However, if the goal is to maintain the const nature of the function for encapsulation or design reasons, alternative solutions are recommended.

2. Utilize a Mutable Member:

   This solution allows the function to be const while still enabling modification of the m_timestamp member. We can use the mutable keyword for the m_timestamp member.

   class TrackTimeElapsed {
   // ... other code ...
   private:
       mutable chrono::time_point<chrono::steady_clock> m_timestamp;
       uint64_t m_interval;
   };
   

   Explanation:

   • The mutable keyword allows a member to be modified even within a const member function. This means that the trackTime function can update the m_timestamp member even though the function itself is marked const.
   • This approach helps maintain the const nature of the function, ensuring that the class's internal state is not modified by accident.

Practical Considerations and Improvements

• Precision and Accuracy: While using std::chrono for time tracking is excellent, consider the precision needed for your application. If high precision is critical, you might need to explore other libraries or techniques for precise time measurements.
• Performance: The frequency of updating m_timestamp can impact performance. If performance is a concern, optimize the update logic.
• Error Handling: Consider incorporating robust error handling mechanisms to handle unexpected time-related scenarios, such as time jumps or time drift.

Further Learning

• Chrono Documentation: Dive deeper into std::chrono for a comprehensive understanding: https://en.cppreference.com/w/cpp/chrono
• Stack Overflow: Utilize Stack Overflow for additional insights and solutions from experienced developers: https://stackoverflow.com/

Remember: The choice of approach depends on your application's specific requirements. Carefully analyze your needs and select the best solution for accurate, reliable, and efficient time tracking.