CODEWITHSUNDEEP
c++lambdamemory-managementsmart-pointers
João Lucas Gouveia
João Lucas Gouveia

Reputation: 266

Problem with reference inside lambda expr

I'm trying to replace a raw pointer implementation to smart pointer, so in this function call

int id = std::stoi(idToken->second);
auto newNode = std::find_if(_nodes.begin(), _nodes.end(), [&id](GraphNode *node) { return node->GetID() == id; });

I'm trying to change the raw pointer (GraphNode) to GraphNode unique ptr, to set the exclusive ownership of graphnode to the class it is declared, so in it's declaration I changed to

typedef std::unique_ptr<GraphNode> GraphNodeUniquePtr;
std::vector<GraphNodeUniquePtr> _nodes;

and in the function call I put

int id = std::stoi(idToken->second);
auto newNode = std::find_if(_nodes.begin(), _nodes.end(), [&id](GraphNodeUniquePtr node){ return node.get()->GetID() == id; });

on compiling, The compiler throws an error

error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = GraphNode; _Dp = std::default_delete<GraphNode>]’

Then I thought it could be the case to make a unique pointer with id and pass it to the lambda using the move function like

int id = std::stoi(idToken->second);
std::unique_ptr<int> idPtr;
*idPtr = id;
auto newNode = std::find_if(_nodes.begin(), _nodes.end(), [idPtr = std::move(idPtr)](GraphNodeUniquePtr node){ return node.get()->GetID() == *idPtr.get(); });

and now the compiler throws me another error:

error: use of deleted function ‘ChatLogic::LoadAnswerGraphFromFile(std::string)::<lambda(ChatLogic::GraphNodeUniquePtr)>::<lambda>(const ChatLogic::LoadAnswerGraphFromFile(std::string)::<lambda(ChatLogic::GraphNodeUniquePtr)>&)’

So I tried other approaches which I saw why it was not working and these two I could not understand why it isnt. I thought to make a shared pointer on GraphNode but with this I lost the exclusive ownership

How can I adapt this raw pointer implementation to unique pointer?

Thanks

Upvotes: 0

Views: 55

Answers (1)

Jarod42
Jarod42

Reputation: 217810

You cannot pass std::unique_ptr by copy. You might use reference instead:

[&id](const GraphNodeUniquePtr& node){ return node->GetID() == id; });

or, with const auto& which would work for both versions

[&id](const auto& node){ return node->GetID() == id; });

Upvotes: 2

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