system_error错误处理库学习

在程序开发时，我们有时遇到程序出错了，但不知道具体的原因，莫名其妙的就崩溃了，其实标准库提供了一个系统错误的库，我们可以使用这个system_error库来了解错误的提示。它不仅提供错误码code()，还提供错误类别category()，错误信息message()等相关接口。

成员函数

(构造函数)	构造一个 error_code (公开成员函数)
operator=	赋值为另一 error_code (公开成员函数)
assign	赋值为另一 error_code (公开成员函数)
修改器
clear	设 error_code 为 system_category 中的值 0 (公开成员函数)
观察器
value	获得 error_code 的值 (公开成员函数)
category	获得此 error_code 的 error_category (公开成员函数)
default_error_condition	获得此 error_code 的 error_condition (公开成员函数)
message	获得此 error_code 的解释性字符串 (公开成员函数)
operator bool	检查值是否非零 (公开成员函数)

下面是具体的示例：

//error_code observers: value, category and message
#include <iostream>         //std::cout, std::ios
#include <system_error>     //std::system_error
#include <fstream>          //std::ifstream
#include <string>           //std::string


using std::cout;
using std::ios;
using std::endl;
using std::system_error;
using std::ifstream;
using std::string;

int main()
{
    std::error_code code;
    cout << "Code: " << code.value() << endl;
    cout << "Category: " << code.category().name() << endl;
    cout << "Message: " << code.message() << endl;
    cout << "default_error_condition: " << code.default_error_condition().message() << endl;
    cout << "bool: " << code.operator bool() << endl;

    cout << "Hello World!" << endl;
    return 0;
}

运行结果：

 上面是创建一个error_code 对象，程序正常运行，根据上面的结果可以看没程序正常没有错误。

下面我们来看一个异常的程序。

//error_code observers: value, category and message
#include <iostream>         //std::cout, std::ios
#include <system_error>     //std::system_error
#include <fstream>          //std::ifstream
#include <string>           //std::string

using std::cout;
using std::ios;
using std::endl;
using std::system_error;
using std::ifstream;
using std::string;

int main()
{
    ifstream is;
    is.exceptions(std::ios::failbit);
    try {
        is.open("unexistent.txt");
    } catch (const std::system_error &e) {
        cout << "Exception caught (system_error):\n" << endl;
        cout << "Error: " << e.what() << endl;
        cout << "Code: " << e.code().value() << endl;
        cout << "Category: " << e.code().category().name() << endl;
        cout << "Message: " << e.code().message() << endl;
        cout << "default_error_condition: " << e.code().default_error_condition().message() << endl;
        cout << "bool: " << e.code().operator bool() << endl;
        cout << endl;

    }

    cout << "Hello World!" << endl;
    return 0;
}

运行结果：

 打开一个不存在的文件，出现了异常，从上面的异常提示我们知道这是输入输出流错误。

接着，我们把这个错误对象赋给第一个程序创建的一个空的错误类对象。查看他的信息。

//error_code observers: value, category and message
#include <iostream>         //std::cout, std::ios
#include <system_error>     //std::system_error
#include <fstream>          //std::ifstream
#include <string>           //std::string

using std::cout;
using std::ios;
using std::endl;
using std::system_error;
using std::ifstream;
using std::string;

int main()
{

    std::error_code code;
    cout << "Code: " << code.value() << endl;
    cout << "Category: " << code.category().name() << endl;
    cout << "Message: " << code.message() << endl;
    cout << "default_error_condition: " << code.default_error_condition().message() << endl;
    cout << "bool: " << code.operator bool() << endl;
    cout << endl;

    ifstream is;
    is.exceptions(std::ios::failbit);
    try {
        is.open("unexistent.txt");
    } catch (const std::system_error &e) {
        cout << "Exception caught (system_error):\n" << endl;
        cout << "Error: " << e.what() << endl;
        cout << "Code: " << e.code().value() << endl;
        cout << "Category: " << e.code().category().name() << endl;
        cout << "Message: " << e.code().message() << endl;
        cout << "default_error_condition: " << e.code().default_error_condition().message() << endl;
        cout << "bool: " << e.code().operator bool() << endl;
        cout << endl;

