std::vector<int> v;
v.emplace_back(123);
v.emplace_back(v[0]);

unique_ptr<T>(new T(std::forward<Args>(args)...))

Forwards lvalues as either lvalues or as rvalues, depending on T

abc(std::unique_ptr<A>(new A()), std::unique_ptr<A>(new A()), some_other_function())

std::auto_ptr<A> a1 = std::auto_ptr<A>(new A());
std::auto_ptr<A> a2 = std::auto_ptr<A>(new A());
abc(a1.get(), a2.get(), some_other_function())

Only non-const unique_ptr can transfer the ownership of the managed object to another unique_ptr. If an object's lifetime is managed by a const std::unique_ptr, it is limited to the scope in which the pointer was created.

auto a1 = std::unique_ptr<A>(new A());
auto a2 = std::unique_ptr<A>(new A());
abc(a1, a2, some_other_function())

a1.get() // null ptr

std::unique_ptr<A> myA1 = a1;

Obtain the address represented by p without forming a reference to the object pointed to by p.

Тё>>std::vector<int> v;
Тё>>v.emplace_back(123);
Тё>>v.emplace_back(v[0]);
Тё>>

1. Проверить size() < capacity(). Если выполнено перейти к пункту 3.
2. Позвать reserve(size() × 2)
3. Вызвать in-place конструктор для нового элемента по смещению size();
4. Сделать ++size;

1. Выделяет новый буфер;
2. Переносит в него элементы из текущего буфера;
3. Разрушает элементы в старом буфере и освобождает память;

Some library implementations of emplace_back do not behave as specified in the C++ standard including the version that ship with Visual Studio 2012, 2013 and 2015.

The call to emplace_back() uses perfect forwarding to perform construction in place and as such v[0] is not evaluated until after the vector has been resized (at which point v[0] is invalid). push_back constructs the new element and copies/moves the element as needed and v[0] is evaluated prior to any reallocation. – Marc Mar 6 '15 at 16:01

void abc(A const *a1, A const *a2, int b)

void abc(A const &a1, A const &a2, int b)
...
abc(*a1.get(), *a2.get(), some_other_function())