exception错误处理库学习

exception是c++标准库组件抛出的各种异常的基类，在c++开发的过程中我们经常会遇到各种各样的异常比如，打开文件失败，数组越界，等等，如果不想让程序崩溃，那么就需要使用一个异常类来处理这些异常错误，使得程序继续运行。下面我们来学习exception的基本使用。

1.异常的使用示例

我们先来看一个不使用exception的程序

#include 
#include 
#include 
 
using namespace std;
 
class Polymorphic
{
    virtual void member() {}
};
 
 
int main()
{
 
    Polymorphic *pb = nullptr;
    typeid(*pb);
 
    cout << "Hello World!" << endl;
    return 0;
}

运行

 程序运行就崩溃了，然后我们再使用异常处理

#include 
#include 
#include 
 
using namespace std;
 
class Polymorphic
{
    virtual void member() {}
};
 
 
int main()
{
 
    try {
        Polymorphic *pb = nullptr;
        typeid(*pb);
    } catch (std::exception &e) {
        std::cerr << "exception caught: " << e.what() << endl;
    }
    cout << "Hello World!" << endl;
    return 0;
}

运行结果：

程序抛出空指针的异常，但程序不会崩溃，因此使用exception时程序出现错误时，可以抛出来，避免程序崩溃，程序员需要处理抛出的异常。

2.current_exception捕获当前异常到exception_ptr中

#include 
#include 
 
using namespace std;
 
 
//1.current_exception捕获当前异常到exception_ptr中
void handle_exception(std::exception_ptr eptr) //按值传递
{
    try {
        if(eptr)
            std::rethrow_exception(eptr);
    } catch (const std::exception &e) {
        std::cout << "Caught exception=====" << e.what() << endl;
    }
}
 
int main()
{
    std::exception_ptr eptr;
    try {
        std::string().at(1);//知成一个std::out_of_range
    } catch (...) {
        //捕获当前异常
        eptr = std::current_exception();
    }
 
    handle_exception(eptr);
    cout << "Hello World!" << endl;
    return 0;
}

运行结果：

 3.make_exception_ptr 创建一个异常对象

#include 
#include 
#include 
 
using namespace std;
 
 
int main()
{
    auto p = std::make_exception_ptr(std::logic_error("logic_error"));
    try {
        std::rethrow_exception(p);
    } catch (const std::exception& e) {
        std::cout << "exception caught:" << e.what() << endl;
    }
    cout << "Hello World!" << endl;
    return 0;
}

运行结果：

4.rethrow_exception 从一个 std::exception_ptr 抛出异常

#include 
#include 
#include 
#include 
 
using namespace std;
 
 
int main()
{
    std::exception_ptr p;
    try {
        std::vector<int> myvector(10);
        myvector.at(20) = 100; //vector::at throws on out-of-range
    } catch (const std::exception& e) { //捕获异常
        cout << "e.what ======== " << e.what() << endl;
        p = current_exception();
        cout << "exception caught, but continuing...  typeid(p).name ====" << typeid(p).name() << endl;
    }
    cout << "(after exception)" << endl;
    try {
        std::rethrow_exception(p);
    } catch (const std::exception& e) {
        std::cout << "exception caught: " << e.what() << endl;
    }
    cout << "Hello World!" << endl;
 
    return 0;
}

运行结果：

5.terminate 异常终止

#include 
#include 
#include 
#include 
 
using namespace std;
 
int main()
{
    char *p;
    cout << "Attempting to allocate 2GB..." << endl;
    try {
        p = new char[2147483647]; //2147483648====>2GB
    } catch (const exception &e) {
        cout << "excepter=======e.what()======= " << e.what() << endl;
        cout << "Error: could not allocate storage." << endl;
        terminate();
    }
    delete[] p;
    cout << "Hello World!" << endl;
    return 0;
}

 运行结果：

6.设置终止异常函数

#include 
#include 
#include 
#include 
 
using namespace std;
 
 
void myterminate()
{
    std::cerr << "terminate handler called" << endl;
    abort(); //forces abnormal termination
}
 
int main()
{
    std::set_terminate(myterminate);
    throw 0; //unhandled exception: calls terminate handler
    cout << "Hello World!" << endl;
    return 0;
}

运行结果：

7.throw_width_nested嵌套异常

#include 
#include 
#include 
#include 
#include 
#include 
 
using namespace std;
 
 
void openFile(const std::string &fileName)
{
    try {
        std::ifstream file(fileName);
        file.exceptions(std::ios_base::failbit);
    } catch (...) {
        std::cout << "openFile=====fileName===" << fileName << endl;
        std::throw_with_nested(std::runtime_error("Couldn't open " + fileName));
    }
}
 
//捕捉异常并将其包装于nested_exception
void throwWithNested()
{
    try {
        openFile("nonexistent.txt");
    } catch (const exception &e) {
        cout << "throwWithNested()====e.what===" << e.what() << endl;
    }
}
 
int main()
{
    try {
        throwWithNested();
    } catch (const std::exception &e) {
        cout << "exception =====e.what====== " << e.what() << endl;
    }
    cout << "Hello World!" << endl;
    return 0;
}

运行结果：

 

8.rethrow_if_nested如果是嵌套抛出异常

#include 
#include 
#include 
#include 
#include 
#include 
 
using namespace std;
 
 
// 打印异常的解释性字符串。若异常内嵌，则递归打印其保有的异常的解释
void printException(const std::exception &e, int level = 0)
{
    cout << std::string(level, ' ') << "exception: " << e.what() << endl;
    try {
        std::rethrow_if_nested(e);
    } catch (const std::exception &e) {
        printException(e, level + 1);
    }catch(...) {
        cout << "printException=======e.what== " << e.what() << " level = " << level << endl;
    }
}
 
 
void openFile(const std::string &fileName)
{
    try {
        std::ifstream file(fileName);
        file.exceptions(std::ios_base::failbit);
    } catch (...) {
        std::cout << "openFile=====fileName===" << fileName << endl;
        std::throw_with_nested(std::runtime_error("Couldn't open " + fileName));
    }
}
 
//捕捉异常并将其包装于nested_exception
void throwWithNested()
{
    try {
        openFile("nonexistent.txt");
    } catch (const exception &e) {
        cout << "throwWithNested()====e.what===" << e.what() << endl;
        std::throw_with_nested(e);
    }
}
 
int main()
{
    
    try {
        throwWithNested();
    } catch (const std::exception &e) {
        printException(e);
    }
    cout << "Hello World!" << endl;
    return 0;
}

运行结果：

 

参考：

- C++ Reference

标准库头文件 - cppreference.com