C++ – Is the Default Move Constructor Defined as noexcept?

The implicit generation of move constructors and assignment operators has been contentious and there have been major revisions in recent drafts of the C++ Standard, so currently available compilers will likely behave differently with respect to implicit generation.

For more about the history of the issue, see the 2010 WG21 papers list and search for "mov"

The current specification (N3225, from November) states (N3225 12.8/8):

If the definition of a class X does not explicitly declare a move constructor, one will be implicitly declared as defaulted if and only if

• X does not have a user-declared copy constructor, and

• X does not have a user-declared copy assignment operator,

• X does not have a user-declared move assignment operator,

• X does not have a user-declared destructor, and

• the move constructor would not be implicitly defined as deleted.

There is similar language in 12.8/22 specifying when the move assignment operator is implicitly declared as defaulted. You can find the complete list of changes made to support the current specification of implicit move generation in N3203: Tightening the conditions for generating implicit moves , which was based largely on one of the resolutions proposed by Bjarne Stroustrup's paper N3201: Moving right along.

[dcl.fct.def.default]/p2:

If a function is explicitly defaulted on its first declaration,

• it is implicitly considered to be constexpr if the implicit declaration would be, and,
• it has the same exception specification as if it had been implicitly declared (15.4).

These rules do not apply if the function is explicitly defaulted on a later declaration, as in your later example, so instead, except for destructors, the function is considered noexcept(false) by default like most other functions.

Since the explicit defaulting can be in a different translation unit - and in the pimpl case, is in a different TU - there's no general way for the compiler to figure out after seeing the class definition only whether the move constructor will throw, unless the function is explicitly defaulted in the class definition (i.e., at its first declaration).