David Abrahams wrote:
Ian McCulloch <ianmcc@lorentz.leidenuniv.nl> writes:
Probably ;-) It's all there right under "One definition rule" in the standard.
3 Basic concepts 3.2 One definition rule
-5- There can be more than one definition of a class type (clause class), enumeration type (dcl.enum), inline function with external linkage (dcl.fct.spec), class template (clause temp), non-static function template (temp.fct), static data member of a class template (temp.static), member function [core 249: template of a class template ] (temp.mem.func), or template specialization for which some template parameters are not specified (temp.spec, temp.class.spec) in a program provided that each definition appears in a different translation unit, and provided the definitions satisfy the following requirements. Given such an entity named D defined in more than one translation unit, then
- each definition of D shall consist of the same sequence of tokens; and
- in each definition of D, corresponding names, looked up according to basic.lookup, shall refer to an entity defined within the definition of D, or shall refer to the same entity, after ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
To my reading of the standard, different typedef-names that are synonyms for the same type are the same entity with respect to the ODR, so I don't think any of this applies here.
If you limit the question to typedefs, you're right, but I'm not limiting the question to typedefs. The problem I'm discussing is this one:
namespace { template <class T> typeof (T() + T()) add(T*, T*) { typeof(T() + T()) x; ... return x; } }
If the declaration of x (or add, for that matter, I'm pretty sure) uses any names that refer to different entities in different TUs, it's an ODR violation.
Interesting. The purpose of typeof (or BOOST_TYPEOF) is to name a type, and that type will be the same in all TU's. But the declaration itself might involve a different type in different translation units, although the full declaration will end up naming the same entity. An interesting interaction with typedefs and templates. Or would you also claim that this would happen even in the absense of templates? If so, what makes the template case different? // header.h namespace { struct typeof_foo { typedef int type; }; } // namespace typeof_foo::type bar();// ODR violation here??? // Why is this different to previous examples? // does anything change when templates are involved? // end header.h Cheers, Ian