1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
|
The ti(map) class offers a (sorted) i(associative array). Before using a
tt(map) container the tthi(map) header file must be included.
A tt(map) is filled with emi(key, value) pairs, which may be of any
container-accepted type. Since types are associated with both the key and the
value, we must specify em(two types) in the angle bracket notation, comparable
to the specification we've seen with the tt(pair) container (cf. section
ref(PAIR)). The first type represents the key's type, the second type
represents the value's type. The emi(key) is used to access its associated
information. The tt(map) sorts the keys using their tt(operator<), where the
smallest key values appears as first element in the map. For example, a
tt(map) in which the key is a tt(string) and the value is a tt(double) can be
defined as follows:
verb( map<string, double> object;)
That information is called the emi(value). For example, a phone book uses the
names of people as the key, and uses the telephone number and maybe other
information (e.g., the zip-code, the address, the profession) as value. Since
a tt(map) sorts its keys, the tt(key)'s ti(operator<) must be defined, and it
must be sensible to use it. For example, it is generally a bad idea to use
pointers for keys, as sorting pointers is something different than sorting the
values pointed at by those pointers. In addition to the em(key, value) types,
a third type defines the comparison class, used to compare two keys. By
default, the comparison class is tt(std::less<KeyType>) (cf. section
ref(RELATIONAL)), using the key type's tt(operator<) to compare two key
values. For key type tt(KeyType) and value type tt(ValueType) the map's type
definition, therefore, looks like this:
verb( map<KeyType, ValueType, std::less<KeyType>>)
The two fundamental operations on maps are the storage of em(key, value)
combinations, and the retrieval of values, given their keys. The index
operator using a key as the index, can be used for both. If the index operator
is used as em(lvalue), the expression's tt(rvalue) is inserted into the
map. If it is used as em(rvalue), the key's associated value is retrieved.
Each key can be stored only once in a tt(map). If the same key is entered
again, the new value replaces the formerly stored value, which is lost.
A specific em(key, value) combination can implicitly or explicitly be inserted
into a tt(map). If explicit insertion is required, the em(key, value)
combination must be constructed first. For this, every tt(map) defines a
ti(value_type) which may be used to create values that can be stored in the
tt(map). For example, a value for a tt(map<string, int>) can be constructed as
follows:
verb( map<string, int>::value_type siValue{ "Hello", 1 };)
The tt(value_type) is associated with the tt(map<string, int>): the
type of the key is tt(string), the type of the value is tt(int). Anonymous
tt(value_type) objects are also often used. E.g.,
verb( map<string, int>::value_type{ "Hello", 1 };)
Instead of using the line tt(map<string, int>::value_type(...)) over and
over again, a using declaration is frequently used to reduce typing and to
improve readability:
verb( using StringIntValue = map<string, int>::value_type;)
Now values for the tt(map<string, int>) may be specified this way:
verb( StringIntValue{ "Hello", 1 };)
Alternatively, tt(pairs) may be used to represent em(key, value)
combinations used by maps:
verb( pair<string, int>{ "Hello", 1 };)
|
