为什么 std::map::insert 在以下示例中失败？

Question

感谢 Bartlomiej Filipek 的好书 C++17 详解。我发现了一些在 std::map.

上下文中使用 insert 的示例

所以我访问了 cppreference.com 以更好地了解它的工作原理。

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

但是，在页面底部，有一个很有价值的长示例，它为我们提供了 std::map::insert 的不同用法示例。我自己在我的电脑上测试过，试图理解为什么 overload 1 和 overload 4，插入失败。

老实说，我不明白为什么。

非常感谢您解释这两个具体示例中发生的情况（插入失败的原因），因为它似乎是一个非常有趣的示例，它同时使用了“”的概念结构化绑定".

#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 所述 std::map::insert:

Inserts element(s) into the container, if the container doesn't already contain an element with an equivalent key.

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 key-value 对，插入失败，因此 success2 == false.

在// Overload 4中：

迭代器 it_hinata 仍然指向相同的（首先）插入的 key-value 对（即相同的 {"Hinata"s, 162.8}）。所以，和上面的情况一样，插入失败。这意味着，地图的大小（即 karasunoPlayerHeights）在插入调用后保持不变，并且条件 std::size(karasunoPlayerHeights) != n 评估为 false.

以下是 OP 发布的minimal, complete reproducible example：

#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";
}

输出：

Insertion of Hinata succeeded
Insertion of Hinata failed
Insertion of Hinata failed

Hinata     | 162.8cm

为什么 std::map::insert 在以下示例中失败？

Why the std::map::insert failed in the following example?

c++

insert

stdmap

c++11

c++17