Difference between new A and new A()

柔光的暖阳◎ 2022-08-22 13:28 250阅读 0赞

**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();

但是如果我这么写呢：

A *pa = new A;

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.

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;
    };

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';
    }

输出：  
true  
false  
false