9.5 Afternotes

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The first compiler really to parse template definitions was developed by a company called Taligent in the mid-1990s. Before that ”and even after that ”most compilers treated templates as a sequence of tokens to be played back through the parser at instantiation time. Hence no parsing was done, except for a minimal amount sufficient to find the end of a template definition. Bill Gibbons was Taligent's representative to the C++ committee and was the principal advocate for making templates unambiguously parsable. The Taligent effort was not released until the compiler was acquired and completed by Hewlett-Packard (HP), to become the aC++ compiler. Among its competitive advantages, the aC++ compiler was quickly recognized for its high quality diagnostics. The fact that template diagnostics were not always delayed until instantiation time undoubtedly contributed to this perception.

Relatively early during the development of templates, Tom Pennello ”a widely recognized parsing expert working for Metaware ”noted some of the problems associated with angle brackets. Stroustrup also comments on that topic in [StroustrupDnE] and argues that humans prefer to read angle brackets rather than parentheses. However, other possibilities exist, and Pennello specifically proposed braces (for example, List { ::X }) at a C++ standards meeting in 1991 (held in Dallas). ^[8] At that time the extent of the problem was more limited because templates nested inside other templates ”so-called member templates ”were not valid and thus the discussion of Section 9.3.3 on page 132 was largely irrelevant. As a result, the committee declined the proposal to replace the angle brackets.

^[8] Braces are not entirely without problems either. Specifically, the syntax to specialize class templates would require nontrivial adaptation.

The name lookup rule for nondependent names and dependent base classes that is described in Section 9.4.2 on page 136 was introduced in the C++ standard in 1993. It was described to the "general public" in Bjarne Stroustrup's [StroustrupDnE] in early 1994. Yet the first generally available implementation of this rule did not appear until early 1997 when HP incorporated it into their aC++ compiler, and by then large amounts of code derived class templates from dependent bases. Indeed, when the HP engineers started testing their implementation, they found that most of the programs that used templates in nontrivial ways no longer compiled. ^[9] In particular, all implementations of the STL ^[10] broke the rule in many hundreds ”and sometimes thousands ”of places. To ease the transition process for their customers, HP softened the diagnostic associated with code that assumed that nondependent names could be found in dependent base classes as follows . When a nondependent name used in the scope of a class template is not found using the standard rules, aC++ peeks inside the dependent bases. If the name is still not found, a hard error is issued and compilation fails. However, if the name is found in a dependent base, a warning is issued, and the name is marked to be treated as if it were dependent, so that lookup will be reattempted at instantiation time.

^[9] Fortunately, they found out before they released the new functionality.

^[10] Ironically, the first of these implementations had been developed by HP as well.

The lookup rule that causes a name in nondependent bases to hide an identically named template parameter (Section 9.4.1 on page 135) is an oversight, and it is not impossible that this will be changed in a revision of the standard. In any case, it is probably wise to avoid code with template parameter names that are also used in nondependent base classes.

Andrew Koenig first proposed ADL for operator functions only (which is why ADL is sometimes called Koenig lookup ). The motivation was primarily esthetic: explicitly qualifying operator names with their enclosing namespace looks awkward at best (for example, instead of a+b we may need to write N::operator+(a, b) ) and having to write using declarations for every operator can lead to unwieldy code. Hence, it was decided that operators would be looked up in the namespaces associated with arguments. ADL was later extended to ordinary function names to accommodate a limited kind of friend name injection and to support a two-phase lookup model for templates and their instantiations (Chapter 10). The generalized ADL rules are also called extended Koenig lookup .

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