Diagnostics

Overview

In C/C+++, error reporting is done in different mechanisms. C Runtime functions return error numbers where as C++ throws exceptions. The following looks into this in detail.

Details

POSIX CRT functions such as log() sets error value in a global integer variable, errno. In addition in Windows environment, the error values set by Win32 APIs are retrieved from GetLastError().

The common convention is 0 means success and non zero indicate failures.  

In C++, exceptions could be thrown in a try block and caught in catch blocks. Any value such as int or a user defined object can be thrown from the try block which will be caught by corresponding catch block as shown in the example.

void printerror(int result)
{
    try
    {
        if (result == 1)    
            throw 10;
        else if (result == 2)
            throw "test";
        else if (result == 3)
            throw 2.0f;
        else if (result >= 4)
            throw 'a';
    }
    catch(const int& result)
    {
         cout << "int thrown: " << result << endl;
    }
    catch (const char*& result)
    {
        cout << "string thrown: " << result << endl;
    }
    catch (const float& result)
    {
        cout << "float thrown: " << result << endl;
    }
    catch (...)
    {
        cout << "something else thrown" << endl;
    }
}

    //prints:int thrown: 10
    printerror(1);
    //prints:string thrown: test
    printerror(2);
    //prints:float thrown: 2
    printerror(3);
    //prints:something else thrown
    printerror(4);

exception class

In C++ 11, a new family of classes were introduced to handle exceptions with exception class as its root.  The class exception is defined as below.

class exception 
{
public:
  exception () noexcept;
  exception (const exception&) noexcept;
  exception& operator= (const exception&) noexcept;
  virtual ~exception();
  virtual const char* what() const noexcept;
}

The derived classes overrides what() to return error string as seen in this example 2. 

struct ooops : exception 
{
  const char* what() const noexcept {return "unexpected error!";}
};

    /*prints
        unexpected error!
    */
    try 
    {
        //throws ooops
        throw ooops();
    } 
    catch (exception& ex) 
    {
        cerr << ex.what() << endl;
    }

    /*prints
        std::exception
    */
    try 
    {
        //throws exception
        throw static_cast<exception>(ooops());
    } 
    catch (exception& ex) 
    {
        cerr << ex.what() << endl;
    }

In addition, exceptions future_error and system_error return additional error information in error_code.

Standard Library Exception hierarchy

The following shows the hierarchy of exceptions. Some of these are thrown by the standard library. It can be grouped into three categories. User applications can also throw them.

Standard Library Exception Categories➹

This discusses various standard library exceptions with examples.

Advanced Exception Handling➹
The following discusses how to handle an exception, nested exceptions, uncaught and unexpected exceptions, termination. Also setting exception and termination handlers.

Handling Exception Failures➹
When an exception is unhandled, it leads to termination of the application. This discusses setting up custom handlers that's called before before termination.

System Errors➹
System error numbers are different in different platforms. e.g., file system errors. In C++11, a new exception class system_error along with helper classes - error_category, error_code, error_condition
were introduced to the diagnostic infrastructure to bring uniformity and develop cross platform applications.

Predefined System error enumerations➹
The standard library provides predefined enumeration classes to define error conditions for these categories: generic_category, iostream_category and future_categoty.

Summary of Examples

Name	Category	Description	Github	Wandbox
Example	C++ Exception	generic exception handler	source    output	source + output
Example 2	exception class	std. library exception handler	source    output	source + output
Example 3	exception_ptr	Usage	source    output	source + output
Example 4	exception_ptr	make_exception	source    output	source + output
Example 5	nested_exception	explicit	source    output	source + output
Example 6	nested_exception	implicit	source    output	source + output
Example 7	terminate()	Usage	source    output	source + output
Example 8	uncaught_exception()	Usage	source    output	source + output
Example 9	unexpected()	Usage	source    output	source + output
Example 10	system_error	using error_code and error_condition (POSIX)	source    output	source + output
Example 11	system_error	using error_code and error_condition (Windows)	source    output	source + output
Example 12	system_error	future_category	source    output	source + output
Example 13	system_error	iostream_category	source    output	source + output
Example 14	system_error	generic_category	source    output	source + output
Example 15	system_error	custom error_category	source    output	source + output
Example 16	errc	Usage	source    output	source + output
Example 17	future_errc	Usage	source    output	source + output
Example 18	io_errc	Usage	source    output	source + output

Included in Header file: <exception>   <future>  <iostream> <system_error>