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# std::vector::emplace_back

From cppreference.com
< cpp‎ | container‎ | vector

[edit template]

template< class... Args > void emplace_back( Args&&... args ); |  | (since C++11) (until C++17)
template< class... Args > reference emplace_back( Args&&... args ); |  | (since C++17)

Appends a new element to the end of the container. The element is constructed through std::allocator_traits::construct, which typically uses placement-new to construct the element in-place at the location provided by the container. The arguments `args...` are forwarded to the constructor as std::forward<Args>(args)....

If the new size() is greater than capacity() then all iterators and references (including the past-the-end iterator) are invalidated. Otherwise only the past-the-end iterator is invalidated.

### [edit] Parameters

args | - | arguments to forward to the constructor of the element
Type requirements
- `T (the container's element type)` must meet the requirements of `MoveInsertable` and `EmplaceConstructible`.

### [edit] Return value

(none) | (until C++17)
A reference to the inserted element. | (since C++17)

### [edit] Complexity

Amortized constant.

### [edit] Exceptions

If an exception is thrown, this function has no effect (strong exception guarantee). If `T`'s move constructor is not noexcept and is not `CopyInsertable` into `*this`, vector will use the throwing move constructor. If it throws, the guarantee is waived and the effects are unspecified.

### Notes

The specialization std::vector<bool> did not have `emplace_back()` member until C++14.

### [edit] Example

The following code uses `emplace_back` to append an object of type `President` to a std::vector. It demonstrates how `emplace_back` forwards parameters to the `President` constructor and shows how using `emplace_back` avoids the extra copy or move operation required when using `push_back`.

Run this code

    #include <vector>
    #include <string>
    #include <iostream>

    struct President
    {
        std::string name;
        std::string country;
        int year;

        President(std::string p_name, std::string p_country, int p_year)
            : name(std::move(p_name)), country(std::move(p_country)), year(p_year)
        {
            std::cout << "I am being constructed.\n";
        }
        President(President&& other)
            : name(std::move(other.name)), country(std::move(other.country)), year(other.year)
        {
            std::cout << "I am being moved.\n";
        }
        President& operator=(const President& other) = default;
    };

    int main()
    {
        std::vector<President> elections;
        std::cout << "emplace_back:\n";
        elections.emplace_back("Nelson Mandela", "South Africa", 1994);

        std::vector<President> reElections;
        std::cout << "\npush_back:\n";
        reElections.push_back(President("Franklin Delano Roosevelt", "the USA", 1936));

        std::cout << "\nContents:\n";
        for (President const& president: elections) {
            std::cout << president.name << " was elected president of "
                      << president.country << " in " << president.year << ".\n";
        }
        for (President const& president: reElections) {
            std::cout << president.name << " was re-elected president of "
                      << president.country << " in " << president.year << ".\n";
        }
    }

Output:

    emplace_back:
    I am being constructed.

    push_back:
    I am being constructed.
    I am being moved.

    Contents:
    Nelson Mandela was elected president of South Africa in 1994.
    Franklin Delano Roosevelt was re-elected president of the USA in 1936.

### [edit] See also

push_back | adds an element to the end (public member function) [edit]

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