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
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
|
/*
The way to remove elements from a hash_map isn't intuitively obsious:
a for_each construction can't be used, as the erase() during the for_each
fucks up the for_each process.
This also holds true for other gerneric algorithms in which iterators are
used, like remove_if etc.
The problem is probably caused by the fact that the keys in a hash_map
aren't ordered, and therefore reordering keys isn't a well defined
operation.
the approach taken here is to collect the relevant keys in a set, and then
iterate over the set of keys, removing the corresponding elements from the
hash_map
*/
#include <algorithm>
#include <iostream>
#include <hash_map>
#include <set>
class Eraser
{
public:
Eraser(set <char const *> &keys)
:
keys(keys)
{}
void operator()(pair<char const *, int> const &it)
{
if (*it.first == 'a' || *it.first == 'e')
keys.insert(set<char const *>::value_type(it.first));
}
private:
set<char const *>
&keys;
};
class Remove
{
public:
Remove(hash_map <char const *, int> &hm)
:
hm(hm)
{}
void operator()(char const *cp)
{
hm.erase(cp);
}
private:
hash_map<char const *, int>
&hm;
};
class List
{
public:
bool operator()(pair<char const *, int> const &it)
{
cerr << it.first << " = " << it.second << '\n';
return true;
}
};
int main(int argc, char **argv)
{
hash_map<char const *, int>
hm;
hm["a"] = 1;
hm["b"] = 2;
hm["c"] = 3;
hm["d"] = 4;
hm["e"] = 5;
hm["f"] = 6;
hm["g"] = 7;
hm["h"] = 8;
set<char const *> keys;
for_each(hm.begin(), hm.end(), Eraser(keys));
for_each(keys.begin(), keys.end(), Remove(hm));
for_each(hm.begin(), hm.end(), List());
}
|
