Question

感谢 Bartlomiej Filipek 的好书 C ++ 17 。我发现一些在insert上下文中使用std::map的示例。

因此，我访问了 cppreference.com ，以更好地了解其工作原理。

https://en.cppreference.com/w/cpp/container/map/insert

但是，在页面底部，有一个有价值且冗长的示例，为我们提供了std::map::insert用法的不同示例。我已经在计算机上亲自对其进行了测试，以尝试理解为什么使用重载1 和重载4 插入失败。

老实说，我不明白为什么。

我非常感谢您解释在这两个特定示例中发生的情况（为什么插入失败），因为这似乎是一个非常有趣的示例，它同时使用了“ 结构化绑定”的概念”。

#include <iomanip>
#include <iostream>
#include <map>
#include <string>
using namespace std::literals;

template<typename It>
void printInsertionStatus(It it, bool success)
{
    std::cout << "Insertion of " << it->first << (success ? " succeeded\n" : " failed\n");
}

int main()
{
    std::map<std::string, float> karasunoPlayerHeights;

    // Overload 3: insert from rvalue reference
    const auto [it_hinata, success] = karasunoPlayerHeights.insert({ "Hinata"s, 162.8 });
    printInsertionStatus(it_hinata, success);

    {
        // Overload 1: insert from lvalue reference
        const auto [it, success2] = karasunoPlayerHeights.insert(*it_hinata);
        printInsertionStatus(it, success2);
    }
    {
        // Overload 2: insert via forwarding to emplace
        const auto [it, success] = karasunoPlayerHeights.insert({ "Kageyama", 180.6 });
        printInsertionStatus(it, success);
    }
    {
        // Overload 6: insert from rvalue reference with positional hint
        const std::size_t n = std::size(karasunoPlayerHeights);
        const auto it = karasunoPlayerHeights.insert(it_hinata, { "Azumane"s, 184.7 });
        printInsertionStatus(it, std::size(karasunoPlayerHeights) != n);
    }
    {
        // Overload 4: insert from lvalue reference with positional hint
        const std::size_t n = std::size(karasunoPlayerHeights);
        const auto it = karasunoPlayerHeights.insert(it_hinata, *it_hinata);
        printInsertionStatus(it, std::size(karasunoPlayerHeights) != n);
    }
    {
        // Overload 5: insert via forwarding to emplace with positional hint
        const std::size_t n = std::size(karasunoPlayerHeights);
        const auto it = karasunoPlayerHeights.insert(it_hinata, { "Tsukishima", 188.3 });
        printInsertionStatus(it, std::size(karasunoPlayerHeights) != n);
    }

    auto node_hinata = karasunoPlayerHeights.extract(it_hinata);
    std::map<std::string, float> playerHeights;

    // Overload 7: insert from iterator range
    playerHeights.insert(std::begin(karasunoPlayerHeights),
        std::end(karasunoPlayerHeights));

    // Overload 8: insert from initializer_list
    playerHeights.insert({ {"Kozume"s, 169.2}, {"Kuroo", 187.7} });


    // Overload 9: insert node
    const auto status = playerHeights.insert(std::move(node_hinata));
    printInsertionStatus(status.position, status.inserted);

    node_hinata = playerHeights.extract(status.position);
    {
        // Overload 10: insert node with positional hint
        const std::size_t n = std::size(playerHeights);
        const auto it = playerHeights.insert(std::begin(playerHeights),
            std::move(node_hinata));
        printInsertionStatus(it, std::size(playerHeights) != n);
    }

    // Print resulting map
    std::cout << std::left << '\n';
    for (const auto& [name, height] : playerHeights)
        std::cout << std::setw(10) << name << " | " << height << "cm\n";
}

Answer 1

如 cppreference.com std::map::insert所述：

将元素插入容器，如果容器不插入 已包含具有等效键的元素。

在// Overload 1中：

迭代器it_hinata指向最后插入的条目{"Hinata"s, 162.8}，如果您尝试输入相同的键值对，则插入失败，因此{ {1}}。

在success2 == false中：

迭代器// Overload 4仍指向相同（首先）插入的键值对（即，相同的it_hinata）。因此，与上述情况相同的原因导致插入失败。也就是说，插入调用后，映射的大小（即{"Hinata"s, 162.8}）保持不变，条件karasunoPlayerHeights的求值结果为std::size(karasunoPlayerHeights) != n。

以下是OP发布的内容minimal, complete reproducible example：

false

输出：

#include <iomanip>
#include <iostream>
#include <map>
#include <string>
using namespace std::literals;

template<typename It> void printInsertionStatus(It it, bool success)
{
    std::cout << "Insertion of " << it->first << (success ? " succeeded\n" : " failed\n");
}

int main()
{
    std::map<std::string, float> karasunoPlayerHeights;

    // Overload 3: insert from rvalue reference
    const auto [it_hinata, success] = karasunoPlayerHeights.insert({ "Hinata"s, 162.8f });
    printInsertionStatus(it_hinata, success);    
    {
        // Overload 1: insert from lvalue reference
        const auto [it, success2] = karasunoPlayerHeights.insert(*it_hinata);
        printInsertionStatus(it, success2);
    }
    {
        // Overload 4: insert from lvalue reference with positional hint
        const std::size_t n = std::size(karasunoPlayerHeights);
        const auto it = karasunoPlayerHeights.insert(it_hinata, *it_hinata);
        printInsertionStatus(it, std::size(karasunoPlayerHeights) != n);
    }
    // Print resulting map
    std::cout << std::left << '\n';
    for (const auto& [name, height] : karasunoPlayerHeights)
        std::cout << std::setw(10) << name << " | " << height << "cm\n";
}

为什么在以下示例中std :: map :: insert失败？

1 个答案: