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Difference between new A and new A()

栏目：php教程时间：2016-06-22 08:37:26

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

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