问题：
A是1个类，我们常常new1个A的对象，你常常会这么写：
A *pa = new A();但是如果我这么写呢：
A *pa = new A;二者有甚么区分吗？ 这就是我们今天要讨论的问题。
先给出答案，再渐渐解释
If the class has a default constructor defined,then both are equivalent; the object will be created by calling that constructor.
If the class only has an implicit default constructor,then there is a difference. The first will leave any members of POD type uninitialised; the second will value-initialise them .
甚么是POD
POD的意思是 Plain Old Data，即1个class或struct没有构造函数、析构函数、虚函数。维基百科上这么描写：
A Plain Old Data Structure in C++ is an aggregate class that contains only PODS as members,has no user-defined destructor,no user-defined copy assignment operator,and no nonstatic members of pointer-to-member type.int,char,wchar_t,bool,float,double are PODs,as are long/short and signed/unsigned versions of them.
pointers (including pointer-to-function and pointer-to-member) are PODs,
enums are PODs
a const or volatile POD is a POD.
a class,struct or union of PODs is a POD provided that all non-static data members are public,and it has no base class and no constructors,destructors,or virtual methods. Static members don’t stop something being a POD under this rule.
我们就定义1个POD的类：
class A
{
int m;
};再定义两个非POD类：
class B
{
~B();
};
class C
{
C() : m() {};
int m;
}; 接下来就是使用了：
#include<iostream>
using namespace std;
class A
{
public:
int m;
};
class B
{
public:
~B() {};
public:
int m;
};
class C
{
public:
C() : m() {};
public:
int m;
};
int main()
{
A *aObj1 = new A;
A *aObj2 = new A();
cout << aObj1->m << endl;
cout << aObj2->m << endl;
B *bObj1 = new B;
B *bObj2 = new B();
cout << bObj1->m << endl;
cout << bObj2->m << endl;
C *cObj1 = new C;
C *cObj2 = new C();
cout << cObj1->m << endl;
cout << cObj2->m << endl;
delete aObj1;
delete aObj2;
delete bObj1;
delete bObj2;
delete cObj1;
delete cObj2;
return 0;
}输出：
18780112
0
4522067
0
0
0
在所有C++版本中，只有当A是POD类型的时候，new A和new A()才会有区分
就是再看看如何判断是否是POD，不用认为判断：
#include <iostream>
#include <type_traits>
struct A {
int m;
};
struct B {
int m1;
private:
int m2;
};
struct C {
virtual void foo();
};
int main()
{
std::cout << std::boolalpha;
std::cout << std::is_pod<A>::value << '\n';
std::cout << std::is_pod<B>::value << '\n';
std::cout << std::is_pod<C>::value << '\n';
}
输出：
true
false
false
