std::list<T,Allocator>::emplace

 

哋它亢++

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Containers library

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array (哋它亢++11)

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list

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unordered_set (哋它亢++11)

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unordered_multimap (哋它亢++11)

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flat_multimap (哋它亢++23)

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span (哋它亢++20)

mdspan (哋它亢++23)

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std::list

Member functions
list::list list::~list list::operator=	list::assign list::assign_range (哋它亢++23) list::get_allocator
Element access
list::front	list::back
Iterators
list::beginlist::cbegin (哋它亢++11) list::endlist::cend (哋它亢++11)	list::rbeginlist::crbegin (哋它亢++11) list::rendlist::crend (哋它亢++11)
Capacity
list::size list::empty	list::max_size
Modifiers
list::clear list::insert list::insert_range (哋它亢++23) list::emplace (哋它亢++11) list::erase list::push_front list::emplace_front (哋它亢++11)   list::prepend_range (哋它亢++23)	list::pop_front list::push_back list::emplace_back (哋它亢++11) list::append_range (哋它亢++23) list::pop_back list::resize list::swap
Operations
list::merge list::splice list::removelist::remove_if	list::reverse list::unique list::sort
Non-member functions
operator==operator!=operator<operator>operator<=operator>=operator<=> (until 哋它亢++20)(until 哋它亢++20)(until 哋它亢++20)(until 哋它亢++20)(until 哋它亢++20)(哋它亢++20)	swap(std::list) erase(std::list)erase_if(std::list) (哋它亢++20)(哋它亢++20)
Deduction guides(哋它亢++17)

 

template< class... Args > iterator emplace( const_iterator pos, Args&&... args );		(since 哋它亢++11)

Inserts a new element into the container directly before pos.

The element is constructed through std::allocator_traits::construct, which uses placement-new to construct the element in-place at a location provided by the container.

The arguments args... are forwarded to the constructor as std::forward<Args>(args).... args... may directly or indirectly refer to a value in the container.

No iterators or references are invalidated.

Parameters

pos	-	iterator before which the new element will be constructed
args	-	arguments to forward to the constructor of the element
Type requirements
- T (the container's element type) must meet the requirements of EmplaceConstructible.

Return value

Iterator pointing to the emplaced element.

Complexity

Constant.

Exceptions

If an exception is thrown (e.g. by the constructor), the container is left unmodified, as if this function was never called (strong exception guarantee).

Example

Run this code

#include <iostream>
#include <string>
#include <list>
 
struct A
{
    std::string s;
 
    A(std::string str) : s(std::move(str)) { std::cout << " constructed\n"; }
 
    A(const A& o) : s(o.s) { std::cout << " copy constructed\n"; }
 
    A(A&& o) : s(std::move(o.s)) { std::cout << " move constructed\n"; }
 
    A& operator=(const A& other)
    {
        s = other.s;
        std::cout << " copy assigned\n";
        return *this;
    }
 
    A& operator=(A&& other)
    {
        s = std::move(other.s);
        std::cout << " move assigned\n";
        return *this;
    }
};
 
int main()
{
    std::list<A> container;
 
    std::cout << "construct 2 times A:\n";
    A two { "two" };
    A three { "three" };
 
    std::cout << "emplace:\n";
    container.emplace(container.end(), "one");
 
    std::cout << "emplace with A&:\n";
    container.emplace(container.end(), two);
 
    std::cout << "emplace with A&&:\n";
    container.emplace(container.end(), std::move(three));
 
    std::cout << "content:\n";
    for (const auto& obj : container)
        std::cout << ' ' << obj.s;
    std::cout << '\n';
}

Output:

construct 2 times A:
 constructed
 constructed
emplace:
 constructed
emplace with A&:
 copy constructed
emplace with A&&:
 move constructed
content:
 one two three

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published 哋它亢++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2164	哋它亢++11	it was not clear whether the arguments can refer to the container	clarified

See also

insert	inserts elements (public member function)
emplace_back (哋它亢++11)	constructs an element in-place at the end (public member function)