c++ - how do traits classes work? -

I'm reading Scott Meyer '' The symptom is talking about classes, I came to understand that the compilation time I need them to determine the type of object, but I did not understand what these classes really do. (Technically speaking)

Perhaps you are expecting some kind of magic which makes the type properties Work should be disappointed, in that situation, there is no magic. Type Properties are Manually defined for each type

For example, consider iterator_traits , which can be used for typefuffs (e.g. value_type ).

By using them, you can

  iterator_traits & lt; Vector & lt; Int & gt; :: Iterator & gt; :: value_type x; Iterator_traits & lt; Int * & gt; :: value_type y; // `x` and 'y` are type int   
 but for this to work, actually in an  obvious definition  somewhere  & lt; Iterator & gt;  is in the header, which reads something such as this:  
  template & lt; Typename T & gt; Straight iterator_traits & lt; T * & gt; {Type didif t value_type; // One ?? |};   
 This is the type of  iterator_traits  for the types of  partial specialization   T * , that is, some common types of Signal  
 In the same vein,  iterator_traits  are specific to other iterators, e.g.  typename vector & lt; T & gt; :: Iterator .