.dp-highlighter { font-family: "Consolas", "Monaco", "Courier New", Courier, monospace; font-size: 12px; background-color: #E7E5DC; width: 99%; overflow: auto; margin: 18px 0 18px 0 !important; padding-top: 1px; /* adds a little border on top when controls are hidden */ } /* clear styles */ .dp-highlighter ol, .dp-highlighter ol li, .dp-highlighter ol li span { margin: 0; padding: 0; border: none; } .dp-highlighter a, .dp-highlighter a:hover { background: none; border: none; padding: 0; margin: 0; } .dp-highlighter .bar { padding-left: 45px; } .dp-highlighter.collapsed .bar, .dp-highlighter.nogutter .bar { padding-left: 0px; } .dp-highlighter ol { list-style: decimal; /* for ie */ background-color: #fff; margin: 0px 0px 1px 45px !important; /* 1px bottom margin seems to fix occasional Firefox scrolling */ padding: 0px; color: #5C5C5C; } .dp-highlighter.nogutter ol, .dp-highlighter.nogutter ol li { list-style: none !important; margin-left: 0px !important; } .dp-highlighter ol li, .dp-highlighter .columns div { list-style: decimal-leading-zero; /* better look for others, override cascade from OL */ list-style-position: outside !important; border-left: 3px solid #6CE26C; background-color: #F8F8F8; color: #5C5C5C; padding: 0 3px 0 10px !important; margin: 0 !important; line-height: 14px; } .dp-highlighter.nogutter ol li, .dp-highlighter.nogutter .columns div { border: 0; } .dp-highlighter .columns { background-color: #F8F8F8; color: gray; overflow: hidden; width: 100%; } .dp-highlighter .columns div { padding-bottom: 5px; } .dp-highlighter ol li.alt { background-color: #FFF; color: inherit; } .dp-highlighter ol li span { color: black; background-color: inherit; } /* Adjust some properties when collapsed */ .dp-highlighter.collapsed ol { margin: 0px; } .dp-highlighter.collapsed ol li { display: none; } /* Additional modifications when in print-view */ .dp-highlighter.printing { border: none; } .dp-highlighter.printing .tools { display: none !important; } .dp-highlighter.printing li { display: list-item !important; } /* Styles for the tools */ .dp-highlighter .tools { padding: 3px 8px 3px 10px; font: 9px Verdana, Geneva, Arial, Helvetica, sans-serif; color: silver; background-color: #f8f8f8; padding-bottom: 10px; border-left: 3px solid #6CE26C; } .dp-highlighter.nogutter .tools { border-left: 0; } .dp-highlighter.collapsed .tools { border-bottom: 0; } .dp-highlighter .tools a { font-size: 9px; color: #a0a0a0; background-color: inherit; text-decoration: none; margin-right: 10px; } .dp-highlighter .tools a:hover { color: red; background-color: inherit; text-decoration: underline; } /* About dialog styles */ .dp-about { background-color: #fff; color: #333; margin: 0px; padding: 0px; } .dp-about table { width: 100%; height: 100%; font-size: 11px; font-family: Tahoma, Verdana, Arial, sans-serif !important; } .dp-about td { padding: 10px; vertical-align: top; } .dp-about .copy { border-bottom: 1px solid #ACA899; height: 95%; } .dp-about .title { color: red; background-color: inherit; font-weight: bold; } .dp-about .para { margin: 0 0 4px 0; } .dp-about .footer { background-color: #ECEADB; color: #333; border-top: 1px solid #fff; text-align: right; } .dp-about .close { font-size: 11px; font-family: Tahoma, Verdana, Arial, sans-serif !important; background-color: #ECEADB; color: #333; width: 60px; height: 22px; } /* Language specific styles */ .dp-highlighter .comment, .dp-highlighter .comments { color: #008200; background-color: inherit; } .dp-highlighter .string { color: blue; background-color: inherit; } .dp-highlighter .keyword { color: #069; font-weight: bold; background-color: inherit; } .dp-highlighter .preprocessor { color: gray; background-color: inherit; }

Thursday, November 27, 2008

Grub error 22 nightmare

So two nights ago, while trying to free up some space on my windows partition, I just decided to remove the linux SUSE partition which I rarely use. So I launched up partition magic, and off I go. I deleted the Linux partition. When I started my machine again, the nightmare started...

