C++异常(5) - 异常处理与对象析构

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1.try-catch中的析构行为

预测下面程序输出结果：

#include <iostream>using namespace std;class Test {public:  Test() { cout << "Constructing an object of Test " << endl; }  ~Test() { cout << "Destructing an object of Test "  << endl; }};int main() {  try {    Test t1;    throw 10;  } catch(int i) {    cout << "Caught " << i << endl;  }}

输出:
Constructing an object of Test
Destructing an object of Test
Caught 10

当抛出一个异常时，在catch代码块得到执行前，try中的对象会被析构。这就是为何上述程序在“Caught 10″之前打印“Destructing an object of Test”的原因.

2.构造函数中抛出异常时的析构行为

如果是在构造函数中抛出异常，程序如何处理？参考下面例子。

#include <iostream>using namespace std;class Test1 {public:  Test1() { cout << "Constructing an Object of Test1" << endl; }  ~Test1() { cout << "Destructing an Object of Test1" << endl; }}; class Test2 {public:  // 这个构造函数会抛出整形异常  Test2() { cout << "Constructing an Object of Test2" << endl;            throw 20; }  ~Test2() { cout << "Destructing an Object of Test2" << endl; }}; int main() {  try {    Test1 t1;  // 构造与析构都被执行    Test2 t2;  // 部分被构造    Test1 t3;  // t3不会被构造，因为这个表达式一直不会执行。  } catch(int i) {    cout << "Caught " << i << endl;  }}

输出:

  Constructing an Object of Test1  Constructing an Object of Test2  Destructing an Object of Test1  Caught 20

析构函数只会运行在已经完整的构造好的对象中。当某个对象在构造时抛出异常，则析构函数不会被调用到。

3.try-catch中对象的析构顺序

预测下面程序结果：

#include <iostream>using namespace std;class Test {  static int count;  int id;public:  Test() {    count++;    id = count;    cout << "Constructing object number " << id << endl;    if(id == 4)       throw 4;  }  ~Test() { cout << "Destructing object number " << id << endl; }}; int Test::count = 0; int main() {  try {    Test array[5];  } catch(int i) {    cout << "Caught " << i << endl;  }}

输出：
Constructing object number 1
Constructing object number 2
Constructing object number 3
Constructing object number 4
Destructing object number 3
Destructing object number 2
Destructing object number 1

观察上面结果，可以看到析构的顺序与构造顺序相反。为什么会这样？
这是因为这些对象都是存储在栈内存上的，当析构时，是按照后进先出的顺序来析构对象。

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