trivial default constructors

构造函数

A default constructor is trivial if it is not user-provided and if:

• its class has ** no virtual functions **
• and ** no virtual base classes **
• and ** no non-static ** data member of its class has a brace-or-equal-initializer
• and all the direct base classes of its class have ** trivial default constructors **
• and for all the non-static data members of its class that are of class type (or array thereof),each such class has a trivial default constructor.

For a default constructor and destructor being "trivial" means literally ** "do nothing at all" **

If you define a constructor yourself, it is considered non-trivial, even if it doesn't do anything, so a trivial constructor must be implicitly defined by the compiler.