(转贴)Java与C++函数参数传递比较

学好C++，为祖国四化做贡献。

简言之：Java都是值传递(pass-by-value)，而C++中包括值传递(pass-by-value)和引用传递(pass-by-reference)。

先说Java，先做几点说明：

在Java中，无非就是两种类型，即基本类型和从Object继承下来的对象类型，而对象类型又包括String这种一旦初始化就不可改变内容的类型和BufferString这种可以初始化后可

以改变内容的类型。

然后看一下代码示例：

 

java 代码

package test;

public class Test {
 public static void main(String args[]) {
  Integer interger1, interger2;
  int i, j;
  interger1 = new Integer(10);
  interger2 = new Integer(50);
  i = 5;
  j = 9;
  System.out.println("Before Swap, Interger1 is " + interger1);
  System.out.println("Before Swap, Interger2 is " + interger2);
  swap(interger1, interger2);
  System.out.println("After Swap Interger1 is " + interger1);
  System.out.println("After Swap Interger2 is " + interger2);
  System.out.println("Before Swap i is " + i);
  System.out.println("Before Swap j is " + j);
  swap(i, j);
  System.out.println("After Swap i is " + i);
  System.out.println("After Swap j is " + j);

  StringBuffer sb = new StringBuffer("I am StringBuffer");
  System.out.println("Before change, sb is <" + sb + ">");
  change(sb);
  System.out.println("After change sb is <" + sb + ">");
 }

 public static void swap(Integer ia, Integer ib) {
  Integer temp = ia;
  ia = ib;
  ib = temp;
 }

 public static void swap(int li, int lj) {
  int temp = li;
  li = lj;
  lj = temp;
 }

 public static void change(StringBuffer ia) {
  ia.append(", but my content can be changed");
  //ia = new StringBuffer(",but my content can be changed");
 }
}

输出：

Before Swap, Interger1 is 10
Before Swap, Interger2 is 50
After Swap Interger1 is 10
After Swap Interger2 is 50
Before Swap i is 5
Before Swap j is 9
After Swap i is 5
After Swap j is 9
Before change, sb is <I am StringBuffer>
After change sb is <I am StringBuffer, but my content can be changed>

这很好解释，对于基本类型诸如int，传递进去的是存放int值的“内存单元”的一个copy，所以函数swap里面的int和外面的int根本就不是一个东西，当然不能反射出去影响外面

的int。而对于对象类型，我们同样可以这样认为，传递进去的是存放对象类型的指针的“内存单元”一个copy（虽然Java里面没有指针的概念，但这并不妨碍我们理解）。这样，

在swap函数里面，对其指针本身的值做任何操作当然不会影响外面的Integer，因为interger1和interger2的“内存单元”里面的值是不变的，其指向的对象类型也是没有变的。

然后这里需要说明一个问题，就是StringBuffer这种类型的对象了。因为其内容是可以改变的，所以change函数里面的“指针”通过类似“*”的操作，改变了StringBuffer对象的

本身，就显而易见了。(StringBuffer对象本身只有一个副本)

然后说C++了，里面的基本类型的诸如int的值传递大家都了然于胸，就不在这里废话了。然后另一种值传递可以称为指针引用传递(pass-by-value argument of pointer)(这个类

似上文说的Java中的对象类型的值传递)，可以通过*操作，改变指针指向的值。示例程序如下，一看便知：

cpp 代码

#include<iostream.h>

int main(){
 void test(int*, const char*);
 int i = 1;
 int* iptr = &i;
 cout<<"Before pass-by-value:"<<"\n\n";
 cout<<"i = "<<i<<", It's value of i"<<endl;
 cout<<"&i = "<<&i<<", It's address of i and value of iptr"<<endl;
 cout<<"*iptr = "<<*iptr<<", It's value of i"<<endl;
 cout<<"iptr = "<<iptr<<", It's value of iptr and address of i"<<endl;
 cout<<"&iptr = "<<&iptr<<", It's address of iptr-self"<<"\n\n";

 test(iptr, "pass-by-iptr");

 test(&i, "pass-by-&i");

 return 0;
}

void test(int* iiptr, const char* string){
 cout<<"When pass-by-value and :"<<"\n\n";
 cout<<"*iiptr = "<<*iiptr<<", It's value of i"<<endl;
 cout<<"iiptr = "<<iiptr<<", It's value of iiptr and address of i"<<endl;
 cout<<"&iiptr = "<<&iiptr<<", It's address of iiptr-self, different with iptr!"<<"\n\n";
}

输出：

Before pass-by-value:

i = 1, It's value of i
&i = 0x0012FF7C, It's address of i and value of iptr
*iptr = 1, It's value of i
iptr = 0x0012FF7C, It's value of iptr and address of i
&iptr = 0x0012FF78, It's address of iptr-self

When pass-by-value and :

*iiptr = 1, It's value of i
iiptr = 0x0012FF7C, It's value of iiptr and address of i
&iiptr = 0x0012FF24, It's address of iiptr-self, different with iptr!

When pass-by-value and :

*iiptr = 1, It's value of i
iiptr = 0x0012FF7C, It's value of iiptr and address of i
&iiptr = 0x0012FF24, It's address of iiptr-self, different with iptr!

在C++里面的第二种就是引用传递了(pass-by-reference)。见如下示例：

cpp 代码

#include<iostream.h>
int main(){
 void test(int&, const char*);
 int i = 1;
 int &iref = i;
 cout<<"Before pass-by-reference:"<<"\n\n";
 cout<<"i = "<<i<<", It's value of i"<<endl;
 cout<<"&i = "<<&i<<", It's address of i and value of iptr"<<endl;
 cout<<"iref = "<<iref<<", It's value of iref and value of i"<<endl;
 cout<<"&iref = "<<&iref<<", It's address of iref-self, the same as i!"<<"\n\n";

 test(iref, "pass-by-iref");

 test(i, "pass-by-i");

 return 0;
}

void test(int &iiref, const char* string){
 cout<<"When pass-by-reference and "<<string<<"\n\n";
 cout<<"iiref = "<<iiref<<", It's value of iiref and value of i"<<endl;
 cout<<"&iiref = "<<&iiref<<", It's address of iiref-self, the same as i!"<<"\n\n";
}

输出：

Before pass-by-reference:

i = 1, It's value of i
&i = 0x0012FF7C, It's address of i and value of iptr
iref = 1, It's value of iref and value of i
&iref = 0x0012FF7C, It's address of iref-self, the same as i!

When pass-by-reference and pass-by-iref

iiref = 1, It's value of iiref and value of i
&iiref = 0x0012FF7C, It's address of iiref-self, the same as i!

When pass-by-reference and pass-by-i

iiref = 1, It's value of iiref and value of i
&iiref = 0x0012FF7C, It's address of iiref-self, the same as i!

这里的引用(reference)说的明白一些，就是被传递参数的一个别名，或者更直接的理解就是被传递参数自己了，只是名字不同而已。那么既然自己都被pass过去了，那当然可以在function里面为所欲为了。赫赫。

renki_z对本文亦有贡献