为什么在以下示例中 std::map::insert 失败?

感谢Bartlomiej Filipek的好书C++17。我发现了一些在std::map上下文中使用insert的例子。

所以我访问了 cppreference.com，以更好地了解它是如何工作的。

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

但是，在页面底部，有一个有价值的长示例，它为我们提供了不同的std::map::insert使用示例。 我已经在我的计算机上自己测试了它，试图理解为什么过载1和重载 4会导致插入失败。

老实说，我不明白为什么。

我希望您解释一下这两个具体示例中发生了什么(为什么插入失败(，因为它似乎是一个非常有趣的示例，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 ? " succeededn" : " failedn");
}
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 << "cmn";
}