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The suffix _t, which in C originally means typedef, is also widely used for (private) typedefs standing for template instances: template class SomeContainer { // informally means: // within this class, a pair always denotes a pair of scalars private: typedef std::pair pair_t; }; On the other hand, a public typedef name usually is composed of lowercase regular English words (such as iterator_category). 6. Metafunctions We often meet stateless class templates whose members are only enumerations (as a rule, anonymous), static constants, types (typedefs or nested classes), and static member functions.

B : a; } template struct SomeClass { static const int value = sq(N)/max(N, 1); Finally, consider a special class of macros. A macro directive is a macro whose usage logically takes an entire line of code. 1415926535897932384626433832795029 // the use of MXT_PI does not take the whole line // so it is not a directive. const double x = std::cos(MXT_PI); // directive MXT_NULL_NAMESPACE_END() The definition of a macro directive, in general, should not end with a semicolon, so the user is forced to close the line manually (when appropriate), as if it were a standard function call.

Macros Macros play a special role in TMP. Some programmers consider them a necessary evil and indeed they are necessary, but it’s not obvious they are also evil. Macros must: • Allow the reader to recognize them • Prevent name collisions As discussed in [5], usually member function names should be actions. Thus empty should be a synonym for make_empty and not for is_empty. However, STL convention is established and universally understood. When in doubt, do as std::vector does. 27 41 Chapter 1 ■ Templates The easiest way to satisfy both requirements is to choose a unique and sufficiently ugly common prefix for all macros and play with lower/uppercase to give extra meaning to the name.