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

Question

Thanks to the good book C++17 in detail of Bartlomiej Filipek. I am discovering some examples which use insert in the context of std::map.

So I visited cppreference.com to have a better idea about how it works.

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

However, at the bottom of the page, there is a valuable and long example which gives us different examples of usage of std::map::insert. I have tested it by myself on my computer to try to understand why with overload 1 and overload 4, the insertion fails.

And honestly, I did not understand why.

I would grateful to you to explain what happens in those 2 specific examples (why the insertion fails), as it seems to be a very interesting example which uses at the same time the notion of "structured bindings".

#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"; }

Something's missing here. When you use hinted insert, you need a call to lower_bound to get that iterator. — Steven Sudit
– Steven Sudit, Commented Aug 9, 2019 at 21:41
can you show the complete code? - something seems to be missing. — skeller
– skeller, Commented Aug 9, 2019 at 21:43
@D.K. "it seems to be a very interesting example which uses at the same time the notion of "structured bindings"." Keep in mind that, the question, has nothing to do with structured bindings here. — JeJo
– JeJo, Commented Aug 9, 2019 at 22:24
@D.K. It's kind of obsolete ever since C++17's insert_or_assign(), but there's an idiom when you use lower_bound() followed by a key comparison to do the equivalent of find(). Then, if it's not found, the iterator is used for the hinted insert. — Steven Sudit
– Steven Sudit, Commented Aug 11, 2019 at 1:03

JeJo · Accepted Answer · 2019-08-09 22:15:03Z

As stated on the cppreference.com the std::map::insert:

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

In // Overload 1:

The iterator it_hinata is pointing to the lastly inserted entry which is {"Hinata"s, 162.8} and if you try to enter the same key-value pair, the insertion fails and hence success2 == false.

In // Overload 4:

The iterator it_hinata is still pointing to the same (firstly) inserted key-value pair(i.e. same {"Hinata"s, 162.8}). Therefore, the same reason in the above case, insertion fails. That means, the size of the map(i.e. karasunoPlayerHeights) remains the same after the insertion call, and the condition std::size(karasunoPlayerHeights) != n evaluated to false.

Following is the minimal, complete reproducible example from what OP posted:

#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"; }

which outputs:

Insertion of Hinata succeeded Insertion of Hinata failed Insertion of Hinata failed Hinata | 162.8cm

Collectives™ on Stack Overflow

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

1 Answer 1

Comments

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Related