一、异常处理的思想与程序实现

异常处理的基本思想

异常处理的语法

注意这里的cath异常类型是不支持隐式转化的

例12-1处理除零异常

//12_1.cpp
#include <iostream>
using namespace std;
int divide(int x, int y) {
    if (y == 0)
        throw x;
    return x / y;
}
int main() {
    try {
        cout << "5 / 2 = " << divide(5, 2) << endl;
        cout << "8 / 0 = " << divide(8, 0) << endl;
        cout << "7 / 1 = " << divide(7, 1) << endl;
    } catch (int e) {
        cout << e << " is divided by zero!" << endl;
    }
    cout << "That is ok." << endl;
    return 0;
}

异常接口声明

• 一个函数显式声明可能抛出的异常，有利于函数的调用者为异常处理做好准备

• 可以在函数的声明中列出这个函数可能抛掷的所有异常类型。void fun() throw(A，B，C，D);

• 若无异常接口声明，则此函数可以抛掷任何类型的异常。

• 不抛掷任何类型异常的函数声明如下：void fun() throw();

二、异常处理中的构造与析构

自动的析构

• 找到一个匹配的catch异常处理后
  • 初始化异常参数。
  • 将从对应的try块开始到异常被抛掷处之间构造（且尚未析构）的所有自动对象进行析构。
• 从最后一个catch处理之后开始恢复执行。

例12-2 带析构语义的类的C++异常处理

//12_2.cpp
#include <iostream>
#include <string>
using namespace std;
class MyException {
public:
    MyException(const string &message) : message(message) {}
    ~MyException() {}
    const string &getMessage() const { return message; }
private:
    string message;
};
class Demo {
public:
    Demo() { cout << "Constructor of Demo" << endl; }
    ~Demo() { cout << "Destructor of Demo" << endl; }
};
void func() throw (MyException) {
    Demo d;
    cout << "Throw MyException in func()" << endl;
    throw MyException("exception thrown by func()");
}
int main() {
    cout << "In main function" << endl;
    try {
        func();
    } catch (MyException& e) {
        cout << "Caught an exception: " << e.getMessage() << endl;
    } 
    cout << "Resume the execution of main()" << endl;
    return 0;
}
运行结果：
In main function
Constructor of Demo
Throw MyException in func()
Destructor of Demo
Caught an exception: exception thrown by func()
Resume the execution of main()

三、标准库异常处理

标准异常类的继承关系

C++标准库各种异常类所代表的异常

标准异常类的基础

• exception：标准程序库异常类的公共基类
• logic_error表示可以在程序中被预先检测到的异常
• • 如果小心地编写程序，这类异常能够避免
• runtime_error表示难以被预先检测的异常

例12-3 三角形面积计算

• 编写一个计算三角形面积的函数，函数的参数为三角形三边边长a、b、c，可以用Heron公式计算：```cpp //12_3.cpp

  include

  include

  include

  using namespace std; //给出三角形三边长，计算三角形面积 double area(double a, double b, double c) throw (invalid_argument) { //判断三角形边长是否为正 if (a <= 0 || b <= 0 || c <= 0)

    throw invalid_argument("the side length should be positive");

  //判断三边长是否满足三角不等式 if (a + b <= c || b + c <= a || c + a <= b)

    throw invalid_argument("the side length should fit the triangle inequation");

  //由Heron公式计算三角形面积 double s = (a + b + c) / 2; return sqrt(s (s - a) (s - b) * (s - c)); } int main() { double a, b, c; //三角形三边长 cout << “Please input the side lengths of a triangle: “; cin >> a >> b >> c; try {

    double s = area(a, b, c);   //尝试计算三角形面积
    cout << "Area: " << s << endl;

  } catch (exception &e) {

    cout << "Error: " << e.what() << endl;

  } return 0; } • 运行结果1： Please input the side lengths of a triangle: 3 4 5 Area: 6 • 运行结果2： Please input the side lengths of a triangle: 0 5 5 Error: the side length should be positive • 运行结果2： Please input the side lengths of a triangle: 1 2 4 Error: the side length should fit the triangle inequation ```

#include <iostream>
using namespace std;
struct CException
{void reason() const{ cout<< "a expected exception accured"<<endl;}};
void func(int n) {
    throw(CException());
    cout<<n<<endl;
}
int main(){  
    try
    {func(666);}
    catch(const CException& e)
    { e.reason();}
    return 0;
}