# std::erase_if (std::unordered_map)

< cpp‎ | container‎ | unordered map

定义于头文件 `<unordered_map>` |  |
template< class Key, class T, class Hash, class KeyEqual, class Alloc, class Pred > typename std::unordered_map<Key,T,Hash,KeyEqual,Alloc>::size_type erase_if(std::unordered_map<Key,T,Hash,KeyEqual,Alloc>& c, Pred pred); |  | (C++20 起)

从容器中擦除所有满足谓词 `pred` 的元素。等价于

    auto old_size = c.size();
    for (auto i = c.begin(), last = c.end(); i != last; ) {
      if (pred(*i)) {
        i = c.erase(i);
      } else {
        ++i;
      }
    }
    return old_size - c.size();

### 参数

c | - | 要从中擦除的元素
pred | - | 若应该擦除元素则对它返回 true 的谓词

### 返回值

擦除的元素数。

### 复杂度

线性。

### 示例

    #include <unordered_map>
    #include <iostream>

    template<typename Os, typename Container>
    inline Os& operator<<(Os& os, Container const& cont)
    {
        os << "{";
        for (const auto& item : cont) {
            os << "{" << item.first << ", " << item.second << "}";
        }
        return os << "}";
    }

    int main()
    {
        std::unordered_map<int, char> data {{1, 'a'},{2, 'b'},{3, 'c'},{4, 'd'},
                                            {5, 'e'},{4, 'f'},{5, 'g'},{5, 'g'}};
        std::cout << "Original:\n" << data << '\n';

        const auto count = std::erase_if(data, [](const auto& item) {
            auto const& [key, value] = item;
            return (key & 1) == 1;
        });

        std::cout << "Erase items with odd keys:\n" << data << '\n'
                  << count << " items removed.\n";
    }

输出：

    Original:
    {{5, e}{4, d}{3, c}{2, b}{1, a}}
    Erase items with odd keys:
    {{4, d}{2, b}}
    3 items removed.

### 参阅

remove remove_if | 移除满足特定判别标准的元素 (函数模板)