如何重载类成员变量上的赋值运算符

Question

I am trying to track the value of a variable that I will input in an API function. One option is to overload the assignment operator and put some code there. But how would I overload an assigment operator on a member variable of a class?

#include <iostream>
using namespace std;

template <class T>
class MonitoredVariable1
{

public:
    MonitoredVariable1() {  }
    MonitoredVariable1(const T& value) : m_value(value) {}

    operator T() const { return m_value; }

    T val;

    T& operator = (const T& value)
    {
        val = value;
        m_value = value;
        std::cout << "value updated" << " \n";  //THIS NEVER GET PRINTED!!!

        return val;
    }

private:
    T m_value;
};


int main()
{

    MonitoredVariable1<double> MonitoredVariable;
    MonitoredVariable.val = 10.2;

    std::cout << "main done..."  << " \n";
    return 0;
}

Answer 1

To monitor changes to the variable, you need to be assigning to the class, not the contained variable.

First, get rid of val . Only have the private m_value value. This way all accesses have to go thru your member functions that can track the changes.

operator= should return a reference to the class ( return *this; ), not the value.

Assignment is to the class object:

MonitoredVariable = 10.2;

Answer 2

You can only overload assignment on a class. But you can make that variable to be of a class type with overloaded assignment, like:

class Monitor {
    class Monitored {
        double x;
    public:
        Monitored &operator= (double v) {
            std::cout << "Assigned " << v << std::endl;
            x = v;
            return *this; // don’t forget this!
        }

        operator double() const {
            std::cout << "Accessed " << x << std::endl;
            return x;
        }
    };
    Monitored val;
};

You may need to overload more operators, and also to pass a reference to Monitor into val (there are tricks to calculate it instead if you're short on memory).

You can (in modern C++) even overload the & operator, but unless the API function is a template, it has to return pointer. Watching for access through it is very environment-specific.

During debugging, you can usually set a memory watchpoint that will pause program execution on writing to, or even on reading from, a particular memory location (for GDB, see Setting Watchpoints ; VS should have a similar feature). That requires hardware support (or debugger-interpreter which is insanely slow), though, so the overall number of watchpoints is often very limited.

Without a debugger, you may be able to make a one-shot watch using memory protection tricks (like protecting the page containing the variable, and unprotecting it on first SEGV) but that's all too fragile for normal use.