Explicit call of constructor and destructor

1. First, read the following code to explicitly call the Destructor:

C ++ code

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# Include <iostream> Using namespace STD; Class myclass { Public: Myclass () { N _ = 1; Cout <"Constructors" <Endl; } ~ Myclass () { Cout <"Destructors" <Endl; } Void display () { Cout <n _ <Endl; } PRIVATE: Int N _; }; Int main (void) { Myclass * pmyclass = new myclass; Pmyclass-> ~ Myclass (); Delete pmyclass; Return 0; }

Output:

Constructors

Destructors

Destructors

Confirmed some arguments:

Two things were actually done in new,

First, call malloc to allocate the required memory (actually calling operator new), and second, call the constructor.

Two things were done during the delete operation,

One is to call the analysis function, and the other is to call free to release the memory (actually calling operator delete ).

This is only for demonstration. Normally, the Destructor is called only once. If it is called twice, the Destructor has a delete operation, this may cause two errors when the memory is released.

2. Then let's look at it again: explicitly calling the constructor (the first method ):

C ++ code

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# Include <iostream> Using namespace STD; Class myclass { Public: Myclass () { N _ = 1; Cout <"Constructors" <Endl; } ~ Myclass () { Cout <"Destructors" <Endl; } Void display () { Cout <"n =" <n _ <Endl; } PRIVATE: Int N _; }; Int main (void) { Myclass * pmyclass = (myclass *) malloc (sizeof (myclass )); Pmyclass-> myclass: myclass (); // method 1 Pmyclass-> display (); Free (pmyclass); // you cannot use delete. It corresponds to malloc and does not call destructor. Return 0; }

Output:

Constructors

N = 1

3. Display call Constructor (method 2): Placement new

C ++ code

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# Include <iostream> Using namespace STD; Class myclass { Public: Myclass () { N _ = 1; Cout <"Constructors" <Endl; } ~ Myclass () { Cout <"Destructors" <Endl; } Void display () { Cout <"n =" <n _ <Endl; } PRIVATE: Int N _; }; Int main (void) { Char TMP [10]; Myclass * pmyclass = new (TMP) myclass; // placement new usage Pmyclass-> display (); Pmyclass-> ~ Myclass (); // It is not the heap memory and cannot be deleted. Return 0; }

Constructors

N = 1

Destructors

PlacementThe role of new is to create an object (call the constructor of this class) but create an object on an existing memory block without allocating memory. It can be used on objects that need to be created and deleted repeatedly to reduce the performance consumption of allocated and released memory.

Reference: http://www.cnblogs.com/fangyukuan/archive/2010/08/28/1811119.html