(轉貼)Java與C++函數參數傳遞比較

學好C++，爲祖國四化做貢獻。

簡言之：Java都是值傳遞(pass-by-value)，而C++中包括值傳遞(pass-by-value)和引用傳遞(pass-by-reference)。

先說Java，先做幾點說明：

在Java中，無非就是兩種類型，即基本類型和從Object繼承下來的對象類型，而對象類型又包括String這種一旦初始化就不可改變內容的類型和BufferString這種可以初始化後可

以改變內容的類型。

然後看一下代碼示例：

 

java 代碼

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");
 }
}

輸出：

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>

這很好解釋，對於基本類型諸如int，傳遞進去的是存放int值的“內存單元”的一個copy，所以函數swap裏面的int和外面的int根本就不是一個東西，當然不能反射出去影響外面

的int。而對於對象類型，我們同樣可以這樣認爲，傳遞進去的是存放對象類型的指針的“內存單元”一個copy（雖然Java裏面沒有指針的概念，但這並不妨礙我們理解）。這樣，

在swap函數裏面，對其指針本身的值做任何操作當然不會影響外面的Integer，因爲interger1和interger2的“內存單元”裏面的值是不變的，其指向的對象類型也是沒有變的。

然後這裏需要說明一個問題，就是StringBuffer這種類型的對象了。因爲其內容是可以改變的，所以change函數裏面的“指針”通過類似“*”的操作，改變了StringBuffer對象的

本身，就顯而易見了。(StringBuffer對象本身只有一個副本)

然後說C++了，裏面的基本類型的諸如int的值傳遞大家都瞭然於胸，就不在這裏廢話了。然後另一種值傳遞可以稱爲指針引用傳遞(pass-by-value argument of pointer)(這個類

似上文說的Java中的對象類型的值傳遞)，可以通過*操作，改變指針指向的值。示例程序如下，一看便知：

cpp 代碼

#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";
}

輸出：

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++裏面的第二種就是引用傳遞了(pass-by-reference)。見如下示例：

cpp 代碼

#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";
}

輸出：

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!

這裏的引用(reference)說的明白一些，就是被傳遞參數的一個別名，或者更直接的理解就是被傳遞參數自己了，只是名字不同而已。那麼既然自己都被pass過去了，那當然可以在function裏面爲所欲爲了。赫赫。

renki_z對本文亦有貢獻