C++ 那些被遗漏的细节2 map emplace emplace_hint

说明

• 主要关注map的emplace、insert的返回类型 std::pair<iterator, bool> 和 emplace_hint的返回类型iterator。

template< class... Args >
std::pair<iterator,bool> emplace( Args&&... args );(since C++11)

• 如果插入成功，则iterator指向新插入元素，否则指向已存在的相同key元素；插入成功bool为true,否则为false。

template <class... Args>
iterator emplace_hint( const_iterator hint, Args&&... args ); (since C++11)

• iterator指向新插入元素或者已存在的相同key元素

例子

#include <iostream>
#include <string>
#include <map>
#include <algorithm>
#include <iterator>

template <typename C>
void print(const std::string &pre, const C &c)
{
  std::cout << pre;
  for (auto it = c.begin(); it != c.end(); ++it)
    std::cout << " {" << it->first << ", " << it->second << "}";
  std::cout << std::endl;
}

int main()
{
  std::map<int, std::string> mp{{1, "john"}, {2, "tom"}};
  print("map: ", mp);

  auto pr = mp.emplace(std::pair<const int, std::string>(1, "ketty"));
  print("map: ", mp);

  if (pr.second)
  {
    std::cout << "aready empalced: return data: " << pr.first->second << std::endl;
  }
  else
  {
    std::cout << "not empalced: return data: " << pr.first->second << std::endl;
  }

  pr = mp.emplace(std::pair<const int, std::string>(3, "lily"));
  print("map: ", mp);

  if (pr.second)
  {
    std::cout << "aready empalced: return data: " << pr.first->second << std::endl;
  }
  else
  {
    std::cout << "not empalced: return data: " << pr.first->second << std::endl;
  }

  return 0;
}

• 结果

lee@leedeMacBook-Pro map % ./main
map:  {1, john} {2, tom}
map:  {1, john} {2, tom}
not empalced: return data: john
map:  {1, john} {2, tom} {3, lily}
aready empalced: return data: lily

参考

• emplace
• emplace_hint