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Ali Boulhajat edited this page Nov 22, 2022 · 1 revision

Table of contents

Nested classes

Example

class Cat
{
public:
    class Leg
    {
          //[...]
    };
};

int main()
{
	Cat somecat; // instantiate the Cat class
	Cat::Leg somecatsleg; // instantiate the Leg class
}

Exceptions

  • Exceptions are an additional approach to signal errors or unexpected conditions in a program.

  • Handling errors. The quality of the error-handling code determines how robust a program is and it is usually a major factor in making a program user-friendly.

    • Not all errors are equal though the nature of the error determines how best to deal with it.
    • Exceptions are to deal with error conditions that you don't expect to occur in the normal course of events.
    • A primary advantage of using exceptions to signal these errors is that the error-handling code is separated completely from the code that caused the error

  • Exceptions need to be used exceptionally. If you expect the function to fail often, you don't want to use exception, as calling exception is more resource consuming than returning an error value.

  • With catch, you can catch a specific type of error messages. To catch any type of error message, you can use catch(...) to catch.

  • If a statement that is not within a try block throws an exception, or a statement within a try block throws an exception that is not caught, the program terminates.

  • A try block can be followed by several catch blocks, each of which handles an exception of a different type. The code in a catch block only executes when an exception of a matching type is thrown.

  • Throwing an exception leaves the try block immediately, so at that point all the automatic objects that have been defined within the try block prior to the exception being thrown are destroyed.

  • An exception object must be of a type that can be copied. An object of a class type that has a private copy constructor can't be used as an exception.

  • None of the automatic objects created in the try block exists by the time the handler code is executed is very important. It implies that you must not throw an exception object that's a pointer to an object that is local to the try block. It's also why the exception object is copied in the throw process. You can throw objects that are local to the try block, but not pointers to local objects.

  • Catch exception by reference:

Example

#include <stdexcept>

void test1()
{
	try
    {
        // Do some stuff here
        if (/*there is an error*/)
        {
            throw std::exception();
        }
        else
        {
        	// Do some more stuff
        }
    }
	catch (std::exception e)
	{
	    // handle the error here
    }
}

void test2()
{
    // Do some stuff here
    if (/*there is an error*/)
    {
        throw std::exception();
    }
    else
    {
    	// Do some other stuff
    }
}

void test3()
{
    try
    {
    	test2();
    }
    catch (std::exception& e) // catch exception by reference
    {
    	// handle error
    }
}

void test4()
{
    class PEBKACException: public std::exception
    {
    	public:
    	    virtual const char* what() const throw() // understand better
            {
                return ("Problem exists between keyboard and chair");
            }
    };
    try
    {
    	test3();
    }
    catch (PEBKACException& e) // specific catch
    {
    	// handle the fact that the user did something stupid
    }
    catch (std::exception& e) // generic catch
    {
    	// handle other exceptions that are like std::exception
    }
}

Code that causes an exeception to be thrown

  • The try blocks enclose code that may throw an exception. However, this doesn't mean that the code that throws an exception must be physically between the braces bounding the try block.

  • It only needs to be logically within the try block. If a function is called within a try block, any exception that is thrown and not caught within that function can be caught by one of the catch blocks for the try block.

Nested try blocks

  • You can nest a try block inside another try block. Each try block has its own set of catch blocks to handle exceptions that may be thrown within it, and the catch blocks for a try block are only invoked for exceptions thrown within that try block.

nested_try_blocks

Add custom exception class

Example

class myexception: public std::exception
{
public:
  virtual const char* what() const throw()
  {
    return "My exception happened";
  }
};
  • virtual: It adds nothing, as the method being overridden is already virtual. It can be omitted.
  • const char* what(): A member function named what() that takes no arguments and returns a pointer to const char
  • const: The member function can be called via a const pointer or reference to an instance of this class or a derived class
  • throw(): throws no exceptions

resources

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