13.10 Static Properties

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In Chapter 15 and Chapter 19 we discuss various ways to categorize types "at compile time." Such traits are useful in selecting specializations of templates based on the static properties of the type. (See, for example, our CSMtraits class in Section 15.3.2 on page 279, which attempts to select optimal or near-optimal policies to copy, swap, or move elements of the argument type.)

Some language designers have observed that if such "specialization selections" are commonplace, they shouldn't require elaborate user -defined code if all that is sought is a property that the implementation knows internally anyway. The language could instead provide a number of built-in type traits. The following could be a valid complete C++ program with such an extension:

 #include <iostream>  int main()  {      std::cout << std::type<int>::is_bit_copyable << '\n';      std::cout << std::type<int>::is_union << '\n';  } 

Although a separate syntax could be developed for such a construct, fitting it in a user-definable syntax may allow for a more smooth transition from the current language to a language that would include such facilities. However, some of the static properties that a C++ compiler can easily provide may not be obtainable using traditional traits techniques (for example, determining whether a type is a union), which is an argument in favor of making this a language element. Another argument is that it can significantly reduce the amount of memory and machine cycles required by a compiler to translate programs that rely on such properties.

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