Difference between new A and new A

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问题：
A是一个类，我们经常new一个A的对象，你经常会这么写：

A *pa = new A();
  
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但是如果我这么写呢：

A *pa = new A;
  
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两者有什么区别吗？ 这就是我们今天要讨论的问题。

先给出答案，再慢慢解释

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 .

什么是POD

POD的意思是 Plain Old Data，即一个class或struct没有构造函数、析构函数、虚函数。维基百科上这么描述：

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.
  
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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.

我们就定义一个POD的类：

class A{  int m;};
  
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再定义两个非POD类：

class B {   ~B(); }; class C {   C() : m() {};   int m; }; 
  
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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;}
  
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输出：
18780112
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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()才会有区别

就是再看看如何判断是不是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';}
  
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输出：
true
false
false

           

分享一下我老师大神的人工智能教程。零基础！通俗易懂！风趣幽默！还带黄段子！希望你也加入到我们人工智能的队伍中来！https://blog.youkuaiyun.com/jiangjunshow