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Document number: N3948

Date: 2014-02-20

Feature-testing for C++ Technical Specifications

by Clark Nelson

Excerpt:

Introduction

One of the primary reasons for defining the feature-test macros for C++14 in a document separate from the standard was to avoid even the impression that standard features are optional. Because features described in a Technical Specification are by definition optional, it is not clear that there is good justification for putting feature-testing facilities for a TS in a different document.

It was important for someone to “back-fill” feature-testing facilities for the new features of C++14, so SG10 undertook that task. But going forward, it would probably be better for feature-testing to be considered as part of the design of a new feature, and a descriptive and unique macro name selected, when necessary, by those who understand it best.

In addition, it is not clear that SG10 has yet adequately considered what to do about issues like ABI compatibility and versioning of interfaces for a library. It may well be that it would be more efficient for such discussions to happen in a library WG.

For all these reasons, it seems that discussion of feature-testing should not be happening exclusively within SG10.

The remainder of this document consists mainly of tables such as were used to drive discussions within SG10, to serve as starting points for discussions in other groups...