Booting grub stage 1.5
Grub error 22

What happened is that grub is looking for linux startup files that are not there anymore. One way to resolve this is to startup your computer with a windows CD, go to the recovery console, and execute fixmbr, which should restore the original MBR, windows only. I have a Toshiba Sattelite A100 laptop which comes only with a recovery CD (no Windows CD), so I had no luck with that.

I had valuable data on my hard-drive and simply wiping it out was not an option. So I did some research online and finally resolved my problem using the following:

1- Install the Ultimate Boot CD (free from www.ultimatebootcd.com). Get the self-extracting EXE as it is smaller and installs faster
2- Extract the ultimate boot cd into an ISO image and use Nero or some other CD writing software to burn it onto a blank CD
3- Insert this blank CD into your computer and startup.
4- Click F-12 as your machine is starting up (or the equivalent on your machine) to go to the Boot Menue. Choose to boot from CD/DVD
5- Once in the command list of the UltimateBootCD (UBCD), choose filesystem tools.
6- Go down to the MBRtool and select it. You should see a screen like the following:


7- Choose Option 4: Work with a MBR ...
8- From the next screen select option 9 (write/refresh boot code). It will ask you what Hard disk you want to select, choose 0 (or whichever is right for you). It will ask you whether you want to work with it on a file/sector or original, select 0 for original. Or simply execute this command /RBC /DSK:0
9- This usually just takes a second. Once this is done, reboot your machine again and it should boot normally this time.

Tuesday, November 11, 2008

boost::any (2) - storing pointers in boost::any

We know that we can retrieve the value stored in a boost::any using one of the two overloaded boost::any_cast by either passing our any by reference or as a pointer. What if we store a pointer in boost::any?

The following example assumes the declaration from the previous boost::any(1) post:

int main()
{
typedef std::vector<boost::any> MixedContainer_t;
MixedContainer_t mixedContainer;

A* aPtr1 = new A(2);
A* aPtr2 = new A(3);

mixedContainer.push_back(aPtr1);
mixedContainer.push_back(aPtr2);

//would not work. What is stored at front() is a pointer to A
//and not an object of type A.
if(A* a = boost::any_cast<A> (&mixedContainer.front()))
a->print();

//we can even confirm it by testing the types
if(typeid(A*) == mixedContainer.back().type())
{
std::cout << "object at front() is a pointer to A" << std::endl;
}
//to get a pointer to it, we need to retrieve a A**, and pass
//the type of object we think stored at front as A*
if(A** a = boost::any_cast<A*> (&mixedContainer.front()))
{
std::cout << "pointer-> ";
(*a)->print();
}

}


Storing pointers in boost::any is problematic, because boost::any stores only a value of the pointer, and when the boost::any is destroyed, the only memory that is released is that occupied by the pointer, and not the memory it points to. That is, delete or delete[] is not called. What to do? Use smart pointers. As with standard containers, avoid using auto_ptr because of its 'weird' copy semantics.

#include <iostream>
#include <vector>
#include <boost/any.hpp>
#include <boost/shared_ptr.hpp>


class A
{
int value_;
public:
A(int value) : value_(value) {}
void print() const {std::cout << "A's value: " << value_ << std::endl;}
~A() {std::cout << "A(" << value_ <<") destructor" << std::endl; }
};

class B
{
int value_;
public:
B(int value) : value_(value) {}
void print() const {std::cout << "B's value: " << value_ << std::endl;}
~B() {std::cout << "B(" << value_ <<") destructor" << std::endl; }
};

int main()
{
typedef std::vector<boost::any> MixedContainer_t;
MixedContainer_t mixedContainer;

boost::shared_ptr<A> aPtr0(new A(1));
A* aPtr1 = new A(2);
A* aPtr2 = new A(3);

mixedContainer.push_back(aPtr0);
mixedContainer.push_back(aPtr1);
mixedContainer.push_back(aPtr2);

//and to retrieve the value from shared Ptr is simple
try
{
boost::shared_ptr<A> ptr = boost::any_cast<boost::shared_ptr<A> > (mixedContainer.front());
ptr->print();

