I'm using base-from-member idiom and I now stuck with copy/move constructors for them. Suppose following code:
#include <iostream>
#include <string>
#include <boost/lexical_cast.hpp>
using namespace std;
struct A // base class
{
A(string &s) : s(s), c(0) { };
A(const A &a) : s(a.s), c(a.c) { };
void print() { cout << s << endl; c++; }
int print_count() { return c; }
string &s;
int c;
};
struct B_base // this class will be initialized before A
{
B_base(int i)
{
s = boost::lexical_cast<string>(i);
}
B_base(const B_base &other) : s(other.s) { };
string s;
};
struct B : B_base, A // main class
{
B(int i) : B_base(i), A(B_base::s) { }
B(const B &other) : B_base(other), A(other) { } // <-- problem here
using A::print;
using A::print_count;
};
int main()
{
B b(10);
b.print(); // prints '10'
cout << b.print_count() << endl; // prints '1'
B b1(b);
b1.print(); // prints '10'
A &a = b;
a.s = "FAIL"; // we modify b, not b1 here!
b1.print(); // but b1 prints 'FAIL' here --- error (it should still print '10')
cout << b.print_count() << " " << b1.print_count() << endl; // prints '1 3'
return 0;
}
Problem here is that reference A.s (which is point to B_base::s) is copied from one instance to another, while it should be modified to point to another B_base::s. Things may be even worse, if previous instance is go out of scope ending with dangling references.
My question is: how can I make correct copy of class with base-from-member idiom? (I think move constructor will be similar to copy one, right?)
The base-from-member idiom you are exercising in this case means: you want a class B derived from A with have to be initialized using a member of B (with is: string s).
B b(10);
B b1(b); // B(const B &other) : B_base(other), A(other) { }
// now A::s in b1 is a ref to b.s
A &a = b;// and a.s is a ref to b.s too.
a.s = "FAIL"; // we modify b, and A::s in b1 to!
This problem can be solved making the copy constructor:
B(const B &other) : B_base(other), A(B_base::s) { }
Also, having A::s and B_base::s with the same name, make thing more difficult to understand.
EDIT: As a class designer, you have to decide with is the exact meaning of your copy constructor.
For example, in this case you may want to keep track (with A::c) of the number of printing of each newly create object A. The copy constructor I proposed do it.
But if you want to keep track of all printing of the original string thing are more complex. Just note that if you copy the old A::c to the new A it will be correct initialized, bur not cross actualized when the the printing of the same original string is made using different copies of A. If this is not a problem you can modify the constructors:
A(string &s, int _c=0) : s(s), c(_c) { };
...
B(const B &other) : B_base(other), A(B_base::s, other.c) { }
Related
I would like to have a unique_ptr class member that points to the base class, but later in the constructor through polymorphism can be changed to point to a sister class that also derives from the same base class.
While I don't get any errors in the constructor setting this polymorphism, it does not seem to work correctly, since I get error messages that my polymorphic pointer can't find a member of the sister class to which I thought the pointer was now pointing.
How do I correctly achieve polymorphism here?
class A {
int bar;
};
class B : public A {
int foo;
};
class C: public A {
C();
std::unique_ptr<A> _ptr; // changing to std::unique_ptr<B> _ptr removes the "class A has no member 'foo'" error
};
C::C() : A()
{
_ptr = std::make_unique<B>(); // no errors here
int w = _ptr->foo; // class A has no member 'foo'
}
When you assign
_ptr = std::make_unique<B>();
This works because B is a derived class of A, however _ptr is still a unique_ptr to the base class. You can't change the type of a variable after it's declared.
So what are your options?
Because you know that _ptr stores a pointer to the derived class B, you can do a cast after dereferencing it:
_ptr = std::make_unique<B>();
// derefence the pointer, and cast the reference to `B&`.
B& reference_to_sister = (B&)(*_ptr);
int w = reference_to_sister.foo;
If you take this approach, you'll have to somehow keep track of which derived class is in _ptr, or you'll run the risk of running into bugs.
