Thread: vector derived class, help

Hi!

I am going to make a vector derived class

Code:

class flagvec: public vector<bool> {
public:
	//Constructors
	flagvec(unsigned int);
};

It is gonna take the ones and zeroes from the unsigned int and extract them to flags. The last one in the variable will be the last element in the flagvec.

Now I'm not sure how the vector part of my class is going to behave when I use my own constructor. If a constructor that I have declared is called, how is the vector initiated? How long will it be, or isn't it initiated at all?

Here's my constructor:

Code:

flagvec::flagvec(unsigned int a) {
	//Here I want the vector to get a start length at 0, but I'm not sure how to call it's constructor
	while (a) {
		push_back(a & 1);
		a >>= 1;
	}
}

Thanks!

You are aware that there may be issues with deriving from the standard containers? http://cboard.cprogramming.com/showthread.php?t=94187

If you don't call any vector constructor specifically, then the default constructor will be used, which will initialize the vector with a length of zero, so your code should work.

But why are you creating a whole new type just for a different constructor?

It seems like you're trying to do something similar to a bitset: http://www.cppreference.com/cppbitse...structors.html

Originally Posted by dwks

You are aware that there may be issues with deriving from the standard containers?

Is it better to use an internal vector variable instead of having the class derived?

Originally Posted by dwks

If you don't call any vector constructor specifically, then the default constructor will be used, which will initialize the vector with a length of zero, so your code should work.

How do I call a constructor? Calling vector<unsigned int>(...);?

Originally Posted by dwks

But why are you creating a whole new type just for a different constructor?

Well, it was just an example. Actually, my vector derived class is supposed to do something completely else. ^_^

Originally Posted by Daved

>> Is it better to use an internal vector variable instead of having the class derived?
Yes. Making it a member is usually the way to go.

>> How do I call a constructor?

Code:

vector<int> myvec; // no parentheses when using the default constructor.

BTW, if you're actually using vector<bool>, you might consider std::bitset instead, depending on your real requirements.

Thanks!

I'm not sure if that is something you are trying to achieve but here's something I threw together out of boredom. I just used as an unsigned as storage because this is probably most versatile. For example, would bitset or vector<bool> let you test combinations of patterns (any_of, all_of) - without a loop?

Code:

#ifndef BINFLAGS_H
#define BINFLAGS_H

#include <limits>

/*
    BinFlags represents a collection of options together with
    some basic functionality to test and set/reset options.

    Template argument must be an enumeration where the largest value
    does not exceed number of bits in unsigned.
*/
template <typename Enum>
class BinFlags
{
    public:
        BinFlags(): state(0) {}
        void set(Enum n)
        {
            state |= (1 << n);
        }
        void reset(Enum n)
        {
            state &= (std::numeric_limits<unsigned>::max() ^ ( 1 << n));
        }
        void toggle(Enum n)
        {
            state ^= (1 << n);
        }
        bool is_set(Enum n) const
        {
            return state & (1 << n);
        }
        bool any_of(const BinFlags flags) const
        {
            return state & flags.state;
        }
        bool all_of(const BinFlags flags) const
        {
            return state & flags.state == flags.state;
        }
    private:
        unsigned state;
};

#endif

Don't use vector<bool>. It's an ill-conceived abomination. Use std::bitset if you know how many bits you need (the typical case), and Boost.DynamicBitset if you don't.

Another thing, I have made a constructor for the class which translates an integer into the vector. But I also want to be able to set an already existing object of this class to a value by typecasting, do I have to overload the = operator to do the exactly same thing as the constructor? Or can I somehow use the = operator in the constructor to save code?

So,

Code:

//External constructor for flagvec from an unsigned int
flagvec::flagvec(unsigned int a) {
	operator=(a);
}

You se what I mean? Is this right?

>> Don't use vector<bool>

Once again, this is only an example. I should never use vector<bool> for real!

Originally Posted by cpjust

You should avoid using vector<bool> whether it's a base class or not. You can read Iten 18 of "Effective STL" by Scott Meyers for more info, but here are some notable quotes:

So you mean that vector<bool> becomes smaller than expected, almosed as if it would have been compressed? Does it have anything with the definition of bool to do?

Originally Posted by Daved

It stores the bools as bits to save space. Because it does that, it breaks some other rules that work for all other vectors.

If you expect it to work like every other vector (especially if you expect it to be convertible to bool*), then don't use it.

Of course, if you're not counting on the fact that actual bools are stored, then I don't see anything wrong with using it.

Wow, i actually didn't know of that ...

Well, C++09 isn't going to remove vector<bool>. I'm not even sure if they accept the proposals to deprecate it.

I haven't heard of a replacement yet. Consider, however, this: the need for dynamic bitsets is actually quite rare. You can always use dynamic_bitset from Boost, though.