I have the following structs
struct A
{
int i;
A() { i = 0; }
A(int _i) : i(_i) {}
virtual void f() { cout << i; }
};
struct B1 : virtual A {
B1() : A(1) { f(); }
void f() { cout << i+10; }
};
struct B2 : virtual A {
B2(int i) : A(2) { f(); }
};
struct C : B1, B2 {
C() : B2(3) {}
};
please explain why the following code prints 100: (10+0)
C* c = new C();
I think it should print 1111:
first A() : i = 0
then B1() : i = 1 and prints B1::f() - 11
then B2() : prints prints B1::f() - 11
Where I go wrong?
Thanks
Due to the inheritance, the C object "contains" a B1 object and a B2 object. Both the B1 and the B2 object "contain" an A object, but since you have virtual inheritance of A, you only have one A object in each C object, not two. This single A object is initialized using A's default constructor.
Public by default in a struct.
It doesn't print 100, it prints 10 followed by 0.
Struct means that the fields are private by default or public?
Public.
Related
While working with templates I ran into a need to make a base class constructors accessible from inherited classes for object creation to decrease copy/paste operations.
I was thinking to do this through using keyword in same manner with functions case, but that not work.
class A
{
public:
A(int val) {}
};
class B : public A
{
};
class C : public A
{
public:
C(const string &val) {}
};
class D : public A
{
public:
D(const string &val) {}
using A::A; // g++ error: A::A names constructor
};
void main()
{
B b(10); // Ok. (A::A constructor is not overlapped)
C c(10); // error: no matching function to call to 'C::C(int)'
}
So my question: Is there any way to import a base class constructors after new ones in inherited class been declared?
Or there is only one alternative to declare new constructors and call a base ones from initializer list?
Yes, Since C++11:
struct B2 {
B2(int = 13, int = 42);
};
struct D2 : B2 {
using B2::B2;
// The set of inherited constructors is
// 1. B2(const B2&)
// 2. B2(B2&&)
// 3. B2(int = 13, int = 42)
// 4. B2(int = 13)
// 5. B2()
// D2 has the following constructors:
// 1. D2()
// 2. D2(const D2&)
// 3. D2(D2&&)
// 4. D2(int, int) <- inherited
// 5. D2(int) <- inherited
};
For additional information see http://en.cppreference.com/w/cpp/language/using_declaration
Prefer initialization:
class C : public A
{
public:
C(const string &val) : A(anInt) {}
};
In C++11, you can use inheriting constructors (which has the syntax seen in your example D).
Update: Inheriting Constructors have been available in GCC since version 4.8.
If you don't find initialization appealing (e.g. due to the number of possibilities in your actual case), then you might favor this approach for some TMP constructs:
class A
{
public:
A() {}
virtual ~A() {}
void init(int) { std::cout << "A\n"; }
};
class B : public A
{
public:
B() : A() {}
void init(int) { std::cout << "B\n"; }
};
class C : public A
{
public:
C() : A() {}
void init(int) { std::cout << "C\n"; }
};
class D : public A
{
public:
D() : A() {}
using A::init;
void init(const std::string& s) { std::cout << "D -> " << s << "\n"; }
};
int main()
{
B b; b.init(10);
C c; c.init(10);
D d; d.init(10); d.init("a");
return 0;
}
No, that's not how it is done. Normal way to initialize the base class is in the initialization list :
class A
{
public:
A(int val) {}
};
class B : public A
{
public:
B( int v) : A( v )
{
}
};
void main()
{
B b(10);
}
You'll need to declare constructors in each of the derived classes, and then call the base class constructor from the initializer list:
class D : public A
{
public:
D(const string &val) : A(0) {}
D( int val ) : A( val ) {}
};
D variable1( "Hello" );
D variable2( 10 );
C++11 allows you to use the using A::A syntax you use in your decleration of D, but C++11 features aren't supported by all compilers just now, so best to stick with the older C++ methods until this feature is implemented in all the compilers your code will be used with.
class A
{
public:
A(int val) {}
A(string name) {}
};
class B : public A
{
using A::A;
};
Addition to other answers, in case there are several constuctors with the base class, and you just want to inherit some of them, you can delete the unwanted.
class B : public A
{
using A::A;
B(string) = delete;
};
Here is a good discussion about superclass constructor calling rules. You always want the base class constructor to be called before the derived class constructor in order to form an object properly. Which is why this form is used
B( int v) : A( v )
{
}
When I construct an object D I need to include the constructors for A, B, and C in the initializer list. Is there any way to make it so that I don't need all three in the initializer list or not?
If I try to initialize D using only a constructor for B I get an error because I don't have a default constructor for A or C. If I add a default constructor for A and C I get issues with "i" being reinitialized with no value.
