How to insert a key/value pair in unordered_map in C++?

Question

I want to construct a graph whose Vertex is defined as follows:

struct Vertex {
    string key;              // key of the vertex.
    vector<string> adj;      // keys of adjacent vertices.
    vector<double> weights;  // weights of edges to adjacent vertices.

    Vertex(string x) : key(x) {}
    Vertex(const Vertex &other) {
        cout << "Copy Constructor\n";
        this->key = other.key;
        this->adj = other.adj;
        this->weights = other.weights;
    }
    Vertex &operator=(const Vertex &other) {
        cout << "Copy Assignment Operator\n";
        if (this != &other) {
            this->key = other.key;
            this->adj = other.adj;
            this->weights = other.weights;
        }
        return *this;
    }
};

First I define an unordered_map which has the pair of <string, Vertex> and string a = "a";.

I can construct the graph as follow:

graph.insert(make_pair(a, Vertex(a)));

But the following

graph[a] = Vertex(a);

doesn't work. And it outputs the error:

In file included from /usr/include/c++/9/bits/hashtable_policy.h:34,
                 from /usr/include/c++/9/bits/hashtable.h:35,
                 from /usr/include/c++/9/unordered_map:46,
                 from construct_graph.cpp:3:
/usr/include/c++/9/tuple: In instantiation of ‘std::pair<_T1, _T2>::pair(std::tuple<_Args1 ...>&, std::tuple<_Args2 ...>&, std::_Index_tuple<_Indexes1 ...>, std::_Index_tuple<_Indexes2 ...>) [with _Args1 = {const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&}; long unsigned int ..._Indexes1 = {0}; _Args2 = {}; long unsigned int ..._Indexes2 = {}; _T1 = const std::__cxx11::basic_string<char>; _T2 = Vertex]’:
/usr/include/c++/9/tuple:1663:63:   required from ‘std::pair<_T1, _T2>::pair(std::piecewise_construct_t, std::tuple<_Args1 ...>, std::tuple<_Args2 ...>) [with _Args1 = {const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&}; _Args2 = {}; _T1 = const std::__cxx11::basic_string<char>; _T2 = Vertex]’
/usr/include/c++/9/ext/new_allocator.h:146:4:   required from ‘void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = std::pair<const std::__cxx11::basic_string<char>, Vertex>; _Args = {const std::piecewise_construct_t&, std::tuple<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&>, std::tuple<>}; _Tp = std::__detail::_Hash_node<std::pair<const std::__cxx11::basic_string<char>, Vertex>, true>]’
/usr/include/c++/9/bits/alloc_traits.h:483:4:   required from ‘static void std::allocator_traits<std::allocator<_CharT> >::construct(std::allocator_traits<std::allocator<_CharT> >::allocator_type&, _Up*, _Args&& ...) [with _Up = std::pair<const std::__cxx11::basic_string<char>, Vertex>; _Args = {const std::piecewise_construct_t&, std::tuple<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&>, std::tuple<>}; _Tp = std::__detail::_Hash_node<std::pair<const std::__cxx11::basic_string<char>, Vertex>, true>; std::allocator_traits<std::allocator<_CharT> >::allocator_type = std::allocator<std::__detail::_Hash_node<std::pair<const std::__cxx11::basic_string<char>, Vertex>, true> >]’
/usr/include/c++/9/bits/hashtable_policy.h:2086:36:   required from ‘std::__detail::_Hashtable_alloc<_NodeAlloc>::__node_type* std::__detail::_Hashtable_alloc<_NodeAlloc>::_M_allocate_node(_Args&& ...) [with _Args = {const std::piecewise_construct_t&, std::tuple<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&>, std::tuple<>}; _NodeAlloc = std::allocator<std::__detail::_Hash_node<std::pair<const std::__cxx11::basic_string<char>, Vertex>, true> >; std::__detail::_Hashtable_alloc<_NodeAlloc>::__node_type = std::__detail::_Hash_node<std::pair<const std::__cxx11::basic_string<char>, Vertex>, true>]’
/usr/include/c++/9/bits/hashtable_policy.h:701:8:   required from ‘std::__detail::_Map_base<_Key, _Pair, _Alloc, std::__detail::_Select1st, _Equal, _H1, _H2, _Hash, _RehashPolicy, _Traits, true>::mapped_type& std::__detail::_Map_base<_Key, _Pair, _Alloc, std::__detail::_Select1st, _Equal, _H1, _H2, _Hash, _RehashPolicy, _Traits, true>::operator[](const key_type&) [with _Key = std::__cxx11::basic_string<char>; _Pair = std::pair<const std::__cxx11::basic_string<char>, Vertex>; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, Vertex> >; _Equal = std::equal_to<std::__cxx11::basic_string<char> >; _H1 = std::hash<std::__cxx11::basic_string<char> >; _H2 = std::__detail::_Mod_range_hashing; _Hash = std::__detail::_Default_ranged_hash; _RehashPolicy = std::__detail::_Prime_rehash_policy; _Traits = std::__detail::_Hashtable_traits<true, false, true>; std::__detail::_Map_base<_Key, _Pair, _Alloc, std::__detail::_Select1st, _Equal, _H1, _H2, _Hash, _RehashPolicy, _Traits, true>::mapped_type = Vertex; std::__detail::_Map_base<_Key, _Pair, _Alloc, std::__detail::_Select1st, _Equal, _H1, _H2, _Hash, _RehashPolicy, _Traits, true>::key_type = std::__cxx11::basic_string<char>]’
/usr/include/c++/9/bits/unordered_map.h:986:20:   required from ‘std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::mapped_type& std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::operator[](const key_type&) [with _Key = std::__cxx11::basic_string<char>; _Tp = Vertex; _Hash = std::hash<std::__cxx11::basic_string<char> >; _Pred = std::equal_to<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, Vertex> >; std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::mapped_type = Vertex; std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::key_type = std::__cxx11::basic_string<char>]’
construct_graph.cpp:34:12:   required from here
/usr/include/c++/9/tuple:1674:70: error: no matching function for call to ‘Vertex::Vertex()’
 1674 |         second(std::forward<_Args2>(std::get<_Indexes2>(__tuple2))...)
      |                                                                      ^
construct_graph.cpp:14:5: note: candidate: ‘Vertex::Vertex(const Vertex&)’
   14 |     Vertex(const Vertex &other) {
      |     ^~~~~~
construct_graph.cpp:14:5: note:   candidate expects 1 argument, 0 provided
construct_graph.cpp:13:5: note: candidate: ‘Vertex::Vertex(std::string)’
   13 |     Vertex(string x) : key(x) {}
      |     ^~~~~~
construct_graph.cpp:13:5: note:   candidate expects 1 argument, 0 provided

