CODEWITHSUNDEEP
c++volatile
Pete D.
Pete D.

Reputation: 177

C++ assign operator to copy between volatile and non-volatile instances of a class

I would like to copy, in both directions, between volatile and non-volatile instances of a class. The following uses the assignment operator for the copy. If the macro "ordinary_cpp" is defined everything compiles as expected. If the macro is not defined, which enables "volatile", it copies a non-volatile to a volatile, but errors on the opposite, and always errors on the return of *this.

I hope someone can tell me the correct syntax to return *this, and how I can assign a volatile to a non-volatile. Thanks.

#define ordinary_cpp
#ifdef ordinary_cpp
  #define cv_qual
  #define thiserrormacro(arg) arg
  #define hopefulmacro(arg) arg
#else
  #define cv_qual volatile
  #define thiserrormacro(arg)
  #define hopefulmacro(arg)
#endif

struct Class {
  int data;
  Class operator = ( Class lhs ) cv_qual {
    data = lhs.data;
    thiserrormacro(return *this;)
  }
};

void test(){
  Class         nonvol;
  Class cv_qual vol;

  vol = nonvol;

  hopefulmacro(nonvol = vol;)
}

Upvotes: 1

Views: 878

Answers (1)

Caleth
Caleth

Reputation: 63039

You need 4 overloads, for 

nonvol = nonvol;
nonvol = vol;
vol = nonvol;
vol = vol;

It would be something like

struct Class {
  int data;
  Class& operator = ( const Class& lhs ) {
    data = lhs.data;
    return *this;
  }
  volatile Class& operator = ( const Class& lhs ) volatile {
    data = lhs.data;
    return *this;
  }
  Class& operator = ( const volatile Class& lhs ) {
    data = lhs.data;
    return *this;
  }
  volatile Class& operator = ( const volatile Class& lhs ) volatile  {
    data = lhs.data;
    return *this;
  }
};

But don't bother. volatile is pretty vestigial at this point, and it definitely doesn't mean "safe to use with threads".

Upvotes: 4

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