Introduce boost::exception to me

I am asked to create "customizable exception framework" using boost::exception. Till date I used only simple exceptions defined by me. So std::exception,boost::exception are new to me. The code is below.

#include <iterator>
#include<string>
#include <algorithm>
#include<errno.h>

struct My_exception:public virtual boost::exception
{
};

int main()
{
std::string fileName="tmp.txt";
std::string mode="r";

    try
    {
        if(fopen(fileName.c_str(),mode.c_str()))
            std::cout << " file opened " << std::endl ;
        else
        {
            My_exception e;
            e << boost::errinfo_api_function("fopen") <<   boost::errinfo_file_name(fileName)
            << boost::errinfo_file_open_mode(mode) << boost::errinfo_errno(errno);

       开发者_运维知识库     throw e;
        }
    }
    catch(My_exception e)
    {
    // extract the  details here //
    }
    return 1;
}

Now, I want to know that how to extract the data from that caught exception. Can anybody guide me in the path of boost::exception

First of all, your code has error, for example you cannot write this:

e << boost::errinfo_api_function("fopen")

Because errinfo_api_function can be used with int only. So do something like this:

  e << boost::errinfo_api_function(100) //say 100 is error code for api error

See the second type parameter to errinfo_api_function¹, it's int. Similarly, check other error class templates. I've given the link to each of them you're using, at the end of this post!

_{1. It seems there're two version of this class template, one which takes int, other which takes const char*. Compare version 1.40.0 errinfo_api_function with version 1.45.0 errinfo_api_function. Thanks to dalle who pointed it out in the comment. :-)}

Use get_error_info function template to get data from boost::exception.

See what boost::exception documentation says,

To retrieve data from a boost::exception object, use the get_error_info function template.

Sample code:

//since second type of errinfo_file_name is std::string
std::string fileError = get_error_info<errinfo_file_name>(e); 

//since second type of errinfo_errno is int
int errno = get_error_info<errinfo_errno>(e);

//since second type of errinfo_file_open_mode is std::string
std::string mode = get_error_info<errinfo_file_open_mode>(e);

//since second type of errinfo_api_function is int
int apiError = get_error_info<errinfo_api_function>(e);

See these for better understanding:

• errinfo_file_name
• errinfo_errno
• errinfo_file_open_mode
• errinfo_api_function