Question

我正在尝试将对象插入C ++中的 unordered_map 中。我找到了一种插入它的方法 map.insert（std :: make_pair（key，object））。但是，我不明白为什么我们应该使用配对而不是常规的 map [key] = value 。而且，我想知道如何从键中访问值。 地图[键] 不起作用。

Answer 1

以下是将值放入无序映射的几种方法。

您会很抱歉，您问：）

值得一看cppreference.com上的参考资料，但一定要阅读有关该主题的最新书籍。像高级语言一样，c ++并不容易显而易见。

#include <unordered_map>
#include <string>
#include <cstring>
#include <tuple>
#include <iostream>

using MyMap = std::unordered_map<int, std::string>;

int main()
{
    auto m = MyMap();

    // only allowed if the mapped_type is default-constructible
    m[0] = "zero";   

    // old-school way, with pair
    m.insert(std::make_pair(1, "one"));

    // "nicer" way with emplace
    m.emplace(3, "three");

    // ultra-technical way with piecewise_construct (useful for constructing complex objects without copy/move)
    m.emplace(std::piecewise_construct, std::make_tuple(2), std::make_tuple("two"));

    // another demonstration of piecewise_construct
    const char* four = "four";
    std::size_t four_len = std::strlen(four);

    m.emplace(std::piecewise_construct, 
        std::make_tuple(2),                 // arguments for the key's constructor 
        std::make_tuple(four, four_len));   // arguments for the mapped-object's constructor

    std::string five = "five";
    auto ib = m.try_emplace(5, std::move(five));
    if (std::get<bool>(ib))
    {
        // emplaced - five's contents are valid but unspecified as it has been moved-from
    }
    else
    {
        // not emplaced - five's contents are still defined to be "five"
    }


    // and now let's retrieve the items..

    // this will only work if m is mutable and the mapped type is default-constructible.
    auto& s_zero = m[0];
    std::cout << s_zero << std::endl;


    // map iterators point to pairs of const key, mapped_type
    auto i_one = m.find(1);
    if (i_one != std::end(m))
        std::cout << i_one->second << std::endl;

    // 'at' will throw if the key is not present
    try
    {
        auto& i_two = m.at(2);
        std::cout << i_two << std::endl;
    }
    catch(std::exception& e)
    {
        std::cerr << e.what() << std::endl;
    }

    auto exists = (m.count(3) != 0);
    std::cout << "exists 3: " << exists << std::endl;
}

如何在C ++中将对象插入unordered_map

1 个答案: