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The syntax of moving std::unique_ptr<...>() somewhat eludes me, and i can't find a clear answer(to me at least), as to how i should move the unique_ptr around.

I have some heap-allocated Nodes and would like to create new Nodes, which have two already existing Nodes as children. Which in turn are supposed to be inserted into the vector.

#include <memory> #include <vector> template <typename T> struct Node{ std::unique_ptr<Node<T>> left, right; Node(std::unique_ptr<Node<T>> left, std::unique_ptr<Node<T>> right){ this->left = left; this->right = right; } } template <typename T> void foo(){ std::vector<std::unique_ptr<Node<T>>> vec; //... vec.insert(vec.begin() + idx, std::unique_ptr<Node<T>>(new Node<T>(vec.at(vec.begin() + idx), vec.at(vec.begin() + idx + 1)))); }

I only get the error-meassage, that no matching function call was found.

expression.hpp:69:58: error: no matching function for call to ‘std::vector<std::unique_ptr<Node<int>, std::default_delete<Node<int> > >, std::allocator<std::unique_ptr<Node<int>, std::default_delete<Node<int> > > > >::at(__gnu_cxx::__normal_iterator<std::unique_ptr<Node<int>, std::default_delete<Node<int> > >*, std::vector<std::unique_ptr<Node<int>, std::default_delete<Node<int> > >, std::allocator<std::unique_ptr<Node<int>, std::default_delete<Node<int> > > > > >)’

Cann somebody help, or has an idea, where i can look for the correct syntax, and which move/copy-function i should use?

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    Please include a minimal reproducible example and the error message Commented Jan 24, 2019 at 14:34
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    From the semantical POV: Who should be the owner of that unique instance after the operation? Commented Jan 24, 2019 at 14:37
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    The std::vector's at member function takes an integer (technically a size_t) and not an iterator, so I think you can just use idx and idx + 1 directly in the calls to at. It's not about syntax, but about the types. I suspect that your error message says as much. Commented Jan 24, 2019 at 14:38
  • the owner should be the new Node @πάνταῥεῖ Commented Jan 24, 2019 at 14:42
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    @user463035818 did it, hope this is better Commented Jan 24, 2019 at 14:42

2 Answers 2

Reset to default
3 
vec.at(vec.begin() + idx)

Take a look at declaration of at. The argument type is size_type (an integer type). You're trying to pass an iterator.

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2 Comments

Weirdly enough this has worked for me before ... but ill look into it; thx
okay, found where it worked for me. wasn't at but ``` erease ```
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It sounds like you want to merge two adjacent Nodes in your vector. You will need to do this in multiple steps

template <typename T> struct Node{ std::unique_ptr<Node<T>> left, right; Node(std::unique_ptr<Node<T>> left, std::unique_ptr<Node<T>> right) : left(std::move(left)), right(std::move(right)) {} } template <typename T> void foo(){ std::vector<std::unique_ptr<Node<T>>> vec; //... // Identify first element auto first = vec.begin() + idx; // and second element auto second = first + 1; // make the merged node auto merged = std::make_unique<Node<T>>(std::move(*first), std::move(*second)); // remove the now empty Node pointers - note that this relies on the adjacency of first and second auto pos = vec.erase(first, first + 2); // add the new node to the correct place vec.emplace(pos, std::move(merged)); }
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3 Comments

+1 This will fix the problem of having empty (nullptr) nodes, which Clebo Sevic mentioned he'd rather not have in the vector.
How do i assign left and right to members of Node? = is a deleted function. Should i move again?
@CleboSevic The code in my answer moves when it needs to. In general member initializers are preferable to code in the body of the constructor.

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