Adds forwarding make_optional helpers, resolves #30

[daniel-j-h]

For #30. The varargs overloads probably make no sense with the bool argument at the second position?

I don't quite understand why the test/optional_test_fail_copying_a_moveable_type.cpp fails to compile here (unrelated to this changeset) - shouldn't construction from a moveable-only type work?

Also how can I provide docs for the overload depending on if rvalue refs are enabled or not? I found the quickbook files in doc I'm just wondering what the best way to document the overloads is.

cc @akrzemi1

[akrzemi1]

The code you highlighted triggers uses val as an lvalue: therefore it chooses to copy rather than move.
For the docs and unit tests, you do not have to bother about these. When I get to merge your change (be patient with me), I will update the docs also. My solution in this case is to only document for the case with rvalue references enabled. For the rest I only say: "if rvalues are disabled, the library degrades in a friendly way".

[stevenr3]

Was it intended to remove the l-value methods with this #else?
I'm getting a compile error trying to make an optional from a variable, e.g. float x = 1; make_optional<float>(x);
(without -DBOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES)

[akrzemi1]

Function make_optional is not intended to be used like this, with template arguments specified explicitly. The only purpose of function make_optional is to spare you the trouble of specifying the type of optional value. If you want to specify it yourself, you have a simpler version:

float x = 1; boost::optional<float>(x);

Macro BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES is not intended to be defined by users. The library defines it when it detects old compilers without rvalue reference support.

[stevenr3]

Sorry I missed some detail… I was attempting to use boost::make_optional<T>(bool, T) to simplify the following:

float value;
bool valid = GetValidAndValue(&value);

// return valid ? boost::optional<float>(value) : boost::none;
return boost::make_optional<float>(valid, value);

True it is not a huge simplification - but should that be possible?

[akrzemi1]

Again, same response. If you can afford spelling out the type of optional value use:

return boost::optional<float>(valid, value);

If you want to the type of optional value to be deduced use:

return boost::make_optional(valid, value);

[akrzemi1]

IOW, make_optional is only used for type deduction.

[stevenr3]

Ah ok, got it... thanks!