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Document number: N3882

Date: 2014-01-17

An update to the preprocessor specification

by David Krauss

Excerpt:

The C preprocessor specification inherited by C++ uses undefined behavior to specify latitude for implementation differences. This technically allows one compiler to produce a defective executable from a particular program, while another fills the void with useful features. In theory this makes porting programs dangerous, but in practice consensus has been reached regarding the meaning of most programs invoking undefined behavior, and programmers have little reason to worry. By capturing the status quo in the standard, safety can be properly guaranteed, and the preprocessor may be more completely understood without referring to platform documentation.

Additionally, the preprocessor has not kept pace with C++11 lexical extensions (raw strings, user-defined literals, and new encoding prefixes), resulting in unspecified corner cases.

This proposal updates the preprocessor specification to remove undefined behavior, missing specifications, and possible contradictions, and to better document implementation differences.

Issues related to universal-character-names are not addressed here, but in N3881.