C++ Primer 4/e在Class Definitions and Declarations這個地方有一個關鍵觀念：『Not all types need to be abstract. The library pair class is a good example of a useful, well-designed class that is concrete rather than abstract. A concrete class is a class that exposes, rather than hides, its implementation.
Some classes, such as pair, really have no abstract interface. The pair type exists to bundle two data members into a single object. There is no need or advantage to hiding the data members. Hiding the members in a class like pair would only complicate the use of the type.
Even so, such types often have member functions. In particular, it is a good idea for any class that has data members of built-in or compound type to define constructor(s) to initialize those members. The user of the class could initialize or assign to the data members but it is less error-prone for the class to do so.』
中文版的這樣寫：『並不是所有型別都要設計為抽象。C++標準庫的pair就是一個很有用而且設計良好的具象class。具象class會曝露（而非隱藏）其實作細節。
某些class如pair，並沒有抽象介面。pair用來把兩個成員變數綁為一個單獨物件。隱藏這些成員變數沒有必要，也沒有任何好處。在具象classes如pair，隱藏成員變數會使型別的使用變得複雜。
即便如此，這些型別也常常有成員函式。更明確地說，如果classes擁有內建的、或複合型別的成員變數，那麼定義建構式來初始化這些成員是個好主意。這一類classes的使用者也可以直接初始化或賦值這些成員變數，但讓classes來做比較不容易出錯。』
抽象對我來說是熟悉的，具象就真的是很陌生，如果非抽象即是具象，那我想我反而就容易懂了，可連英文concrete對我來說，我也很陌生，不過多用一些非抽象類別的程式，可能就會懂具象類別是怎麼一回事了！