Alternatively, if you're using C++17, you can use std::variant:
class C : public A {
void initialize(A& a) {
// Do stuff if it's the base class
}
void initialize(B& b) {
// Do different stuff if it's derived
int w = b.foo;
}
C() {
_ptr = std::make_unique<B>(); // This works
// This takes the pointer, and calls 'initialize'
auto initialize_func = [&](auto& ptr) { initialize(*ptr); };
// This will call 'initialize(A&)' if it contains A,
// and it'll call 'initialize(B&)' if it contains B
std::visit(initialize_func, _ptr);
}
std::variant<std::unique_ptr<A>, std::unique_ptr<B>> _ptr;
};
In fact, if you use std::variant this will work even if A and B are completely unrelated classes.
Here's another short variant example
#include <variant>
#include <string>
#include <iostream>
void print(std::string& s) {
std::cout << "String: " << s << '\n';
}
void print(int i) {
std::cout << "Int: " << i << '\n';
}
void print_either(std::variant<std::string, int>& v) {
// This calls `print(std::string&) if v contained a string
// And it calls `print(int)` if v contained an int
std::visit([](auto& val) { print(val); }, v);
}
int main() {
// v is empty right now
std::variant<std::string, int> v;
// Put a string in v:
v = std::string("Hello, world");
print_either(v); //Prints "String: Hello, world"
// Put an int in v:
v = 13;
print_either(v); //Prints "Int: 13"
}
I am trying to understand object oriented programming using c++. The following is a minimal example for which the result is not what I naively expect:
#include <iostream>
class B {
public:
B (int val) : val(val) {;}
int get_val() { return val; }
int set_val(int a) { val = a; }
private:
int val;
};
class A {
public:
A (B b) : b(b) {;}
B get_b() { return b; }
private:
B b;
};
int main(){
B b_main(5);
std::cout << b_main.get_val() << std::endl; // Prints 5, which makes sense
A a_main(b_main);
std::cout << a_main.get_b().get_val() << std::endl; // Prints 5, which makes sense
a_main.get_b().set_val(2);
std::cout << a_main.get_b().get_val() << std::endl; // Why does this not print 2?
return 0;
}
The last cout statement does not make sense to me. In the second to last line, I set the value of the object to be 2, so why does this not print 2? Looking at some similar questions on Stack Exchange, I found some suggestions to make A and B be friends of each other. I tried adding friend class A in class B and friend class B in class A, but this did not work. In my understanding, adding the friend statements should be unnecessary since I have the get_b() method in class A. I found some suggestions to try passing the object of type B in by reference to the constructor of A: A (B& b) : b(b) {;} but this did not work either.
Can anyone explain to me why the program is not producing the intended result and also how to obtain the desired result (that is, the last cout statement prints 2)?
Note: I also experimented with the following. I made the private variable b of class A be public:
#include <iostream>
class B {
public:
B (int val) : val(val) {;}
int get_val() { return val; }
int set_val(int a) { val = a; }
private:
int val;
};
class A {
public:
A (B b) : b(b) {;}
B b; // This is now public
//B get_b() { return b; } // This is no longer needed
private:
};
int main(){
B bmain(5);
std::cout << bmain.get_val() << std::endl;
A amain(bmain);
std::cout << amain.b.get_val() << std::endl;
amain.b.set_val(2);
std::cout << amain.b.get_val() << std::endl; // Works!
return 0;
}
And now I obtain the desired result. Is this how the code should be implemented as opposed to the first code snippet? I would like to have a get_b() method as in the first code snippet, but if this is not the correct way of going about this, please let me know.
In the second to last line, I set the value of the object to be 2, so why does this not print 2?
Because you return a copy of the B object in a_main with the get_b() method. What happens is that the b variable in a_main is copied, i.e. another object of class B, identical to the b member, is created, and returned to the caller. Then, that new B object is modified. But it has no connection to the original b in a_main. This has little to do with visibility and member access.
