C++异常处理（try catch throw）完全攻略

• 做除法的时候除数为 0；
• 用户输入年龄时输入了一个负数；
• 用 new 运算符动态分配空间时，空间不够导致无法分配；
• 访问数组元素时，下标越界；打开文件读取时，文件不存在。

C++异常处理基本语法

throw? 表达式;

try {
??? 语句组
}
catch(异常类型) {
??? 异常处理代码
}
...
catch(异常类型) {
??? 异常处理代码
}

• 执行 try 块中的语句，如果执行的过程中没有异常拋出，那么执行完后就执行最后一个 catch 块后面的语句，所有 catch 块中的语句都不会被执行；
• 如果 try 块执行的过程中拋出了异常，那么拋出异常后立即跳转到第一个“异常类型”和拋出的异常类型匹配的 catch 块中执行（称作异常被该 catch 块“捕获”），执行完后再跳转到最后一个 catch 块后面继续执行。

#include <iostream>
using namespace std;
int main()
{
    double m,n;
    cin >> m >> n;
    try {
        cout << "before dividing." << endl;
        if( n == 0)
            throw -1; //抛出int类型异常
        else
            cout << m / n << endl;
        cout << "after dividing." << endl;
    }
    catch(double d) {
        cout << "catch(double) " << d <<  endl;
    }
    catch(int e) {
        cout << "catch(int) " << e << endl;
    }
    cout << "finished" << endl;
    return 0;
}

能够捕获任何异常的 catch 语句

catch(...) {
??? ...
}

#include <iostream>
using namespace std;
int main()
{
    double m,n;
    cin >> m >> n;
    try {
        cout << "before dividing." << endl;
        if (n == 0)
            throw - 1;  //抛出整型异常
        else if (m == 0)
            throw - 1.0;  //拋出 double 型异常
        else
            cout << m / n << endl;
        cout << "after dividing." << endl;
    }
    catch (double d) {
        cout << "catch (double)" << d << endl;
    }
    catch (...) {
        cout << "catch (...)" << endl;
    }
    cout << "finished" << endl;
    return 0;
}

异常的再拋出

#include <iostream>
#include <string>
using namespace std;
class CException
{
public:
    string msg;
    CException(string s) : msg(s) {}
};
double Devide(double x,double y)
{
    if (y == 0)
        throw CException("devided by zero");
    cout << "in Devide" << endl;
    return x / y;
}
int CountTax(int salary)
{
    try {
        if (salary < 0)
            throw - 1;
        cout << "counting tax" << endl;
    }
    catch (int) {
        cout << "salary < 0" << endl;
    }
    cout << "tax counted" << endl;
    return salary * 0.15;
}
int main()
{
    double f = 1.2;
    try {
        CountTax(-1);
        f = Devide(3,0);
        cout << "end of try block" << endl;
    }
    catch (CException e) {
        cout << e.msg << endl;
    }
    cout << "f = " << f << endl;
    cout << "finished" << endl;
    return 0;
}

#include <iostream>
#include <string>
using namespace std;
int CountTax(int salary)
{
    try {
        if( salary < 0 )
            throw string("zero salary");
        cout << "counting tax" << endl;

    }
    catch (string s ) {
        cout << "CountTax error : " << s << endl;
        throw; //继续抛出捕获的异常
    }
    cout << "tax counted" << endl;
    return salary * 0.15;
}
int main()
{
    double f = 1.2;
    try {
        CountTax(-1);
        cout << "end of try block" << endl;
    }
    catch(string s) {
        cout << s << endl;
    }
    cout << "finished" << endl;
    return 0;
}

函数的异常声明列表

void func() throw (int,double,A,B,C);

void func() throw (int,C){...}

void func() throw ();

C++标准异常类

图1：常用的异常类

1) bad_typeid

2) bad_cast

#include <iostream>
#include <stdexcept>
using namespace std;
class Base
{
    virtual void func() {}
};
class Derived : public Base
{
public:
    void Print() {}
};
void PrintObj(Base & b)
{
    try {
        Derived & rd = dynamic_cast <Derived &>(b);
        //此转换若不安全，会拋出 bad_cast 异常
        rd.Print();
    }
    catch (bad_cast & e) {
        cerr << e.what() << endl;
    }
}
int main()
{
    Base b;
    PrintObj(b);
    return 0;
}

3) bad_alloc

#include <iostream>
#include <stdexcept>
using namespace std;
int main()
{
    try {
        char * p = new char[0x7fffffff];  //无法分配这么多空间，会抛出异常
    }
    catch (bad_alloc & e)  {
        cerr << e.what() << endl;
    }
    return 0;
}

4) out_of_range

#include <iostream>
#include <stdexcept>
#include <vector>
#include <string>
using namespace std;
int main()
{
    vector<int> v(10);
    try {
        v.at(100) = 100;  //拋出 out_of_range 异常
    }
    catch (out_of_range & e) {
        cerr << e.what() << endl;
    }
    string s = "hello";
    try {
        char c = s.at(100);  //拋出 out_of_range 异常
    }
    catch (out_of_range & e) {
        cerr << e.what() << endl;
    }
    return 0;
}

（编辑：李大同）

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