2.11 Summary

A number of techniques form the building blocks of the components presented in this book. Most of the techniques are related to template code.

• Compile-time assertions (Section 2.1) help libraries to generate meaningful error messages in templated code.

• Partial template specialization (Section 2.2) allows you to specialize a template, not for a specific, fixed set of parameters, but for a family of parameters that match a pattern.

• Local classes (Section 2.3) let you do interesting things, especially inside template functions.

• Mapping integral constants to types (Section 2.4) eases the compile-time dispatch based on numeric values (notably Boolean conditions).

• Type-to-type mapping (Section 2.5) allows you to substitute function overloading for function template partial specialization, a feature missing in C++.

• Type selection (Section 2.6) allows you to select types based on Boolean conditions.

• Detecting convertibility and inheritance at compile time (Section 2.7) gives you the ability to figure out whether two arbitrary types are convertible to each other, are aliases of the same type, or inherit one from the other.

• TypeInfo (Section 2.8) implements a wrapper around std::type_info, featuring value semantics and ordering comparisons.

• The NullType and EmptyType classes (Section 2.9) function as placeholder types in template metaprogramming.

• The TypeTraits template (Section 2.10) offers a host of general-purpose traits that you can use to tailor code to specific categories of types.

Table 2.1. TypeTraits<T> Members

Name	Kind	Comments
isPointer	Boolean constant	True if T is a pointer.
PointeeType	Type	Evaluates to the type to which T points, if T is a pointer. Otherwise, evaluates to NullType.
isReference	Boolean constant	True if T is a reference.
ReferencedType	Type	If T is a reference, evaluates to the type to which T refers. Otherwise, evaluates to the type T itself.
ParameterType	Type	The type that's most appropriate as a parameter of a nonmutable function. Can be either T or const T&.
isConst	Boolean constant	True if T is a const-qualified type.
NonConstType	Type	Removes the const qualifier, if any, from type T.
isVolatile	Boolean constant	True if T is a volatile-qualified type.
NonVolatileType	Type	Removes the volatile qualifier, if any, from type T.
NonQualifiedType	Type	Removes both the const and volatile qualifiers, if any, from type T.
isStdUnsignedInt	Boolean constant	True if T is one of the four unsigned integral types (unsigned char, unsigned short int, unsigned int, or unsigned long int).
isStdSignedInt	Boolean constant	True if T is one of the four signed integral types (signed char, short int, int, or long int).
isStdIntegral	Boolean constant	True if T is a standard integral type.
isStdFloat	Boolean constant	True if T is a standard floating-point type (float, double, or long double).
isStdArith	Boolean constant	True if T is a standard arithmetic type (integral or floating point).
isStdFundamental	Boolean constant	True if T is a fundamental type (arithmetic or void).