However, in the second example, you expose the b member in a_main and directly operate on that object without making a copy of it, thus the successful result. What the public modifier changes is that it allows you to access the b object directly, hence the effect.
I found some suggestions to try passing the object of type B in by reference to the constructor of A: A (B& b) : b(b) {;} but this did not work either.
That isn't going to work. What happens when you do so, is that the A::b is initialized using the value that is passed by reference, true. But the reference only leads to no additional copy of b passed to the constructor being made. This reference does not create a link between the b passed to the constructor and A::b. It's on the other end, so to say.
By the way, A (B& b) : b(b) {;} that the c'tor parameter name is identical to the member name is a bad practice. It's a good idea to have them named similarly, but still, add e.g. an underscore: A (B& _b) : b(_b) {;}
If you want to achieve the same result in the first snippet, return a reference to b like so:
B& get_b() { return b; }
Still, this is undesirable, because you expose a private member of class A just to allow clients of A to modify a certain property of that member. Better provide a method in A to set the val property of A::b without giving full access to A::b.
Definitely see this: What's the difference between passing by reference vs. passing by value?
and maybe this: Java and C++ pass by value and pass by reference
because I have a feel that you're coming from Java and expect pass-by-reference in C++ by default.
get_b returns a copy of your private variable b, not the actual variable. If you want to be able to access it, you need to return a reference to b so that the returned value can be manipulated. Your get_b definition should look like this:
B& get_b() { return b; }
if that is what you expect to do. However, this is not usually a desirable solution. If you are going to be actively changing the value of b you should write a set_b function to manipulate the variable. And if you are really working with the variable a lot, reading and writing values to it, you should keep it public for fast access.
Just for the sake of completeness, you can solve this problem as a C programing problem rather than using all the fancy references in C++ programing. When you get b_main from a_main, the returned object does not occupy the same memory address.
#include <iostream>
class B {
public:
B (int val) : val(val) {;}
int get_val() { return val; }
int set_val(int a) { val = a; }
private:
int val;
};
class A {
public:
A (B b) : b(b) {;}
B get_b() { return b; }
private:
B b;
};
int main(){
B b_main(5);
B* addrb = &b_main;
std::cout << b_main.get_val() << std::endl; // Prints 5, which makes sense
std::cout<<"Address of b_main: "<<addrb<<std::endl;
A a_main(b_main);
B bt = a_main.get_b();
addrb = &(bt);
std::cout << a_main.get_b().get_val() << std::endl; // Prints 5, which makes sense
std::cout<<"Address of a_main.get_b(): "<<addrb<<std::endl;
a_main.get_b().set_val(2);
std::cout << a_main.get_b().get_val() << std::endl; // Why does this not print 2?
return 0;
}
Notice the difference in address of the new cout statements. One way to fix this is to return pointers rather than b itself. i.e.
#include <iostream>
class B {
public:
B (int val) : val(val) {;}
int get_val() { return val; }
int set_val(int a) { val = a; }
private:
int val;
};
class A {
public:
A (B b) : b(b) {;}
B* get_b() { return &b; }
private:
B b;
};
int main(){
B b_main(5);
//B* addrb = &b_main;
std::cout << b_main.get_val() << std::endl; // Prints 5, which makes sense
//std::cout<<"Address of b_main: "<<addrb<<std::endl;
A a_main(b_main);
//B bt = a_main.get_b();
//addrb = &(bt);
std::cout << a_main.get_b()->get_val() << std::endl; // Prints 5, which makes sense
//std::cout<<"Address of a_main.get_b(): "<<addrb<<std::endl;
a_main.get_b()->set_val(2);
std::cout << a_main.get_b()->get_val() << std::endl; // Why does this not print 2?
return 0;
}
I stumbled upon something similar today, and subsequently tried a few things out and noticed that the following seems to be legal in G++:
struct A {
int val_;
A() { }
A(int val) : val_(val) { }
const A& operator=(int val) { val_ = val; return *this; }
int get() { return val_; }
};
struct B : public A {
A getA() { return (((A)*this) = 20); } // legal?
