c++17中map的try_emplace与insert_or_assign

try_emplace

函数原型：

template <class... Args>
pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
(1)	(since C++17)
template <class... Args>
pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
(2)	(since C++17)
template <class... Args>
iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
(3)	(since C++17)
template <class... Args>
iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);
(4)	(since C++17)

如果容器中已经存在等效的键，则不执行任何操作。否则，其行为类似于emplace。

与insert或emplace不同，如果插入没有发生，这些函数不会从右值参数移动，提升效率！

代码示例：

map<int, string> cities = {
    {1, "beijing"},
    {2, "shanghai"},
    {3, "hangzhou"},
    pair<int, string>(4, "tianjin"),
    make_pair(5, "chengdu")};

auto [iterator, inserted] = cities.try_emplace(1, "shenzhen");
if (inserted)
{
    cout << "insert ok!" << endl;
}
else
{
    cout << "error insert" << endl;
}

insert_or_assign

函数原型：

template <class M>
pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
(1)	(since C++17)
template <class M>
pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
(2)	(since C++17)
template <class M>
iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
(3)	(since C++17)
template <class M>
iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);
(4)	(since C++17)

如果容器中已经存在等效的键，则更新key的value，否则，插入新值，和insert一样

代码示例:

map<int, string> cities = {
    {1, "beijing"},
    {2, "shanghai"},
    {3, "hangzhou"},
    pair<int, string>(4, "tianjin"),
    make_pair(5, "chengdu")};

auto [iterator, inserted] = cities.insert_or_assign(1, "shenzhen");
if (inserted)
{
    cout << "insert ok!" << endl;
}
else
{
    cout << "error insert" << endl;
}

for (const auto &[cityNum, cityName] : cities)
{
    cout << cityNum << ":" << cityName << endl;
}