我們在C++程式中經常看到兩種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？
呼叫new分配的記憶體有時候會被初始化，而有時候不會，這依賴於A的型別是否是POD（Plain old data）型別，或者它是否是包含POD成員、使用編譯器生成預設建構函式的類。
附：POD型別
POD是Plain old data的縮寫，它是一個struct或者類，且不包含建構函式、解構函式以及虛擬函式。
維基百科給出了更加詳細的解釋：
C++的POD型別或者是一個標量值，或者是一個POD型別的類。POD class沒有使用者定義的解構函式、拷貝建構函式和非靜態的非POD型別的資料成員。而且，POD class必須是一個aggregate，沒有使用者定義的建構函式，沒有私有的或者保護的非靜態資料，沒有基類或虛擬函式。它只是一些欄位值的集合，沒有使用任何封裝以及多型特性。
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).
接著介紹一下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。
採用如下程式碼可以驗證編譯器遵循的到底是哪一種標準：
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#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++示例程式碼：
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#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; } |
執行結果：
上述測試平臺是VS2008.需要注意的是，VS08只支援C++98。
在這種情況下：
new A：不確定的值
new A()：zero-initialize
new B：預設構造（B::m未被初始化）
new B()：預設構造（B::m未被初始化）
new C：預設構造（C::m被zero-initialize）
new C()：預設構造(C::m被zero-initialize)
如果用相容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型別所以是zero initialization
new B：預設構造（B::m未被初始化）
new B()：value-initialize B，zero-initialize所有欄位，因為使用的預設建構函式
new C：default-initialize C，呼叫預設建構函式
new C()：value-initialize C，呼叫預設建構函式
在所有C++版本中，只有當A是POD型別的時候，new A和new A()才會有區別。而且，C++98和C++03會有區別。
參考資料：