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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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insert vs emplace vs operator[] in c++ map

I'm using maps for the first time and I realized that there are many ways to insert an element. You can use emplace(), operator[] or insert(), plus variants like using value_type or make_pair. While there is a lot of information about all of them…
German Capuano
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Efficiency of C++11 push_back() with std::move versus emplace_back() for already constructed objects

In C++11 emplace_back() is generally preferred (in terms of efficiency) to push_back() as it allows in-place construction, but is this still the case when using push_back(std::move()) with an already-constructed object? For instance, is…
Riot
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std::map emplace without copying value

The C++11 std::map type has an emplace function, as do many other containers. std::map m; std::string val {"hello"}; m.emplace(1, val); This code works as advertised, emplacing the std::pair directly, however it results…
Drew Noakes
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Why emplace_back is faster than push_back?

I thought that emplace_back would be the winner, when doing something like this: v.push_back(myClass(arg1, arg2)); because emplace_back would construct the object immediately in the vector, while push_back, would first construct an anonymous object…
gsamaras
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What is the difference between unordered_map::emplace and unordered_map::insert in C++?

What is the difference between std::unordered_map::emplace and std::unordered_map::insert in C++?
Harsh M. Shah
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why do i need to use piecewise_construct in map::emplace for single arg constructors of noncopyable objects?

The following code will not compile on gcc 4.8.2. The problem is that this code will attempt to copy construct an std::pair which can't happen due to struct A missing copy and move constructors. Is gcc failing here or am I missing…
oli_obk
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Emplacement of a vector with initializer list

i have a std::vector> and would like to add some elements at the end of it so this was my trial: std::vector > vec; vec.emplace_back({0,0}); but this does not compile whereas the following will…
m47h
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Is it safe to use emplace_back with a container of unique_ptrs?

Consider the following: std::vector> ptrsToInts; ptrsToInts.emplace_back(new int); If reallocation occurs in the vector, and that fails (throwing std::bad_alloc), am I "safe" or will I leak an int? C++11 23.3.6.5…
Billy ONeal
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Why doesn't this use of emplace_back with deleted copy constructor work?

I have a type that I have deleted the copy constructor from, and I would like to have a vector of this type, so I need to create all the elements via emplace_back. But, emplace_back seems to require a copy constructor, as the compiler gives a…
Drew
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Insert into vector having objects without copy constructor

I have a class whose copy constructors are explicitly deleted (because A uses pointers internally and I don't want to fall into shallow copy pitfalls): class A { public: A(const A&) = delete; A& operator=(const A&) = delete; A(const…
vigs1990
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Best way to use emplace_back to avoid move constructor call?

I just learned about guaranteed copy elision in C++17. According to the answer on that question: When you do return T();, this initializes the return value of the function via a prvalue. Since that function returns T, no temporary is created;…
user2108462
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Difference between 's emplace and push

What are the differences between 's emplace and push? here's explanation about the std::queue::emplace and std::queue::push . Both methods add element after its current last element, return None.
Dami.h
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Why can an aggreggate struct be brace-initialized, but not emplaced using the same list of arguments as in the brace initialization?

It seems like this code: #include #include struct bla { std::string a; int b; }; int main() { std::vector v; v.emplace_back("string", 42); } could be made to work properly in this case, but it doesn't (and I…
rubenvb
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emplacing a POD

Possible Duplicate: C++11 emplace_back on vector? Is emplacement possible with PODs? It does not seem to work in Visual Studio 2012: struct X { int a; int b; }; void whatever() { std::vector xs; X x = {1, 2}; //…
fredoverflow
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Is there any C# analogue of C++11 emplace/emplace_back functions?

Starting in C++11, one can write something like #include #include struct S { S(int x, const std::string& s) : x(x) , s(s) { } int x; std::string s; }; // ... std::vector v; // add new…
Constructor
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