};
int main() {
A a = 10;
B b;
A c = b.getA();
}
So B::getB returns a type A, after it as assigned the value 20 to itself (via the overloaded A::operator=).
After a few tests, it seems that it returns the correct value (c.get would return 20 as one may expect).
So I'm wondering, is this undefined behavior? If this is the case, what exactly makes it so? If not, what would be the advantages of such code?
After careful examination, with the help of #Kerrek SB and #Aaron McDaid, the following:
return (((A)*this) = 20);
...is like shorthand (yet obscure) syntax for:
A a(*this);
return a.operator=(20);
...or even better:
return A(*this) = 20;
...and is therefore defined behavior.
There are a number of quite separate things going on here. The code is valid, however you have made an incorrect assumption in your question. You said
"B::getA returns [...] , after it as assigned the value 20 to itself"
(my emphasis) This is not correct. getA does not modify the object. To verify this, you can simply place const in the method signature. I'll then fully explain.
A getA() const {
cout << this << " in getA() now" << endl;
return (((A)*this) = 20);
}
So what is going on here? Looking at my sample code (I've copied my transcript to the end of this answer):
A a = 10;
This declares an A with the constructor. Pretty straightfoward. This next line:
B b; b.val_ = 15;
B doesn't have any constructors, so I have to write directly to its val_ member (inherited from A).
Before we consider the next line, A c = b.getA();, we must very carefully consider the simpler expression:
b.getA();
This does not modify b, although it might superfically look like it does.
At the end, my sample code prints out the b.val_ and you see that it equals 15 still. It has not changed to 20. c.val_ has changed to 20 of course.
Look inside getA and you see (((A)*this) = 20). Let's break this down:
this // a pointer to the the variable 'b' in main(). It's of type B*
*this // a reference to 'b'. Of type B&
(A)*this // this copies into a new object of type A.
It's worth pausing here. If this was (A&)*this, or even *((A*)this), then it would be a simpler line. But it's (A)*this and therefore this creates a new object of type A and copies the relevant slice from b into it.
(Extra: You might ask how it can copy the slice in. We have a B& reference and we wish to create a new A. By default, the compiler creates a copy constructor A :: A (const A&). The compiler can use this because a reference B& can be naturally cast to a const A&.)
In particular this != &((A)*this). This might be a surprise to you. (Extra: On the other hand this == &((A&)*this) usually (depending on whether there are virtual methods))
Now that we have this new object, we can look at
((A)*this) = 20
This puts the number into this new value. This statement does not affect this->val_.
It would be an error to change getA such that it returned A&. First off, the return value of operator= is const A&, and therefore you can't return it as a A&. But even if you had const A& as the return type, this would be a reference to a temporary local variable created inside getA. It is undefined to return such things.
Finally, we can see that c will take this copy that is returned by value from getA
A c = b.getA();
That is why the current code, where getA returns the copy by value, is safe and well-defined.
== The full program ==
#include <iostream>
using namespace std;
struct A {
int val_;
A() { }
A(int val) : val_(val) { }
const A& operator=(int val) {
cout << this << " in operator= now" << endl; // prove the operator= happens on a different object (the copy)
val_ = val;
return *this;
}
int get() { return val_; }
};
struct B : public A {
A getA() const {
cout << this << " in getA() now" << endl; // the address of b
return (((A)*this) = 20);
// The preceding line does four things:
// 1. Take the current object, *this
// 2. Copy a slice of it into a new temporary object of type A
// 3. Assign 20 to this temporary copy
// 4. Return this by value
} // legal? Yes
};
int main() {
A a = 10;
B b; b.val_ = 15;
A c = b.getA();
cout << b.get() << endl; // expect 15
cout << c.get() << endl; // expect 20
B* b2 = &b;
A a2 = *b2;
cout << b2->get() << endl; // expect 15
cout << a2.get() << endl; // expect 15
}
It seems the way to construct objects in C++0x avoiding copies/moves (particularly for large stack allocated objects) is "pass by lambda".
