I would like to know if there is a way in C++ to connect two elements of a vector for example std::vector such that if one is changed, the other changes automatically. If not is there any other way to do so?
Thanks.
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You can store `std::shared_ptr`'s in the vectors. As you tagged your question such did you already try that and it didn't work for you? – πάντα ῥεῖ Apr 30 '16 at 08:56
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@πάνταῥεῖ Thanks for the answer. I am relatively new to C++, should I use std::vector
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You should use `std::vector
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Or `std::shared_ptr
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Or, or if you are careful, you could store simple pointers. – perencia Apr 30 '16 at 09:04
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Thanks a lot. Now that I have a clue and I can do a little more search on the internet to find out how it's done. – Eman Apr 30 '16 at 09:05
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@Eman [The reference documentation](http://en.cppreference.com/w/cpp/memory/shared_ptr) is also a good starting point. – πάντα ῥεῖ Apr 30 '16 at 09:09
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Also, references could be a good option too. – perencia Apr 30 '16 at 09:10
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Thanks again. Could you tell me which one is less expensive? The number of elements is large and the number of connected elements is relatively small. – Eman Apr 30 '16 at 09:11
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You should use `std::shared_ptr
`, it will manage the memory correctly for you, especially if you're new. Even if you used the faster raw pointers, you'd have to do additional bookkeeping or have specific conditions to know when it would be safe to free the memory; this isn't as easy as it seems. – bku_drytt Apr 30 '16 at 09:13 -
@perencia _"Also, references ..."_ Nope, you can't store a reference in a `std::vector`. – πάντα ῥεῖ Apr 30 '16 at 09:15
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@perencia Because you cannot. – πάντα ῥεῖ Apr 30 '16 at 09:20
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@πάνταῥεῖ Oh, so that's a dogma. – perencia Apr 30 '16 at 09:24
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@perencia No it's plan syntax. It's syntactically not possible to store a reference in a vector, try it. – πάντα ῥεῖ Apr 30 '16 at 09:25
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Thanks everybody for your insightful comments. – Eman Apr 30 '16 at 09:26
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@πάνταῥεῖ ok , I see. newacct explains it very well here http://stackoverflow.com/questions/922360/why-cant-i-make-a-vector-of-references – perencia Apr 30 '16 at 09:36
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if only a minority of elements are shared, it might be better perf-wise (though less elegant) to make a union of pointer-to-value and value. Then in the majority of cases there is no need to dereference a pointer. (obviously you need a flag to say which version of the union is correct) It should be possible to wrap this in a pointer-like class, which hides the details. – dan-man Apr 30 '16 at 09:36
2 Answers
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Let's say you have two vectors containing instances of a same kind of Object. Then using shared_ptr<Object> you can refer to the same object:
vector<shared_ptr<Object>> v1;
vector<shared_ptr<Object>> v2;
for(int i=0;i<3;i++) {
v1.push_back(shared_ptr<Object>(new Object()));
v2.push_back(v1[i]);
}
Now if you edit a property of an object of one vector, the corresponding object will be updated too:
v1[0]->value = 12;
cout << v2[0]->value << endl; // print 12
dynamic
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Thanks a lot for the answer. I get the idea behind it. However, it is a bit different from what I need. I have only one vector containing doubles and let's say it has the size of 1000 elements. I want to connect for example; the first 10 elements to the last 10 elements respectively. So when I change the first 10 elements, the last 10 elements also change. – Eman Apr 30 '16 at 09:43
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You could also use emplace_back and make_shared `v1.emplace_back(make_shared – Andreas DM May 03 '16 at 20:12
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class A {
... Your class definition
public:
int x;
};
typedef std::shared_ptr<A> APtr;
typedef std::vector<APtr> AVect;
AVect v;
APtr ptr_a = std::make_shared<A>(); //using std::make_shared<> is better than naked new allocations in code.
v.push_back(ptr_a);
v.push_back(ptr_a);
v[0]->x = 3; //modifies v[1] as well
Abdus Khazi
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