        // operator= 操作
        code = e.code();
        cout << "Code: " << code.value() << endl;
        cout << "Category: " << code.category().name() << endl;
        cout << "Message: " << code.message() << endl;
        cout << "default_error_condition: " << code.default_error_condition().message() << endl;
        cout << "bool: " << code.operator bool() << endl;
        cout << endl;
    }

    cout << "Hello World!" << endl;
    return 0;
}

运行结果：

 e和code两个错误对象的信息相同，接下来再调用clear函数，把信息内容清空

//error_code observers: value, category and message
#include <iostream>         //std::cout, std::ios
#include <system_error>     //std::system_error
#include <fstream>          //std::ifstream
#include <string>           //std::string


using std::cout;
using std::ios;
using std::endl;
using std::system_error;
using std::ifstream;
using std::string;

int main()
{

    std::error_code code;
    cout << "Code: " << code.value() << endl;
    cout << "Category: " << code.category().name() << endl;
    cout << "Message: " << code.message() << endl;
    cout << "default_error_condition: " << code.default_error_condition().message() << endl;
    cout << "bool: " << code.operator bool() << endl;
    cout << endl;

    ifstream is;
    is.exceptions(std::ios::failbit);
    try {
        is.open("unexistent.txt");
    } catch (const std::system_error &e) {
        cout << "Exception caught (system_error):\n" << endl;
        cout << "Error: " << e.what() << endl;
        cout << "Code: " << e.code().value() << endl;
        cout << "Category: " << e.code().category().name() << endl;
        cout << "Message: " << e.code().message() << endl;
        cout << "default_error_condition: " << e.code().default_error_condition().message() << endl;
        cout << "bool: " << e.code().operator bool() << endl;
        cout << endl;

        // operator= 操作
        code = e.code();
        cout << "Code: " << code.value() << endl;
        cout << "Category: " << code.category().name() << endl;
        cout << "Message: " << code.message() << endl;
        cout << "default_error_condition: " << code.default_error_condition().message() << endl;
        cout << "bool: " << code.operator bool() << endl;
        cout << endl;

        //调用clear()
        code.clear();
        cout << "Code: " << code.value() << endl;
        cout << "Category: " << code.category().name() << endl;
        cout << "Message: " << code.message() << endl;
        cout << "default_error_condition: " << code.default_error_condition().message() << endl;
        cout << "bool: " << code.operator bool() << endl;
        cout << endl;
    }

    cout << "Hello World!" << endl;
    return 0;
}

运行结果：

 code对象又恢复到初始状态了。

我们也可以对错误码对像进行hash：

//error_code observers: value, category and message
#include <iostream>         //std::cout, std::ios
#include <system_error>     //std::system_error
#include <fstream>          //std::ifstream
#include <string>           //std::string


using std::cout;
using std::ios;
using std::endl;
using std::system_error;
using std::ifstream;
using std::string;


int main()
{
    ifstream is;
    is.exceptions(std::ios::failbit);
    try {
        is.open("unexistent.txt");
    } catch (const std::system_error &e) {
        cout << "Exception caught (system_error):\n" << endl;
        cout << "Error: " << e.what() << endl;
        cout << "Code: " << e.code().value() << endl;
        cout << "Category: " << e.code().category().name() << endl;
        cout << "Message: " << e.code().message() << endl;
        cout << "default_error_condition: " << e.code().default_error_condition().message() << endl;
        cout << "bool: " << e.code().operator bool() << endl;
        cout << endl;

        std::hash<std::error_code> hashErrorCode;
        cout << "hash(error_code): " << hashErrorCode(e.code()) << endl;

    }

    cout << "Hello World!" << endl;
    return 0;
}

运行结果：

 hash这是一个结构体模板，它不仅可以对错误码对象进行hash也可以对数值进行hash，比如int, long long, float, double,string等，下面来看一个示例：

//error_code observers: value, category and message
#include <iostream>         //std::cout, std::ios
#include <system_error>     //std::system_error
#include <fstream>          //std::ifstream
#include <string>           //std::string


using std::cout;
using std::ios;
using std::endl;
using std::system_error;
using std::ifstream;
using std::string;

int main()
{
    ifstream is;
    is.exceptions(std::ios::failbit);
    try {
        is.open("unexistent.txt");
    } catch (const std::system_error &e) {
        cout << "Exception caught (system_error):\n" << endl;
        cout << "Error: " << e.what() << endl;
        cout << "Code: " << e.code().value() << endl;
        cout << "Category: " << e.code().category().name() << endl;
        cout << "Message: " << e.code().message() << endl;
        cout << "default_error_condition: " << e.code().default_error_condition().message() << endl;
        cout << "bool: " << e.code().operator bool() << endl;
        cout << endl;

        std::hash<std::error_code> hashErrorCode;
        cout << "hash(error_code): " << hashErrorCode(e.code()) << endl;

        string str = "hello world!";
        std::hash<string> hashStr;
        cout << "hash(string): " << hashStr(str) << endl;

        int n = 100;
        std::hash<int> hashN;
        cout << "hash(int): " << hashN(n) << endl;

        long long ln = 100;
        std::hash<long long> hashLn;
        cout << "hash(long long): " << hashLn(ln) << endl;

        double dou = 100;
        std::hash<double> hashD;
        cout << "hash(double): " << hashD(dou) << endl;

        float f = 100;
        std::hash<float> hashF;
        cout << "hash(float): " << hashF(f) << endl;
    }

    cout << "Hello World!" << endl;
    return 0;
}

 运行结果：

参考：

<system_error> - C++ Reference

标准库头文件 <system_error> - cppreference.com