See the following code:
#include <iostream>
#define LAMBDA(x) [&] { return x; }
class A
{
public:
A() {};
A(const A&) { std::cout << "Copy "; }
A(A&&) { std::cout << "Move "; }
};
class B1
{
public:
B1(const A& a_) : a(a_) {}
B1(A&& a_) : a(std::move(a_)) {}
A a;
};
class B2
{
public:
B2(const A& a_) : a(a_) {}
B2(A&& a_) : a(std::move(a_)) {}
template <class LAMBDA_T>
B2(LAMBDA_T&& f, decltype(f())* dummy = 0) : a(f()) {}
A a;
};
int main()
{
A a;
std::cout << "B1 b11( a ): ";
B1 b11(a);
std::cout << std::endl;
std::cout << "B2 b12(LAMBDA(a)): ";
B2 b12(LAMBDA(a));
std::cout << std::endl;
std::cout << std::endl;
std::cout << "B1 b21( std::move(a) ): ";
B1 b21(std::move(a));
std::cout << std::endl;
std::cout << "B2 b22(LAMBDA(std::move(a))): ";
B2 b22(LAMBDA(std::move(a)));
std::cout << std::endl;
std::cout << std::endl;
std::cout << "B1 b31(( A() )): ";
B1 b31((A()));
std::cout << std::endl;
std::cout << "B2 b32((LAMBDA(A()))): ";
B2 b32((LAMBDA(A())));
std::cout << std::endl;
std::cout << std::endl;
}
Which outputs the following:
B1 b11( a ): Copy
B2 b12(LAMBDA(a)): Copy
B1 b21( std::move(a) ): Move
B2 b22(LAMBDA(std::move(a))): Move
B1 b31(( A() )): Move
B2 b32((LAMBDA(A()))):
Note the "pass by lambda" removes the move in the case where the parameter is a what I believe is called a "prvalue".
Note that it seems the "pass by lambda" approach only helps when the parameter is a "prvalue", but it doesn't seem to hurt in other cases.
Is there anyway to get functions to accept "pass by lambda" parameters in C++0x, that is nicer than the client having to wrap their parameters in lambda functions themselves? (other than defining a proxy macro that calls the function).
If you're okay with a templated constructor, you might as well use perfect forwarding instead of the obfuscation with lambdas.
class super_expensive_type {
public:
struct token_t {} static constexpr token = token_t {};
super_expensive_type(token_t);
}
constexpr super_expensive_type::token_t super_expensive_type::token;
class user {
public:
template<typename... Args>
explicit
user(Args&&... args)
: member { std::forward<Args>(args)... }
{}
private:
super_expensive_type member;
};
// ...
// only one construction here
user { super_expensive_type::token };
super_expensive_type moved_from = ...;
// one move
user { std::move(moved_from) };
super_expensive_type copied_from = ...;
// one copy
user { copied_from };
Using lambdas can't be better than this because the result from the expression in the lambda body has to be returned.
There's a fundamental problem with what you're doing. You cannot magic an object into existence. The variable must be:
Default constructed
Copy constructed
Move constructed
Constructed with a different constructor.
4 is off the table, since you only defined the first three. Your copy and move constructors both print things. Therefore, the only conclusion one can draw is that, if nothing is printed, the object is being default constructed. IE: filled with nothing.
In short, your Lambda-based transfer mechanism doesn't seem to be transferring anything at all.
After further analysis, I see what's happening. Your lambda isn't actually taking a value by reference; it's constructing a value. If you expand the macro, what you get is this:
B2 b32(([&] {return A()}));
It constructs a temporary; it doesn't actually take anything by reference. So I'm not sure how you can consider this "passing" anything. All you're doing is making a function that constructs an object. You could just as easily pass the arguments for B2::a's constructor to the constructor of B2 and have it use them to create the object, and it would give you the same effect.
You're not passing a value. You're making a function that will always create the exact same object. That's not very useful.
