Problems understanding base and derived

You're on the wrong track. Read this:

http://en.wikibooks.org/wiki/C%2B%2B_Programming/Classes/Inheritance

Thanks man, i'm on it

So i made this new version that actually works but this got me thinking isn't there any other way to use abstract class constructor in derived class without doing this

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D_public(int num) : Base(num){}; 
D_public() : Base(){};

Its almost like writing constructor from scratch and if i want to check if the value given is good and throw exception if its not i bet i have to type that in { . . . } as well so my D_public class constructor would look like this

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D_public(int num) : Base(num){
    if(num <= 0) throw //something
};

In my example i couldn't even use default constructor from base class that is public.
I had to write D_public() : Base(){};

Isn't there any way to write constructor in base class and just forget about it in derived classes?

1 more question - Why would we define private constructor and how could we actually access it from derived class to create object?

#include <iostream>
using namespace std;


class Base{
public:
	int get_x(){ return x; }
	Base(){ x = 20; }		        // public default constructor
protected:
	Base(int num) : x{ num }{}		// protected construcor
	virtual void set_x(int num){ x = num; }
private:
	int x;
};

class D_public : public Base{
public:
	D_public(int num) : Base(num){};    // Can i use Base(int) without this????
	D_public() : Base(){};		    // Can i use Base() without this????

};



int main(){
	D_public pub{ 5 };
	D_public pub2;
	cout << "pub.x = " << pub.get_x() << endl;
	cout << "pub2.x = " << pub2.get_x() << endl;

	system("pause");
}

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20

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Can i use Base(int) without this???

What do you mean? If a default constructor (without parameter) is available you don't need to call it explicitly. You can always call something like set_x(...) later. But in this case it makes sense.

Can i use Base() without this????

You can write it like so:
D_public() {} // Note: the explicit call of the default constructor is not necessary
Actually a default constructor that does nothing don't need to appear at all.

1 more question - Why would we define private constructor and how could we actually access it from derived class to create object?

You cannot access a private constructor. That's the trick. E.g.: If want to disallow copying an object you make the copy constructor (and operator=(...)) private. In that case pub = pub2; would cause a compiler error.

It's generally not a good idea to throw in a constructor (due to initialization that might or might not happen. For instance in the derived class). Rather process the error handling later.

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I'm sorry i didn't put my thoughts out correct, what i mean with

Can i use Base(int) without this???

and

Can i use Base() without this????

is this.
If i have these 2 constructors in my base class
public : Base(){ x = 20; }
protected : Base(int num) : x{ num }{}
Can i somehow use them in my derived class D_public so i could use my derived class like this
D_public pub; or like this
D_public pub2{5};
and get x to be initialized to 20 in pub and 5 in pub2 without need to include these 2 lines in my class D_public : public Base

line 1 : D_public(int num) : Base(num){};
line 2 : D_public() : Base(){};

You cannot access a private constructor. That's the trick

With constructor delegation, it can also be used as common part of initialization not intended to be used outside of class.

It's generally not a good idea to throw in a constructor (due to initialization that might or might not happen. For instance in the derived class).

I disagree. If class cannot establish its invariants in constructor it should throw. All initializated members/base classes will be automatically destroyed, memory allocated — freed, and program will behave as object was never created in first place.

A default constructor is only generated when no other constructors exists. So yes, you need those two line, but you may simplify line 2: D_public() {}

Why is it such a problem that these lines exists?

You can also inherit costructors from base class:

#include <iostream>

struct B
{
	B() = default;
	B(int) { std::cout << "Parametrized B constructor\n"; }
};

struct D : B
{
	using B::B;
};

int main()
{
    D a;
    D b(34);
}

MiiNiPaa wrote:
You can also inherit costructors from base class:

Is this allowed in C++14 or so? The C++ shell doesn't allow this.

It is C++11 feature: http://coliru.stacked-crooked.com/a/39039b399c950b1b
http://en.wikipedia.org/wiki/C%2B%2B11#Object_construction_improvement

I can use using B::B; only if constructor in B is public right?
If its protected i have to use D(int num) : B(num){}; right?

Yes, inheriting constructors inherits access specifiers as well, so you need to manually call protected constructor from public one. Also if you explicitely define any constructor in derived class, default constructor will not be inherited.

#include <iostream>

struct B
{
	int x = 0;
	B() = default;
	B(int num) : x{num} {}
};

struct D : B
{
	using B::B;
};

int main()
{
	D a;
	D b(34);              //error is coz of this line
	std::cout << a.x << std::endl;
	system("pause");
}

When i try to compile this im getting error

Error 1 error C2664: 'D::D(const D &)' : cannot convert argument 1 from 'int' to 'const D &'
2 IntelliSense: no instance of constructor "D::D" matches the argument list
argument types are: (int)

Why is this happening?
I'm sorry for so many questions

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It should work:
http://coliru.stacked-crooked.com/a/d066ec4d78bcc2c1
http://ideone.com/Je6yR9
http://melpon.org/wandbox/permlink/onystSDyvOagg5c5

It might be problem with Visual Studio: inheriting costructors had become avaliable only starting from VS2015: http://blogs.msdn.com/b/vcblog/archive/2014/11/17/c-11-14-17-features-in-vs-2015-preview.aspx

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