overloaded assigment operator not called

user_1094536 0
Viewed 71

Thank you all for your answers. And thanks for the cppinsight and godbolt, did not know these.

I edited my question here what I want to do. Basically I have a SDK acquisition function that has an pointer float argument as input, and I want to monitor that argument in real time to know the status of the acquisition. Now i got this code to work:

#include <iostream>

template <class T>
class MonitoredVariable
{
public:
    MonitoredVariable() {}
   // MonitoredVariable(const T& value) : m_value(value) {}

    operator T&() { return m_value; }
   //operator const T&() const { return m_value; }

    MonitoredVariable& operator = (float value)
    {
        printf("Variable modified\n");
        this->m_value = &value;
        printf("%f \n", *this->m_value);
   
        return *this;
    }

      MonitoredVariable&  operator * () {return *this;}
    
private:
    T m_value;
};

void SDKfunctionIcannotChange(float *a)
{
    *a = (float) 5.2; 
    printf("%f \n", *a); // if I comment this out, x will not update?!?!
    return;
}

int main()
{
    MonitoredVariable<float*> x;
    *x=0;
    SDKfunctionIcannotChange(x);
    printf("%f \n", *x);
    
return 1;
}

But strangely if I comment out that printf command in the SDKfunctionIcannotChange function, then the x value will not change. Any ideas why is this happening?

So with printf (correct):

Variable modified
0.000000 
5.200000 
5.200000

and without

Variable modified
0.000000 
0.000000

thanks..

operator-overloading c++
2 Answers

This answers original question before extensive edit.

Basically your program is equivalent to:

int main()
{
    float *x = nullptr;
    float y = 5;
    *x = y; // here is UB since x == nullptr
}

See cppinsights: problematic line looks like this *static_cast<float *>(x.operator float *&()) = y;. So your code do not use custom operator=, but conversion operator.
Basically problem is code which uses template.

I do not know what was your intent, maybe this:

#include <iostream>

template <class T>
class MonitoredVariable
{
public:
    MonitoredVariable() {}
    MonitoredVariable(const T& value) : m_value(value) {}

    operator T&() { return m_value; }
    operator const T&() const { return m_value; }

    MonitoredVariable& operator = (const T& value)
    {
        printf("Variable modified\n");
        m_value = value; // here was also a problem
        return *this;
    }

int main()
{
    MonitoredVariable<float*> x;
    float y = 5;
    x = &y;

    return;
}

https://godbolt.org/z/js5zdz

user_13874380

The *x expression calls operator* with an argument of x. During the name lookup, a builtin operator* for pointers is found. Given that your class MonitoredVariable has a conversion operator to T (which is a pointer type in your case), that conversion operator is called and the returned pointer, which is null, is then dereferenced causing a crash.

What you probably wanted to write is x = y.

user_46365340
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