当析构函数``virtual时，几乎没有区别，否则： default时：认为是 trivial {}时： 认为是 user-defined

区别

如下为trivial class的两种实现：

struct Trivial
{
  int foo;
};
struct Trivial_Default
{
  int foo;
   Trivial_Default() = default;
};

如下为non-trivial的实现：

struct Non_Trivial
{
  int foo;
  Non_Trivial(){}
};

什么是trivial

The C++ Standard n3337 § 8.5/7 says

To value-initialize an object of type T means: — if T is a (possibly cv-qualified) class type (Clause 9) with a user-provided constructor (12.1), then the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor); — if T is a (possibly cv-qualified) non-union class type without a user-provided constructor, then the object is zero-initialized and, if T’s implicitly-declared default constructor is non-trivial, that constructor is called. — if T is an array type, then each element is value-initialized; — otherwise, the object is zero-initialized.

举例如下

#include <iostream>
class A {
    public:
    A(){}
    int i;
    int j;
};
class B {
    public:
    B() = default;
    int i;
    int j;
};
int main()
{
    for( int i = 0; i < 100; ++i) {
        A* pa = new A();
        B* pb = new B();
        std::cout << <<pa->i << "," << pa->j << std::endl;
        std::cout << pb->i << "," << pb->j << std::endl;
        delete pa;
        delete pb;
    }
  return 0;
}

结果如下：