#include <iostream>
using namespace std;
class A
{
int i;
public:
A(int ii) :
i(ii)
{}
~A() { }
int getI() { return i; }
};
class B : public virtual A
{
public:
B(int ii) :
A(ii)
{ }
~B() { }
};
class C : public virtual A
{
public:
C(int ii) :
A(ii)
{ }
~C() { }
};
class D : public B, public C
{
public:
D(int ii) :
A(ii), B(ii), C(ii)
{ }
~D() { }
};
int main()
{
D d(45);
cout << d.getI() << endl;
}
If you add default constructors to A, B, and C, the implmentation of D becomes a bit simpler.
#include <iostream>
using namespace std;
class A
{
int i;
public:
A() : i(0) {}
A(int ii) : i(ii) {}
~A() { }
int getI() { return i; }
};
class B : public virtual A
{
public:
B() { }
B(int ii) : A(ii) { }
~B() { }
};
class C : public virtual A
{
public:
C() { }
C(int ii) : A(ii) { }
~C() { }
};
class D : public B, public C
{
public:
// Relies on default constructors of the other classes.
D() { }
// Relies on the default constructors of B and C.
D(int ii) : A(ii) { }
~D() { }
};
int main()
{
D d1(45);
D d2;
cout << d1.getI() << endl;
cout << d2.getI() << endl;
}
I'm afraid not. With virtual inheritance, your most-derived class must initialise the virtual base.
Read more here.
How about adding a default value to the A, B, C constructors, i.e.
A(int ii=0) :
My question is very similar like other's, but a bit different. I have a sample code:
class B1 {
private :
int * ptr;
public:
B1() : a{ 1 } { ptr = new int{ 2 }; }
B1(const B1& other) : a{ other.a } { ptr = new int{ *other.ptr }; }
~B1() { delete ptr; }
int a;
virtual void smthing() = 0;
};
class D : B1 {
public:
D(int i) : B1{} {}
D(const D& a) : B1{ a } {}
void smthing() { return; };
};
int main() {
D d { 3 };
D dd { d };
return 0;
}
I am using vs2015, and this code is works, but gives me error: object of abstract class type "B1" is not allowed...
If I remove this line D(const D& a) : B1{ a } {}, the base class copy constructor is called and there's no problem, but if I need the derived class copy constructor how can I make this work without error?
Thanks for the answer!
In the next code, building a C class starts with initializing A then B1 and then B2 and at last C class. However, when initializing B1 and B2, the debugger ignored the initialization of A(1) and A(2) (that appear on the intialization list of B1 and B2 in this order) but didn't ignore the initialization of B2(3) at C initialization list.
Why is that?
Thanks in advance.
Here is the code:
struct A {
int i;
A() { i = 0; }
A(int _i) : i(_i) {}
virtual void f() { cout << i; }
};
struct B1 : virtual A {
B1() : A(1) { f(); }
void f() { cout << i+10; }
};
struct B2 : virtual A {
B2(int i) : A(2) { f(); }
};
struct C : B1, B2 {
C() : B2(3) {}
};
int _tmain(int argc, _TCHAR* argv[])
{
C* c = new C();
return 0;
}
For virtual bases, the most derived class has to provide the constructor arguments, e.g.:
struct C
: B1, B2 {
C()
: A(17)
, B2(3) {}
};
If the most derived class does not mention the virtual base in its initializer list, the default constructor of the virtual base is used (if there is none, it is an error).
The reason this is done is to avoid ambiguity of which of the derived classes should provide the constructor arguments for the shared virtual base: the most derived class knows best what is actually needed.
That's exactly how compiler avoids "diamond of death". If A(1) and A(2) would be called, means there was base class duplicate initialization , this is the purpose of virtual inheritance - remove ambiguous common base elements.
Given the following code in C++:
struct A {
A() { f(0); }
A(int i) { f(i); }
virtual void f(int i) { cout << i; }
};
struct B1 : virtual A {
B1(int i) : A(i) { f(i); }
virtual void f(int i) { cout << i+10; }
};
struct B2 : virtual A {
B2(int i) : A(i) { f(i); }
virtual void f(int i) { cout << i+20; }
};
struct C : B1, virtual B2 {
int i;
C() : B1(6),B2(3),A(1){}
virtual void f(int i) { cout << i+30; }
};
Can someone explain why C* c = new C(); will print 1 23 and then 16 in that order? How does it decide which order to print in? I know that the nonvirtual B1 will be called last but why is A() called first? Thanks for the help and explanation ahead of time.
Because your are virtually inheriting B2, the compiler will construct it first as to identify which variables are virtually inherited in C before it constructs any non-virtual inheritance (B1). Of course, A gets constructed first because B2 needs it before it can be constructed.