Is C++ compiler allowed to optimize out unreferenced local objects

Question

I use following class to automatically set waiting cursor in the beginning of a certain function and reset the cursor when function returns.

class WaitCursorSetter
{
public:
    WaitCursorSetter() {QApplication::setOverrideCursor(Qt::WaitCursor);}
    virtual ~WaitCursorSetter() {QApplication::restoreOverrideCursor();}
};

I create a local WaitCursorSetter object when function begins. Since waiting cursor is reset in the destructor of object, I do not have to reset the cursor before each and every return statement in the method since destructor gets called when function returns and object goes out of scope.

If the compiler optimized out the unreferenced WaitCursorSetter object, this will not work. My problem is, is the compiler allowed to optimize out this object?

Answer 1

The compiler is not allowed to optimize away an automatic object whose destructors or initlization has side effects, we can see this by going to the draft standard section 3.7.3:

If a variable with automatic storage duration has initialization or a destructor with side effects, it shall not be destroyed before the end of its block, nor shall it be eliminated as an optimization even if it appears to be unused, except that a class object or its copy/move may be eliminated as specified in 12.8.