[C++] Object i = Object(0)和Object* i = new Object(0)的区别

标签：生成   returns   str   hold   c++   int   copy   point   har   

  C/C++里指针通常是指向一个对象的，所以不能指向某一变量（Object i，i为某个变量），不过在Java中所有变量都是对象

  举个例子: 

int a=1;

int *p=a;

报错：invalid conversion from ‘int‘ to ‘ int* ‘;

int *a = new int(1);

int *p=a;

正常运行!

再来看一个较为复杂一点的例子（将一个二叉链表的结点类作为我们的Object）：

template<class T>

struct BTNode
{
　　T data;
　　BTNode *left,*right;
　　BTNode(const T& item=T(),BTNode *lptr=NULL,BTNode *rptr=NULL):data(item),left(lptr),right(rptr){}
};

int main()
{
　　BTNode<char>  a=BTNode<char>(‘D‘,NULL,NULL);
　　BTNode<char> *ap=a;
　　return 0;
}

同样地会报错:cannot convert ‘BTNode<char>‘ to ‘BTNode<char>*‘ int initialization

而这样做就可以了：

int main()
{
　　BTNode<char>  *a=new BTNode<char>(‘D‘,NULL,NULL);
　　BTNode<char> *ap=a;
　　return 0;
}

以上问题的原因都是因为指针指向了某一变量而不是对象，而用new Object()则是生成的一个对象并将地址给=前面的指针

再来看StackOverFlow里的一个例子：

struct Foo {
  int value; // Foo stores an int, called value
  Foo(int v):value(v) {}; // Foo can be constructed (created) from an int
  explicit Foo(double d):value(d) {}; // Foo can be constructed (created) from a double
  // ^^^ note that keyword.  It says that it can only be EXPLICITLY created from a double
};

Foo i = Foo(0);

the above creates an int literal, then constructs Foo i with it using the Foo(int v) constructor, then copy-constructs Foo i from it. However, the standard allows the compiler to "elide" (skip) the copy constructor, and instead just construct Foo i directly from the int literal 0.

Foo* i = new Foo(0);

this goes to the free store (typically implemented and called the heap), gets ahold of enough memory to store a Foo, then constructs it with the integer literal 0. It then returns the address of this Foo object, which is then used to initialize the pointer Foo* i.

[C++] Object i = Object(0)和Object* i = new Object(0)的区别

标签：生成   returns   str   hold   c++   int   copy   point   har   

原文地址：http://www.cnblogs.com/yzz0110/p/7667649.html