无法更改volatile变量的值

Question

Description:

I cannot set a variable or change it while it is defined volatile (in Main). Removing volatile solves the problem, but I need my variable to be volatile.

My tries:

Really a lot of tries out there. Overloading operator=, making new operator= volatile, making new volatile method. Nothing actually worked so far.

My main:

int main() {
    volatile PlaceParentConversion s(10.0); // remove volatile = no errors
    std::cout << s.mX << std::endl;
    s = PlaceParentConversion::IDENTITY_CONVERSION;
    std::cout << s.mX << std::endl;
    return 0;
}

My class:

class PlaceParentConversion {
public: //all public, easier to check
    const static PlaceParentConversion IDENTITY_CONVERSION;
    double mX;
    PlaceParentConversion(const double x);
    PlaceParentConversion(const PlaceParentConversion& other);
};

const PlaceParentConversion PlaceParentConversion::IDENTITY_CONVERSION(0);

PlaceParentConversion::PlaceParentConversion(const double x) : mX(x) {}
PlaceParentConversion::PlaceParentConversion(const PlaceParentConversion& other) : mX(other.mX) {}

Error:

‘volatile PlaceParentConversion’ as ‘this’ argument of ‘PlaceParentConversion& PlaceParentConversion::operator=(const PlaceParentConversion&)’ discards qualifiers [-fpermissive] 

Answer 1

Define a volatile assignment operator:

Foo volatile & operator=(Foo const & rhs) volatile
{
    // ...
    return *this;
}

(I've shortened your class name for readability.)

---

Here's a more complete example:

struct Foo
{
    Foo() { }
    Foo(Foo const volatile &) { }
    Foo volatile & operator=(Foo const &) volatile
    { return *this; }
};

int main()
{
    volatile Foo x;
    Foo y;
    static_cast<Foo>(x = y);
}

The static cast in the final line makes GCC not issue a warning that no access is happening to the volatile object which is the result of the evaluation of the assignment expression: the standard says that in a void context there is no lvalue-to-rvalue conversion and thus no access. We make the conversion explicit.

Answer 2

是的，这是正确的，因为你没有volatile PlaceParentConversion& operator =(const PlaceParentConversion&) volatile;