}
catch(boost::bad_any_cast& bac)
{
bac.what();
}
//only the destructor of the A pointed to by shared_ptr is printed.
}

Sunday, November 9, 2008

boost::any (1)

Sometimes you might want to store unrelated types in the same container, and convey the data from one point to another without caring much about it's type. C++ containers are templated on the type and can only store objects of the same type in the same container. Traditionally, this problem has been solved by storing pointers to objects as void pointers, or using discriminated unions. The problem with these two approaches is that they lose type safety. You can access elements using the wrong type and causes disastrous results at run time with no errors from the compiler.

boost::any solves this problem. Internally boost::any uses a templates to create a wrapper class for the type you pass to it, and then create an instance of this class on the heap. It then manages retrieving the object inside using boost::any_cast safely.

Examples:

#include <boost/any.hpp>
#include <iostream>
#include <string>
#include <vector>

class A
{
int value_;
public:
A(int value) : value_(value) {}
void print() const {std::cout << "A's print: " << value_ << std::endl; }
};

class B
{
public:
void print() const {std::cout << "B's print" << std::endl; }
};

class C
{
public:
void print() const {std::cout << "C's print" << std::endl; }
};

int main()
{
typedef std::vector<boost::any> MixedContainer_t;
MixedContainer_t mixedContainer;

mixedContainer.push_back(A(1));

try
{
A a = boost::any_cast<A> (mixedContainer.back());
a.print();
}
catch(boost::bad_any_cast& bac)
{
bac.what();
}

//you can also retrieve the object via a pointer using the
//the overloaded boost::any_cast
if(A* a = boost::any_cast<A> (&mixedContainer.back()))
{
std::cout << "I can retrieve A by pointer" << std::endl;
}

boost::any a1 = mixedContainer.back();
if(typeid(A*) == a1.type())
std::cout << "last element is of A's type " << std::endl;


mixedContainer.push_back(B());
mixedContainer.push_back(C());

//push a string
std::string str("Hello World!");
mixedContainer.push_back(str);

void print_any(boost::any& element); //declaration
std::for_each(mixedContainer.begin(),
mixedContainer.end(),
print_any);
return 0;
}

void print_any(boost::any& element)
{
try
{
A a = boost::any_cast<A> (element);
a.print();
}
catch(boost::bad_any_cast& ex)
{
std::cout << "bad cast exception" << ex.what();
}
try
{
B b = boost::any_cast<B> (element);
b.print();
}
catch(boost::bad_any_cast& ex)
{
std::cout << "bad cast exception" << ex.what();
}
try
{
C c = boost::any_cast<C> (element);
c.print();
}
catch(boost::bad_any_cast& ex)
{
std::cout << "bad cast exception" << ex.what();
}
}




As you can see in the previous example, we can store any type in boost::any. Boost::any preserves the type and will not let you tamper with it without knowing the correct type. You can store pointers too, but retrieving pointers is a bit tricky and I'll discuss it in a later post. For now, we are interested in the two mechanism for retrieving the values stores in boost::any:

template<typename ValueType>
ValueType any_cast(const any & operand);

The argument is the boost::any we want to retrieve, and the ValueType is the type of the stored value. If the type does not correspond to what is stored in boost::any, then this version of any_cast will throw a bad_any_cast exception.

template<typename ValueType>
ValueType * any_cast(any * operand)


This overloaded any_cast takes a pointer to any, and returns a pointer to the stored value. If the type in the any isn 't Value_Type, a NULL is returned. There is also a version of this any_cast for const pointers.

Note the difference between the first mechanism where an exception is thrown, and the second where a null pointer is returned. Also note that we can use any of the overloaded any_cast exception to retrieve the value by either passing our argument by reference (to use the first mechanism) or value (to use the second) as we show in the preceding example:

    mixedContainer.push_back(A(1));

try
{
A a = boost::any_cast<A> (mixedContainer.back());
a.print();
}
catch(...)
{}

if(A* a = boost::any_cast<A> (&mixedContainer.back()))
{
std::cout << "I can retrieve A by pointer" << std::endl;
}



References:
- Introduction to Boost by Bjorn Karlsson
- the boost website