public member function
<vector>
std::vector::emplace
template <class... Args>iterator emplace (const_iterator position, Args&&... args);
Construct and insert element
The container is extended by inserting a new element at position. This new element is constructed in place using args as the arguments for its construction.
This effectively increases the container size by one.
An automatic reallocation of the allocated storage space happens if -and only if- the new vector size surpasses the current vector capacity.
Because vectors use an array as their underlying storage, inserting elements in positions other than the vector end causes the container to shift all the elements that were after position by one to their new positions. This is generally an inefficient operation compared to the one performed by other kinds of sequence containers (such as list or forward_list). See emplace_back for a member function that extends the container directly at the end.
The element is constructed in-place by calling allocator_traits::construct with args forwarded.
A similar member function exists, insert, which either copies or moves existing objects into the container.
Parameters
- position
- Position in the container where the new element is inserted.
Member type const_iterator is a random access iterator type that points to a const element.
- args
- Arguments forwarded to construct the new element.
Return value
An iterator that points to the newly emplaced element.
Member type iterator is a random access iterator type that points to an element.
If a reallocation happens, the storage is allocated using the container's allocator, which may throw exceptions on failure (for the default allocator, bad_alloc is thrown if the allocation request does not succeed).
Example
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// vector::emplace
#include <iostream>
#include <vector>
int main ()
{
std::vector<int> myvector = {10,20,30};
auto it = myvector.emplace ( myvector.begin()+1, 100 );
myvector.emplace ( it, 200 );
myvector.emplace ( myvector.end(), 300 );
std::cout << "myvector contains:";
for (auto& x: myvector)
std::cout << ' ' << x;
std::cout << '\n';
return 0;
}
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myvector contains: 10 200 100 20 30 300
|
Complexity
Linear on the number of elements after position (moving).
If a reallocation happens, the reallocation is itself up to linear in the entire size.
Iterator validity
If a reallocation happens, all iterators, pointers and references related to this container are invalidated.
Otherwise, only those pointing to position and beyond are invalidated, with all iterators, pointers and references to elements before position guaranteed to keep referring to the same elements they were referring to before the call.
Data races
The container is modified.
If a reallocation happens, all contained elements are modified.
Otherwise, none of the elements before position is accessed, and concurrently accessing or modifying them is safe.
Exception safety
If position is end, and no reallocations happen, there are no changes in the container in case of exception (strong guarantee).
If a reallocation happens, the strong guarantee is also given if the type of the elements is either copyable or no-throw moveable.
Otherwise, the container is guaranteed to end in a valid state (basic guarantee).
If allocator_traits::construct is not supported with the appropriate arguments, or if position is not valid, it causes undefined behavior.