I have a class Model:
class Model
{
...
boost::shared_ptr<Deck> _deck;
boost::shared_ptr<CardStack> _stack[22];
};
Deck inherits from CardStack.
I tried to make _stack[0] point to the same thing that _deck points to by going:
{
_deck = boost::shared_ptr<Deck>(new Deck());
_stack[0] = _deck;
}
It seems that the assignment to _deck of _stack[0] results in a copy of _deck being made. (I know this because modifications to _stack[0] do not result in modifications to _deck.) How can I get them to point to the same thing?
Ok - no copy constructor is being called. I have verified this by implementing it and seeing if it gets called - it doesn't.
However - I have a function that operates on CardStack objects:
void TransferSingleCard(CardStack & src, CardStack & dst, Face f)
{
if( !src._cards.empty() )
{
src._cards.back().SetFace(f);
dst.PushCard(src._cards.back());
src._cards.pop_back();
}
}
Now - when I call:
{
TransferSingleCard(*_stack[DECK], _someotherplace, FACEDOWN);
std::cout << *_stack[DECK];
std::cout << *_deck;
}
I get this output (where std::cout on a CardStack will print out the size of that stack):
Num(103) TOP
Num(104) TOP
... so I've concluded (incorrectly?) that _stack[DECK] points to something different.
The Deck
class Deck : public CardStack
{
public:
Deck(int numsuits=2, StackIndex index = NO_SUCH_STACK );
Deck::Deck( const Deck & d);
int DealsLeft() const;
void RecalcDealsLeft();
private:
int _dealsleft;
};
Not clear what you are asking about - consider this code:
#include <iostream>
#include "boost/shared_ptr.hpp"
using namespace std;
struct A {
virtual ~A() {
cout << "destroyed" << endl;
}
};
struct B : public A {
};
int main() {
boost::shared_ptr<B> b( new B );
boost::shared_ptr<A> a;
a = b;
}
Only one "destroy" message appears, indicating that no copy has been made.
This example - derives from #Neil's answer, tries to emulate what you say is happening. Could you check that it works as expected (A and B have the same count) on your system.
Then we could try and modify this code or your code until they match.
#include <boost/shared_ptr.hpp>
#include <iostream>
class A {
public:
virtual ~A()
{
std::cerr << "Delete A" << std::endl;
}
int _count;
void decrement()
{
_count --;
}
};
class B : public A {
public:
virtual ~B()
{
std::cerr << "Delete B" << std::endl;
}
};
int main()
{
boost::shared_ptr<B> b(new B);
b->_count = 104;
boost::shared_ptr<A> a;
a = b;
a->decrement();
std::cerr << "A:" << a->_count << std::endl;
std::cerr << "B:" << b->_count << std::endl;
return 0;
}
EDIT:
So from the comment, we know the original pointers are correct, so now we need to trace.
Either:
log pointers to see when they change.
Use watchpoints in a debugger to see when the pointer changes.
Use a third shared pointer to see which pointer is changed.
Introduce a function that changes both pointers at the same time.
I think the problem is that you're assigning between different types here. boost::shared_ptr is a template and templates are not polymorphic even if the type in them is. So what's happening is that your compiler sees the assignment from boost::shared_ptr<Deck> to boost::shared_ptr<CardStack> and notices that it can make the assignment by calling the copy constructor for CardStack to duplicate the Deck object.
I think what you want the assignment to look like is something like this:
_stack[0] = boost::static_pointer_cast<CardStack>(_deck);
Which will do the conversion the way you expect it to.
I think you may want shared_array for _stack . . . Take a look at the documentation on shared_ptr;from boost.org, specifically:
http://www.boost.org/doc/libs/1_42_0/libs/smart_ptr/shared_ptr.htm
"Normally, a shared_ptr cannot
correctly hold a pointer to a
dynamically allocated array. See
shared_array for that usage."
Also, be aware of the T* get() function (not to be used without good reason) which returns the raw pointer being held by the managed pointer (shared_ptr in this case).