michi7x7 wrote:
I have crafted a general replacement of T(a) as a workaround for dangerous conversions caused by T(a) which may happen in some kinds of templates.
There is already one in boost (Functional/Factory) AFAIK, but it might has the same problem: http://www.boost.org/doc/libs/1_47_0/libs/functional/factory/doc/html/index....
boost::value_factory<T>()(arg1,arg2,arg3) // same as T(arg1,arg2,arg3)
Thank you for pointing out. It is another example of the problem. I verified that boost::value_factory<int>()("") compiles, too. My replacement (unary_initialized) is meant to be used internally to aid these library codes. For the case of value_factory, something like pack_initialized, I've shown later in the story about variadic templates, seems desired.
To avoid the unexpected conversion, I first proposed use of T{...} . But it is not a perfect solution because it changes the meaning in some cases like std::vector<int>. But this is exactly the use-case T{...} was made for. Of course initializer_lists make this approach completely unusable...
FYI, I found a somewhat related paper. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2477.pdf With pack_initialized, I imagine one can still use initializer_list constructors explicitly, like this: pack_initialized<std::vector<int>>(std::initializer_list<int>{...}).value
Another possible solution would be to use static_cast<T>(a) for one constructor-argument, as static_cast is type-safe.
Unfortunately, static_cast involves base* to derived* conversion which needs special assumption to be safe.
Thinking more, I wonder why such dangerous conversion was explicitly injected to T(a) in the first place. I first thought of some C-compatibility issues. But now I think it cannot be, because T(a) is a new construct in C++. Is it possible to remove this dangerous conversion path in a future revision of the standard?
T(a) is a function-style cast which was used in C, removing it would be rather difficult.
"function-style cast which was used in C" ? I still believe that function-style cast is new in C++, meaning that (T)a is the only syntax of casts in C. Could you please give an example of valid function-style cast in C code? -- k_satoda