總結：
????有四種情況，會導致“編譯器必須為未聲明構造函數的classes合成一個默認構造函數”。C++ 標準把那些合成物稱為隱含的有用默認構造函數。被合成出來的構造函數只能滿足編譯器(非程序)的需要。它之所以能夠完成任務，是借著“調用成員對象或基類的默認構造函數”或是“為每一個對象初始化其虛函數機制或虛基類機制”而完成的。至于沒有存在那四種情況而又沒有聲明構造函數的類，我們說它們擁有的是隱含的無用默認構造函數，實際上它們并不被合成出來。
????在合成的默認構造函數中，只有基類子對象和成員對象會被初始化。所有其它的非靜態(tài)數據成員，如整數、整數指針、整數數組等等都不會被初始化。這些初始化操作對程序而言有需要，但對編譯器而言則沒必要。如果程序需要一個“把某指針設為0”的默認構造函數，那么提供它的人應該是程序員。
???
????C++新手一般有兩個常見的誤解：
1）任何類如果沒有定義默認構造函數，編譯器就會合成出它來。
2）編譯器合成出來的默認構造函數會明確設定“類中每一個數據成員的默認值”。
正如你所見，上述兩個沒有一個是真的！
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Summary:?
???There are four characteristics of a class under which the compiler needs to synthesize a default constructor for classes that declare no constructor at all. The Standard refers to these as implicit, nontrivial default constructors. The synthesized constructor fulfills only an implementation need. It does this by invoking member object or base class default constructors or initializing the virtual function or virtual base class mechanism for each object. Classes that do not exhibit these characteristics and that declare no constructor at all are said to have implicit, trivial default constructors. In practice, these trivial default constructors are not synthesized.
Within the synthesized default constructor, only the base class subobjects and member class objects are initialized. All other nonstatic data members, such as integers, pointers to integers, arrays of integers, and so on, are not initialized. These initializations are needs of the program, not of the implementation. If there is a program need for a default constructor, such as initializing a pointer to 0, it is the programmer's responsibility to provide it in the course of the class implementation.
Programmers new to C++ often have two common misunderstandings:
1)
That a default constructor is synthesized for every class that does not define one
2)
That the compiler-synthesized default constructor provides explicit default initializers
?? for each data member declared within the class
As you have seen, neither of these is true.
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