It seems that it can not find the constructor to construct a Vertex object in graph. But I don't understand why it cannot find the constructor.

Here is the complete code:

#include <iostream>
#include <string>
#include <unordered_map>
#include <vector>

using namespace std;

struct Vertex {
    string key;              // key of the vertex.
    vector<string> adj;      // keys of adjacent vertices.
    vector<double> weights;  // weights of edges to adjacent vertices.

    Vertex(string x) : key(x) {}
    Vertex(const Vertex &other) {
        cout << "Copy Constructor\n";
        this->key = other.key;
        this->adj = other.adj;
        this->weights = other.weights;
    }
    Vertex &operator=(const Vertex &other) {
        cout << "Copy Assignment Operator\n";
        if (this != &other) {
            this->key = other.key;
            this->adj = other.adj;
            this->weights = other.weights;
        }
        return *this;
    }
};

void construct_graph() {
    unordered_map<string, Vertex> graph;
    string a = "a";
    // graph[a] = Vertex(a);  // doesn't work.
    graph.insert(make_pair(a, Vertex(a)));
}

// g++ construct_graph.cpp && ./a.out
int main(void) {
    construct_graph();
    return 0;
}

Does this answer your question? Why does the C++ map type argument require an empty constructor when using []? — JaMiT
– JaMiT, Commented Nov 5, 2023 at 9:39

HolyBlackCat · Accepted Answer · 2023-11-05 08:14:13Z

4

When graph[a] is called, it can't know in advance what you're going to do with the result (whether you're going to do = Vertex(a) or something else), so the value will first be constructed using its default constructor. Then = Vertex(a) performs assignment.

Other ways of inserting elements, such as .insert(), immediately receive the constructor arguments, so they can call the correct constructor directly.

The compiler doesn't generate a default constructor for your class, because you added a custom one. Having a default constructor is a good idea (even though using [] on maps is not; prefer .try_emplace() and .insert_or_assign() to all other methods), add it with Vertex() {}.

Also, don't manually define copy constructors/assignments, move constructors/assignments, and destructors, unless you actually need to (in this case you don't), because the compiler will likely do a better job at generating them. E.g. in this case by manually defining the copy operations you lost the move operations.

edited Nov 5, 2023 at 8:14

answered Nov 5, 2023 at 8:03

HolyBlackCat

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2 Comments

Adrian Mole Over a year ago

It's possible that the OP defined their own copy c'tor and assignment so that they can check when/if they're being called (as the cout statements in them would indicate).

HolyBlackCat Over a year ago

@AdrianMole Yep. Warning them just in case.

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