Feature-testing recommendations for C++

Preface

The more time that passes without any sort of feature-testing recommendation, the more confusion will affect programmers and implementers interested in features of C++14, of the sort that has plagued C++11 for years. So whatever action should be taken in this arena should not be delayed any more than can be helped.

SG10 intends to produce its recommendations solely as a WG21 document, without any balloting at higher levels. This is partly to save time, but also to avoid making significant conformance changes. It is hoped that compiler and library implementers will follow these recommendations voluntarily, even without a threat of claims of non-conformance. To improve the chances of that happening, it is considered important to have a record of the endorsement of WG21 – or at least of the C++ technical experts who attend WG21 meetings.

So SG10 would like to bring this document forward for some sort of approval vote at the Chicago meeting. Formally speaking, no action by the committee is requested, so this vote should probably be just a straw poll.

Contents

1. Explanation and rationale for the approach
   1. Problem statement
   2. Characteristics of the proposed solution
2. Recommendations
   1. Introduction
   2. C++14 features
   3. C++11 features
   4. Conditionally-supported constructs

Explanation and rationale for the approach

Problem statement

The pace of innovation in the standardization of C++ makes long-term stability of implementations unlikely. Features are added to the language because programmers want to use those features. Features are added to (the working draft of) the standard as the features become well-specified. In many cases a feature is added to an implementation well before or well after the standard officially introducing it is approved.

This process makes it difficult for programmers who want to use a feature to know whether it is available in any given implementation. Implementations rarely leap from one formal revision of the standard directly to the next; the implementation process generally proceeds by smaller steps. As a result, testing for a specific revision of the standard (e.g. by examining the value of the __cplusplus macro) often gives the wrong answer. Implementers generally don't want to appear to be claiming full conformance to a standard revision until all of its features are implemented. That leaves programmers with no portable way to determine which features are actually available to them.

It is often possible for a program to determine, in a manner specific to a single implementation, what features are supported by that implementation; but the means are often poorly documented and ad hoc, and sometimes complex – especially when the availability of a feature is controlled by an invocation option. To make this determination for a variety of implementations in a single source base is complex and error-prone.

Characteristics of the proposed solution

To preserve implementers' freedom to add features in the order that makes the most sense for themselves and their customers, implementers should indicate the availability of each separate feature by adding a definition of a macro with the name corresponding to that feature.

Important note: By recommending the use of these macros, WG21 is not making any feature optional; the absence of a definition for the relevant feature-test macro does not make an implementation that lacks a feature conform to a standard that requires the feature. However, if implementers and programmers follow these recommendations, portability of code between real-world implementations should be improved.

To a first approximation, a feature is identified by the WG21 paper in which it is specified, and by which it is introduced into the working draft of the standard. Not every paper introduces a new feature worth a feature-test macro, but every paper that is not just a collection of issue resolutions is considered a candidate; exceptions are explicitly justified.

For C++14, it is preferred for the feature-test macro to be named using some combination of words from the title of the paper. In the future, it is hoped that every paper will include its own recommendations concerning feature-test macro names.

The value specified for a feature-test macro is based on the month in which the feature is voted into the working draft. In a case where a feature is subsequently changed in a significant way, but arguably remains the same feature, the value of the macro can be changed to indicate the “revision level” of the specification of the feature. However, in most cases it is expected that the presence of a feature can be determined by the presence of any non-zero macro value; for example:

#if __cpp_binary_literals
int const packed_zero_to_three = 0b00011011;
#else
int const packed_zero_to_three = 0x1B;
#endif

Note: There are cases where the decision between adding a new macro and changing the value of an existing macro will be subjective; constexpr is a good example.

To avoid the user's namespace, names of macros for language features are prefixed by “__cpp_”; for library features, by “__cpp_lib_”. A library feature that doesn't introduce a new header is expected to be defined by the header(s) that implement the feature.

Note: Whether macros for language features and new library headers should be specified as predefined or defined in a new header is still under discussion.

Recommendations

Introduction

For the sake of improved portability between partial implementations of various C++ standards, WG21 (the ISO technical committee for the C++ programming language) recommends that implementers and programmers follow the guidelines in this document concerning feature-test macros.

Implementers who provide a new feature should define a macro with the recommended name, in the same circumstances under which the feature is available (for example, taking into account relevant command-line options), to indicate the presence of support for that feature.

Programmers who wish to determine whether a feature is available in an implementation should base that determination on the state of the macro with the recommended name. (The absence of a tested feature may result in a program with decreased functionality, or the relevant functionality may be provided in a different way. A program that strictly depends on support for a feature can just try to use the feature unconditionally; on an implementation lacking necessary support, translation will presumably fail.)

C++14 features

The following table itemizes all the changes that were made to the working draft for C++14 as specified in a WG21 technical document. (Changes that were made as specified in a core or library issue are not included.) The table is sorted by the section of the standard primarily affected.

C++11 features

This table is not intended to be formally complete; it is intended only to cover variations of current (as of 2013) commercial importance.

Conditionally-supported constructs

The standard requires implementations to document the conditionally-supported constructs it does not support. For consistency, the recommendation is to define a macro for each conditionally-supported construct that is diagnosed (not supported)