CPP1114 — In-place construction of structure types

There are two main motivations:

(1) Performance: IDL types may be large and not all IDL types may have C++11 mapping with efficient move constructors. For example, an IDL structure containing an array does not have an efficient move-ctor. Current mapping for an IDL struct type (section 6.13.1) requires an explicit constructor accepting values for each member by value in the order they are specified in IDL. Although this method is sufficient in most cases, it is not optimal. Particularly, the types that don't support efficient move-ctor.

(2) Usability: Often C+11 standard library containers could be constructed using several alternatives. For instance, a string member in a struct may be constructed using any of its 9 constructors or a vector may be constructed using any of its 7 constructors. Currently, the IDL2C+11 specification allows construction of members of a structure using only two kinds of constructors: copy-ctor and move-ctor. (due to the pass-by-value rule mentioned above)

Resolution: The IDL2C++11 mapping of structures could be enhanced to construct large objects in-place. Provide a way to construct the member objects of a struct in-place using a perfect-forwarding constructor. The intent is to support all the ways of constructing all the member objects from the constructor of the parent object. Moreover, a perfect-forwarding constructor may eliminate the need for a move, which may lead to some performance improvements.

The solution relies on an idiom known as piecewise_construct idiom. It relies on a perfect-forwarding constructor that takes as many tuples as there are members in a struct. Each tuple encapsulates the parameters to construct one member. Tuples are needed because member ctor could take several parameters and the user may be interested in using them. For instance, using an allocator for std::vector. The user of the struct calls std::forward_as_tuple for each member object to group the parameters in a tuple. The special constructor simply forwards the tuples to the respective member.

The C++ standard library uses this idiom for std::map and std::unordered_map to construct complex objects in-place using the emplace operations. However, more general uses have been identified: http://cpptruths.blogspot.com/2012/06/perfect-forwarding-of-parameter-groups.html

Existing alternatives for struct construction appear to be sufficient

Since the issue has been unresolved for a number of RTFs I'm proposing we close it now.
The proposed enhancement could be provided by implementations as an extension to the spec-mandated behavior.
I don't argue that there is some benefit from this approach, but there is also implementation complexity: when generating a struct constructor, the tool will need to know, for each struct member, which constructor parameters that member can take. Those parameters can vary based on versions of the C++ standard library (for types like std::string, vector, array, map).