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Questions tagged [emplace]

For questions that concern constructing objects directly within some container object, as opposed to copying/moving the object into the container.

Use for questions related to the act of doing something in place, like for example inserting a new element in place.

For example, for an std::vector in , one could use std:emplace_back(), which is described like:

Inserts a new element at the end of the vector, right after its current last element. This new element is constructed in place using args as the arguments for its constructor.

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How do I insert an element to a set that is a value in multimap C++

I have a trouble with adding a new values to a set that is in my map>. When I try to add a value like this: Key key(get<0>(key_args), get<1>(key_args)); if (!(get<0>(key_args) < 1 && get<0>(key_args) >…
Artem
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emplace and try_emplace with copy constructor

I have an issue with emplace and try_emplace as they always use the copy constructors when moving an object in. #include #include #include using namespace std; class Too { public: Too(int x, int y):x_(x),…
Quang Le
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Emplace method in own deque

How can I write my own emplace method in my deque? For example, I have this emplace_front function template void emplace_front(Args&& ...val) { ??? } and Node constructor template Node(Args&&... val) :…
Ja4
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Error in implementing emplace_back() binary '=': no operator found which takes a right-hand operand of type 'T *'

Code below, for my linked list implementation and specifically emplace_back below. What am I doing wrong and how to fix? Error I am getting is: Build started... 1>------ Build started: Project: so_list_emplace, Configuration: Debug Win32…
Angus Comber
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c++17 correct moving resource between unique_ptr in containers

How to correct move resource from existed unique_ptr to another created in container? I want to put to container some unique_ptr with resource from another unique_ptr. Or maybe move one unique_ptr from one container to another container. Some code…
Pasha
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What is the difference between std::list::insert() and std::list::emplace()

What is the difference between std::list::insert() and std::list::emplace()? I need to pass a value to either of them. I had expected emplace to create an instance of the list's data type without initializing it with some value I pass but just…
Razzupaltuff
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Building a std::map and issue with using std::emplace

Code: std::map mapHistory; // History list is in ascending date order for (auto& sHistItem : m_listDiscussionItemHist) { if (m_bFullHistoryMode) mapHistory.emplace(sHistItem.strName, sHistItem); else…
Andrew Truckle
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Unable to avoid copying while pushing objects with copy-construcor into a vector

I'm trying to avoid copying with emplace_back() and reserve(). But when I've tried to do it, i caught myself getting 3 copies for reason i cannot really understand. reserve() actually helps to avoid copying, but emplace_back() actually does nothing…
Venci
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Is there ever a time where insert is better than emplace on a map in C++?

To head this off, I've already read through the following question. My question could be seen as a follow up to this. insert vs emplace vs operator[] in c++ map From what info I can find, I don't see any reason why you would ever want to use an…
TheChemist
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Is it still necessary to use std move even if auto && has been used

As we know, STL usually offered two kinds of functions to insert an element: insert/push and emplace. Let's say I want to emplace all of elements from one container to another. for (auto &&element : myMap) { anotherMap.emplace(element); // vs…
Yves
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std::vector::emplace_back error in class with a custom constructor

My program has a class similar to the following: class IOWorker { std::thread thread_; boost::asio::io_service ios_; boost::optional work_; …
void
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Is emplace only advantageous over push when the object being emplaced is a non-POD and an r-value?

From what I've read regarding emplace vs push for STL containers, the benefit of emplace over push is only evident when the object you're emplacing is an r-value and a non-POD type. Is this the only situation in which emplace is faster?
user5965026
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try_emplace doesn't work (as desired) when you have an inheritance structure for the value type?

// A couple of simple structs that inherit AND have different ctor arguments struct A { A(int) {} }; // Note that "B" is a subclass of "A", which is important below struct B : A { B(int, char) : A(0) // The 0 doesn't matter {} // Note…
user5406764
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Can I emplace arguments into a class member without first default-constructing said member?

template class InternalObject { public: using RefCountT = unsigned short; template static void* emplace(CtorTs&&...); Types type; RefCountT reference_count; bool is_immovable; …
Doot
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Confusion about the different efficiency between emplacement and insertion for containers not allowing duplicates

In Item 41 from Effective Modern C++, the following is one of the situations that give a chance for emplacement functions to be more performant than the insertion conterparts: The container is unlikely to reject the new value as a duplicate the…
Enlico
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