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变量、指针、引用与形参初始化

2014-07-07 19:56:39| 分类：默认分类 | 标签： |举报 |字号大中小订阅

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        C++ 关于函数参数是这样规定的：按照形参初始化方式，函数参数有两种：拷贝初始化方式和直接初始化方式。

拷贝初始化：形参初始化是通过拷贝实参来实现；
直接初始化：形参初始化是通过直接赋予变量地址的方式来实现。

        有个认识：对变量的访问都是通过地址访问的，在汇编语言中，变量存储与内存中，变量占据内存块，并有一个首地址。对变量的访问必须首先确定首地址。首地址的确定方式有两种：直接访问和间接访问。直接访问是通过变量地址访问，间接访问是通过内存中所存储的另一个变量的地址进行间接访问。

一、拷贝初始化形参

        拷贝初始化形参有两种：直接形参和指针形参。

1、直接形参如下：

void swap_cpy (int a, int b)
{
  ...
}

直接形参是将实参的值拷贝给形参，对形参的访问与实参没任何关系。

2、指针形参如下：

void swap_ptr (int *a, int *b)
{
  ...
}

指针形参方式是：将实参的地址拷贝给指针变量，而指针的地址才是直接访问指针变量的地址。

二、非拷贝初始化形参

直接初始化形参方式就是引用：

void swap_ref (int &a, int &b)
{
  ...
}

引用就是将实参的地址直接给形参，形参地址就是实参地址。

三、举例说明

#include <iostream>

using namespace std;

void swap_cpy (int a, int b)
{
  int c;

  c = a;
  a = b;
  b = c;

  cout << "--- swap_cpy (int a, int b) begins ---" << endl;
  cout << "a = " << a << ", " << "b = " << b << endl;
  cout << "&a = " << &a << ", " << "&b = " << &b << endl;
  cout << "--- swap_cpy (int a, int b) ends --- " << endl;
}

void swap_ptr (int *a, int *b)
{
  int c;

  c = *a;
  *a = *b;
  *b = c;

  cout << "--- swap_cpy (int *a, int *b) begins ---" << endl;
  cout << "*a = " << *a << ", " << "*b = " << *b << endl;
  cout << "a = " << a << ", " << "b = " << b << endl;
  cout << "&a = " << &a << ", " << "&b = " << &b << endl;
  cout << "--- swap_cpy (int *a, int *b) ends ---" << endl;
}

void swap_ref (int &a, int &b)
{
  int c;

  c = a;
  a = b;
  b = c;

  cout << "--- swap_cpy (int &a, int &b) begins ---" << endl;
  cout << "a = " << a << ", " << "b = " << b << endl;
  cout << "&a = " << &a << ", " << "&b = " << &b << endl;
  cout << "--- swap_cpy (int &a, int &b) ends ---" << endl;
}

void try_ref ()
{
  int x = 1;
  int y = 8;

  cout << "x = " << x << ", " << "y = " << y << endl;
  cout << "&x = " << &x << ", " << "&y = " << &y << endl;
  cout << endl;

  cout << "*** before swap_cpy (int x, int y) *** " << endl;
  cout << "x = " << x << ", " << "y = " << y << endl;
  swap_cpy (x, y);
  cout << "*** after swap_cpy (int x, int y) ***" << endl;
  cout << "x = " << x << ", " << "y = " << y << endl;
  cout << endl;

  cout << "*** before swap_ptr (int *x, int *y) ***" << endl;
  cout << "x = " << x << ", " << "y = " << y << endl;
  swap_ptr (&x, &y);
  cout << "*** after swap_cpy (int *x, int *y) ***" << endl;
  cout << "x = " << x << ", " << "y = " << y << endl;
  cout << endl;

  cout << "*** before swap_ptr (int &x, int &y) ***" << endl;
  cout << "x = " << x << ", " << "y = " << y << endl;
  swap_ref (x, y);
  cout << "*** after swap_cpy (int &x, int &y) ***" << endl;
  cout << "x = " << x << ", " << "y = " << y << endl;
  cout << endl;
}

int main ()
{
  try_ref ();

  return 0;
}

结果：

x = 1, y = 8
&x = 0x7fffee33dfac, &y = 0x7fffee33dfa8  // 实参地址

*** before swap_cpy (int x, int y) *** 
x = 1, y = 8
--- swap_cpy (int a, int b) begins ---
a = 8, b = 1
&a = 0x7fffee33defc, &b = 0x7fffee33def8  // 形参地址，不同于实参地址
--- swap_cpy (int a, int b) ends --- 
*** after swap_cpy (int x, int y) ***
x = 1, y = 8

*** before swap_ptr (int *x, int *y) ***  
x = 1, y = 8
--- swap_cpy (int *a, int *b) begins ---
*a = 8, *b = 1
a = 0x7fffee33dfac, b = 0x7fffee33dfa8     // 指针变量的值，是实参地址
&a = 0x7fffee33def8, &b = 0x7fffee33def0   // 指针变量地址
--- swap_cpy (int *a, int *b) ends ---
*** after swap_cpy (int *x, int *y) ***
x = 8, y = 1

*** before swap_ptr (int &x, int &y) ***
x = 8, y = 1
--- swap_cpy (int &a, int &b) begins ---  
a = 1, b = 8
&a = 0x7fffee33dfac, &b = 0x7fffee33dfa8   // 引用就是实参地址
--- swap_cpy (int &a, int &b) ends ---
*** after swap_cpy (int &x, int &y) ***
x = 1, y = 8

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