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std::map::try_emplace(3) | C++ Standard Libary | std::map::try_emplace(3) |
NAME¶
std::map::try_emplace - std::map::try_emplace
Synopsis¶
template< class... Args > (1) (since C++17)
pair<iterator, bool> try_emplace( const Key& k, Args&&...
args );
template< class... Args > (2) (since C++17)
pair<iterator, bool> try_emplace( Key&& k, Args&&...
args );
template< class... Args >
iterator try_emplace( const_iterator hint, const Key& k,
Args&&... (3) (since C++17)
args );
template< class... Args >
iterator try_emplace( const_iterator hint, Key&& k, Args&&...
args (4) (since C++17)
);
Inserts a new element into the container with key k and value constructed
with args,
if there is no element with the key in the container.
1) If a key equivalent to k already exists in the container, does nothing.
Otherwise, behaves like emplace except that the element is constructed as
value_type(std::piecewise_construct,
std::forward_as_tuple(k),
std::forward_as_tuple(std::forward<Args>(args)...))
2) If a key equivalent to k already exists in the container, does nothing.
Otherwise, behaves like emplace except that the element is constructed as
value_type(std::piecewise_construct,
std::forward_as_tuple(std::move(k)),
std::forward_as_tuple(std::forward<Args>(args)...))
3) If a key equivalent to k already exists in the container, does nothing.
Otherwise, behaves like emplace_hint except that the element is constructed
as
value_type(std::piecewise_construct,
std::forward_as_tuple(k),
std::forward_as_tuple(std::forward<Args>(args)...))
4) If a key equivalent to k already exists in the container, does nothing.
Otherwise, behaves like emplace_hint except that the element is constructed
as
value_type(std::piecewise_construct,
std::forward_as_tuple(std::move(k)),
std::forward_as_tuple(std::forward<Args>(args)...))
No iterators or references are invalidated.
Parameters¶
k - the key used both to look up and to insert if not found
hint - iterator to the position before which the new element will be inserted
args - arguments to forward to the constructor of the element
Return value¶
1,2) Same as for emplace
3,4) Same as for emplace_hint
Complexity¶
1,2) Same as for emplace
3,4) Same as for emplace_hint
Notes¶
Unlike insert or emplace, these functions do not move from rvalue
arguments if the
insertion does not happen, which makes it easy to manipulate maps whose
values are
move-only types, such as std::map<std::string,
std::unique_ptr<foo>>. In addition,
try_emplace treats the key and the arguments to the mapped_type separately,
unlike
emplace, which requires the arguments to construct a value_type (that is, a
std::pair).
Feature-test macro: __cpp_lib_map_try_emplace
Example¶
// Run this code
#include <iostream>
#include <utility>
#include <string>
#include <map>
auto print_node = [](const auto &node) {
std::cout << "[" << node.first << "] =
" << node.second << '\n';
};
auto print_result = [](auto const &pair) {
std::cout << (pair.second ? "inserted: " : "ignored:
");
print_node(*pair.first);
};
int main()
{
using namespace std::literals;
std::map<std::string, std::string> m;
print_result( m.try_emplace("a", "a"s) );
print_result( m.try_emplace("b", "abcd") );
print_result( m.try_emplace("c", 10, 'c') );
print_result( m.try_emplace("c", "Won't be inserted")
);
for (const auto &p : m) { print_node(p); }
}
Output:¶
inserted: [a] = a
inserted: [b] = abcd
inserted: [c] = cccccccccc
ignored: [c] = cccccccccc
[a] = a
[b] = abcd
[c] = cccccccccc
See also¶
emplace constructs element in-place
(C++11) (public member function)
emplace_hint constructs elements in-place using a hint
(C++11) (public member function)
inserts elements
insert or nodes
(since C++17)
(public member function)
2022.07.31 | http://cppreference.com |