Why am I not provided with a default copy constructor from a volatile?

Question

This code:

class X {   int member;   };  volatile X a; X b = a; 

Fails with the error:

prog.cpp:6:7: error: no matching function for call to ‘X::X(volatile X&)’ prog.cpp:6:7: note: candidates are: prog.cpp:1:7: note: X::X() prog.cpp:1:7: note:   candidate expects 0 arguments, 1 provided prog.cpp:1:7: note: X::X(const X&) prog.cpp:1:7: note:   no known conversion for argument 1 from ‘volatile X’ to ‘const X&’ 

Is there any way I can get the compiler to generate a volatile copy constructor for me?

Answer 1

The short answer is: Because the standard says you won't.

The C++ Standard 12.8/9 (Draft N3242) tells:

The implicitly-declared copy constructor for a class X will have the form

• X::X(const X&)

if

• each direct or virtual base class B of X has a copy constructor whose first parameter is of type const B& or const volatile B&, and
• for all the non-static data members of X that are of a class type M (or array thereof), each such class type has a copy constructor whose first parameter is of type const M& or const volatile M&. [Note: 119]

Otherwise, the implicitly-declared copy constructor will have the form

• X::X(X&)

Note 119 says:

This implies that the reference parameter of the implicitly-declared copy constructor cannot bind to a volatile lvalue; see C.1.9.

In C.1.9 you'll find:

The implicitly-declared copy constructor and implicitly-declared copy assignment operator cannot make a copy of a volatile lvalue. For example, the following is valid in ISO C:

struct X { int i; }; volatile struct X x1 = {0}; struct X x2(x1); // invalid C++ struct X x3; x3 = x1; // also invalid C++ 

Rationale: Several alternatives were debated at length. Changing the parameter to volatile const X& would greatly complicate the generation of efficient code for class objects. Discussion of providing two alternative signatures for these implicitly-defined operations raised unanswered concerns about creating ambiguities and complicating the rules that specify the formation of these operators according to the bases and members.