C++new and New () Differences (Learn)

We often see two new ways to use it in C + + programs:NewA and new A (). So what is the difference between these two? The memory that the call to new allocates is sometimes initialized, and sometimes not, depending on whether the type of a is a pod (Plain old data) type, or whether it is a class that contains POD members and uses the compiler to generate a default constructor. Attached: Pod type pod is an abbreviation for plain old data, which is a struct or class and does not contain constructors, destructors, and virtual functions. Wikipedia gives a more detailed explanation: C++The pod type is either a scalar value, or a class of pod type. POD class does not have user-defined destructors, copy constructors, and non-static data members for non-pod types. Also, POD class must be a aggregate, no user-defined constructor, no private or protected non-static data, no base class or virtual function. It is just a collection of some field values, with no encapsulation and polymorphic properties. Attached: aggregate definition: an aggregate isAn array or aclass(clause9) with no user-declared constructors (12.1), noPrivateOrprotectednon-StaticData members (clause One), noBaseClasses (clauseTen), and NoVirtualFunctions (10.3). Then introduce the C++three ways to initialize in: zero-initialization,default-initialization,value-initialization. The first thing to note is the value-initialization is in C + +The newly introduced in the 2003 standard does not exist in the original 1998 standard. C++03 Instructions for these three ways in the standard: to zero-initialize anObjectof type T means:-ifT isA scalar type (3.9), theObject  is SetTo the value of0(zero) converted to t;-ifT isA non-unionclassType, each nonstatic data member and eachBase-classSubobject iszero-initialized;-ifT isA union type, theObject' s first named data member iszero-initialized;-ifT isAn array of type, each element iszero-initialized;-ifT isA reference type, no initialization isperformed. todefault-initialize anObjectof type T means:-ifT isA non-podclassType (clause9), thedefaultConstructor forT isCalled (and the initialization isIll-formedifT has no accessibledefaultconstructor); -ifT isAn array of type, each element is default-Initialized;-otherwise, theObject  iszero-initialized. To value-initialize anObjectof type T means:-ifT isAclassType (clause9) with a user-declared constructor (12.1), then thedefaultConstructor forT isCalled (and the initialization isIll-formedifT has no accessibledefaultconstructor); -ifT isA non-unionclassType without a user-declared constructor, then every non-Staticdata member andBase-classComponent of T isvalue-initialized;-ifT isAn array of type, then each element isvalue-Initialized;-otherwise, theObject  iszero-initialized A program that calls for default-initialization or value-initialization of an entity of reference type isIll-formed. If T isA cv-qualified type, the cv-unqualified version of T isUsed forThese definitions of zero-initialization,default-initialization, and value-initialization. Note: VS2008 follows the 98 standard, while the GCC3.4. 5 followed by the 03 standard. Use the following code to verify which criteria the compiler follows: #include<stdio.h>#include<string.h>#include<New>structAintM };//PODstructB {~b ();intM };//non-pod, compiler generated default ctorstructc {C (): M () {}; ~c ();intM };//non-pod, default-initialising mintMain () {Charbuf[sizeof(B)]; memset (BUF,0x5a,sizeof(BUF)); //Use placement New in the Memset ' Ed buffer to make sure//if we see a zero result it's due to an explicit//Value Initializationb* PB =New(BUF) B ();//c++98 rules-pb->m is uninitialized//c++03 Rules-pb->m is set to 0printf"m is%d\n", pb->m); return 0;} The output in VS008 is not 0, which indicates that the 98 standard is followed. Let's take a look at C++example code: #include<iostream>using namespacestd;structAintM };//PODstructB {~b () {};intM };//non-pod, compiler generated default ctorstructc {C (): M () {}; ~c () {};intM };//non-pod, default-initialising mintMain () {A*aobj1 =NewA; A*aobj2 =NewA (); cout<< aobj1->m <<Endl; cout<< aobj2->m <<Endl; B*bobj1 =NewB; B*bobj2 =NewB (); cout<< bobj1->m <<Endl; cout<< bobj2->m <<Endl; C*cobj1 =NewC; C*cobj2 =NewC (); cout<< cobj1->m <<Endl; cout<< cobj2->m <<Endl;  Delete aObj1;  Delete AObj2;  Delete bObj1;  Delete BObj2;  Delete cObj1;  Delete CObj2; return 0;} Running result: The test platform above is VS2008. Note that the VS08 only supports C++98. In this case:NewA: Indeterminate valueNewA (): zero-InitializeNewB: Default construction (b::m not initialized)NewB (): Default construct (b::m not initialized)NewC: Default construct (C::m is zero-Initialize)NewC (): Default construct (C::m is zero-Initialize) if compatible with C++the 03 compiler should produce the following results:NewA: Indeterminate valueNewA (): value-initialize a, because it is a pod type, so it is zero initializationNewB: Default construction (b::m not initialized)NewB (): Value-initialize b,zero-initialize all fields, because the default constructor usedNewC:default-Initialize C, calling the default constructorNewC (): value-Initialize C, call the default constructor at all C+ + version, only if A is the pod type,NewA and new A () will only make a difference. Also, c++98 and C + +03 will make a difference. Reference: What is POD typesinchC++?What is aggregates and PODs and how/why is they special?