unordered_map::unordered_map

Constructs a container object.

unordered_map(
    const unordered_map& Right
);
explicit unordered_map(
    size_type Bucket_count = N0,
    const Hash& Hash = Hash(),
    const Comp& Comp = Comp(),
    const Allocator& Al = Allocator()
);
unordered_map(
    unordered_map&& Right
);
unordered_map(
    initializer_list<Type> IList
);
unordered_map(
    initializer_list<Type> IList, 
    size_type Bucket_count
);
unordered_map(
    initializer_list<Type> IList, 
    size_type Bucket_count, 
    const Hash& Hash
);
unordered_map(
    initializer_list<Type> IList, 
    size_type Bucket_count, 
    const Hash&  Hash,
    KeyEqual& equal
);
unordered_map(
    initializer_list<Type> IList, 
    size_type Bucket_count,
    const Hash&  Hash,
    KeyEqual& Equal
    const Allocator& Al
);
template<class InIt>
    unordered_map(
        InputIterator First, 
        InputIterator Last,
        size_type Bucket_count = N0,
        const Hash& Hash = Hash(),
        const Comp& Comp = Comp(),
        const Allocator& Al = Alloc()
    );

Parameters

Remarks

The first constructor specifies a copy of the sequence controlled by right. The second constructor specifies an empty controlled sequence. The third constructor inserts the sequence of element values [first, last). The fourth constructor specifies a copy of the sequence by moving right.

All constructors also initialize several stored values. For the copy constructor, the values are obtained from Right. Otherwise:

the minimum number of buckets is the argument Bucket_count, if present; otherwise it is a default value described here as the implementation-defined value N0.

the hash function object is the argument Hash, if present; otherwise it is Hash().

The comparison function object is the argument Comp, if present; otherwise it is Pred().

The allocator object is the argument Al, if present; otherwise, it is Alloc().

Example

// std__unordered_map__unordered_map_construct.cpp 
// compile with: /EHsc 
#include <unordered_map> 
#include <iostream> 
#include <initializer_list>

using namespace std;

using Mymap = unordered_map<char, int>;

int main()
{
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

    // display contents " [c 3] [b 2] [a 1]" 
    for (const auto& c : c1) {
        cout << " [" << c.first << ", " << c.second << "]";
    }
    cout << endl;

    Mymap c2(8,
        tr1::hash<char>(),
        equal_to<char>(),
        allocator<pair<const char, int> >());

    c2.insert(Mymap::value_type('d', 4));
    c2.insert(Mymap::value_type('e', 5));
    c2.insert(Mymap::value_type('f', 6));

    // display contents " [f 6] [e 5] [d 4]" 
    for (const auto& c : c2) {
        cout << " [" << c.first << ", " << c.second << "]";
    }
    cout << endl;

    Mymap c3(c1.begin(),
        c1.end(),
        8,
        tr1::hash<char>(),
        equal_to<char>(),
        allocator<pair<const char, int> >());

    // display contents " [c 3] [b 2] [a 1]" 
    for (const auto& c : c3) {
        cout << " [" << c.first << ", " << c.second << "]";
    }
    cout << endl;

    Mymap c4(move(c3));

    // display contents " [c 3] [b 2] [a 1]" 
    for (const auto& c : c4) {
        cout << " [" << c.first << ", " << c.second << "]";
    }
    cout << endl;
    cout << endl;

    // Construct with an initializer_list
    unordered_map<int, char> c5({ { 5, 'g' }, { 6, 'h' }, { 7, 'i' }, { 8, 'j' } });
    for (const auto& c : c5) {
        cout << " [" << c.first << ", " << c.second << "]";
    }
    cout << endl;

    // Initializer_list plus size
    unordered_map<int, char> c6({ { 5, 'g' }, { 6, 'h' }, { 7, 'i' }, { 8, 'j' } }, 4);
    for (const auto& c : c1) {
        cout << " [" << c.first << ", " << c.second << "]";
    }
    cout << endl;
    cout << endl;

    // Initializer_list plus size and hash
    unordered_map<int, char, tr1::hash<char>> c7(
        { { 5, 'g' }, { 6, 'h' }, { 7, 'i' }, { 8, 'j' } }, 
        4, 
        tr1::hash<char>()
    );
   
    for (const auto& c : c1) {
        cout << " [" << c.first << ", " << c.second << "]";
    }
    cout << endl;

    // Initializer_list plus size, hash, and key_equal
    unordered_map<int, char, tr1::hash<char>, equal_to<char>> c8(
        { { 5, 'g' }, { 6, 'h' }, { 7, 'i' }, { 8, 'j' } }, 
        4, 
        tr1::hash<char>(), 
        equal_to<char>()
    );
    
    for (const auto& c : c1) {
        cout << " [" << c.first << ", " << c.second << "]";
    }
    cout << endl;

    // Initializer_list plus size, hash, key_equal, and allocator
    unordered_map<int, char, tr1::hash<char>, equal_to<char>> c9(
        { { 5, 'g' }, { 6, 'h' }, { 7, 'i' }, { 8, 'j' } },
        4,
        tr1::hash<char>(),
        equal_to<char>(),
        allocator<pair<const char, int> >()
    );
    
    for (const auto& c : c1) {
        cout << " [" << c.first << ", " << c.second << "]";
    }
    cout << endl;
}

[a, 1] [b, 2] [c, 3]
 [d, 4] [e, 5] [f, 6]
 [a, 1] [b, 2] [c, 3]
 [a, 1] [b, 2] [c, 3]
 [5, g] [6, h] [7, i] [8, j]
 [5, g] [6, h] [7, i] [8, j]
 [5, g] [6, h] [7, i] [8, j]
 [5, g] [6, h] [7, i] [8, j]
 [5, g] [6, h] [7, i] [8, j]

Requirements

Header: <unordered_map>

Namespace: std

See Also

Reference

<unordered_map>

unordered_map Class