c++new和new()區別(瞭解)

咱們在C++程序中常常看到兩種new的使用方式：new A以及new A()。那麼這兩種究竟有什麼區別呢？

調用new分配的內存有時候會被初始化，而有時候不會，這依賴於A的類型是不是POD（Plain old data）類型，或者它是不是包含POD成員、使用編譯器生成默認構造函數的類。

附：POD類型

POD是Plain old data的縮寫，它是一個struct或者類，且不包含構造函數、析構函數以及虛函數。

維基百科給出了更加詳細的解釋：

C++的POD類型或者是一個標量值，或者是一個POD類型的類。POD class沒有用戶定義的析構函數、拷貝構造函數和非靜態的非POD類型的數據成員。並且，POD class必須是一個 aggregate ，沒有用戶定義的構造函數，沒有私有的或者保護的非靜態數據，沒有基類或虛函數。它只是一些字段值的集合，沒有使用任何封裝以及多態特性。

附：aggregate的定義：

An aggregate is an array or a class (clause 9) with no user-declared constructors (12.1), no private or protected non-static data members (clause 11), no base classes (clause 10), and no virtual functions (10.3). 

接着介紹一下 C++中的三種初始化方式：

zero-initialization,default-initialization,value-initialization 。

首先須要注意的是 value-initialization是在C++2003標準中新引入的，在原來的1998標準中並不存在。

C++03標準中針對這三種方式的說明：

To zero-initialize an object of type T means: 
— if T is a scalar type (3.9), the object is set to the value of 0 (zero) converted to T; 
— if T is a non-union class type, each nonstatic data member and each base-class subobject is zero-initialized; 
— if T is a union type, the object’s first named data member is zero-initialized; 
— if T is an array type, each element is zero-initialized; 
— if T is a reference type, no initialization is performed. 

To default-initialize an object of type T means: 
— if T is a non-POD class type (clause 9), the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor); 
— if T is an array type, each element is default-initialized; 
— otherwise, the object is zero-initialized. 

To value-initialize an object of type T means: 
— if T is a class type (clause 9) with a user-declared 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 non-union class type without a user-declared constructor, then every non-static data member and base-class component of T is value-initialized; 
— if T is an array type, then each element is value-initialized; 
— otherwise, the object is zero-initialized 
A program that calls for default-initialization or value-initialization of an entity of reference type is ill-formed. If T is a cv-qualified type, the cv-unqualified version of T is used for these definitions of zero-initialization, default-initialization, and value-initialization. 

注意：VS2008遵循的是98標準，而GCC3.4.5遵循的是03標準。

採用以下代碼能夠驗證編譯器遵循的究竟是哪種標準：

#include <stdio.h>
#include <string.h>
#include <new>

struct A { int m; }; // POD
struct B { ~B(); int m; }; // non-POD, compiler generated default ctor
struct C { C() : m() {}; ~C(); int m; }; // non-POD, default-initialising m

int main()
{
    char buf[sizeof(B)];
    memset( buf, 0x5a, sizeof( buf));

    // use placement new on the memset'ed buffer to make sure 
    //  if we see a zero result it's due to an explicit 
    //  value initialization
    B* pB = new(buf) B();   //C++98 rules - pB->m is uninitialized
                            //C++03 rules - pB->m is set to 0
    printf( "m  is %d\n", pB->m);
    return 0;
}
在VS008中輸出就不是0，說明遵循的是98標準。
下面先看一段C++示例代碼：

#include <iostream>

using namespace std;

struct A { int m; }; // POD
struct B { ~B(){}; int m; }; // non-POD, compiler generated default ctor
struct C { C() : m() {}; ~C(){}; int m; }; // non-POD, default-initialising 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;
}
運行結果：
上述測試平臺是VS2008.須要注意的是，VS08只支持C++98。

在這種狀況下：

new A：不肯定的值

new A()：zero-initialize

new B：默認構造（B::m未被初始化）

new B()：默認構造（B::m未被初始化）

new C：默認構造（C::m被zero-initialize）

new C()：默認構造( C::m被zero-initialize )

若是用兼容C++03的編譯器，應該產生以下結果：

new A：不肯定的值

new A()：value-initialize A，因爲是POD類型因此是zero initialization

new B：默認構造（B::m未被初始化）

new B()： value-initialize B，zero-initialize全部字段，由於使用的默認構造函數

new C：default-initialize C，調用默認構造函數

new C()： value-initialize C，調用默認構造函數

在全部C++版本中，只有當A是POD類型的時候，new A和new A()纔會有區別。並且，C++98和C++03會有區別。

參考資料：

What are POD types in C++?

What are Aggregates and PODs and how/why are they special?