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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)

Appends a new element to the end of the container. The element is constructed in-place, i.e. no copy or move operations are performed. The constructor of the element is called with exactly the same arguments that are supplied to the function.

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

### [edit] Return value

(none)

### [edit] Complexity

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 elements to the end (public member function) [edit]

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