轉自http://blog.csdn.net/xiajun07061225/article/details/8796257
我們在C++程序中經常看到兩種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的類型是否是POD(Plain old data)類型,或者它是否是包含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。而且,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。
首先需要注意的是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。
採用如下代碼可以驗證編譯器遵循的到底是哪一種標準:
- #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;
- }
#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;
- }
#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.需要注意的是,VS08只支持C++98。
在這種情況下:
new A:不確定的值
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)
如果用兼容C++03的編譯器,比如G++結果:
new A:不確定的值
new A():value-initialize A,由於是POD類型所以是zero initialization
new B:默認構造(B::m未被初始化)
new B():value-initialize B,zero-initialize所有字段,因爲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
在所有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。而且,C++98和C++03會有區別。