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std::{{{1}}}::emplace_back

من cppreference.com
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 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)....

Information on iterator invalidation is copied from here

محتويات

[تعديل] Parameters

args - arguments to forward to the constructor of the element

قالب:par hreq

[تعديل] Return value

(none)

[تعديل] Complexity

Constant.

[تعديل] Exceptions

If an exception is thrown, this function has no effect (strong exception guarantee).


[تعديل] Example

The following code uses ‎emplace_back‎ to append an object of type ‎President‎ to a std::{{{1}}}. 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.

#include <{{{1}}}>
#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::{{{1}}}<President> elections;
    std::cout << "emplace_back:\n";
    elections.emplace_back("Nelson Mandela", "South Africa", 1994);
 
    std::{{{1}}}<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";
    }
}

الخرج:

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.

[تعديل] See also

قالب:dsc mem fun