[英]transform_iterator not compatible with std::iterators?
I'm having problems using boost::transform_iterator
s where "ordinary" iterators are expected. 我在使用
boost::transform_iterator
遇到问题,在这里需要“普通”迭代器。 For instance, I want to add all keys from a map to a set. 例如,我想将地图中的所有键添加到集合中。 I wrote the following short snippet:
我写了以下简短代码段:
template <typename K, typename V>
struct map_keys {
typedef const K& result_type;
const K& operator()(const std::pair<K,V>& kvp) const {
return kvp.first;
}
};
int main() {
std::map<int, double> my_map;
std::set<int> my_set;
my_map[1]=1.2;
my_map[2]=2.4;
my_map[4]=4.1;
my_map[6]=12.2;
my_map[123]=3;
typedef map_keys<int, double> mk;
auto b = boost::make_transform_iterator(my_map.begin(), mk()),
e = boost::make_transform_iterator(my_map.end(), mk());
my_set.insert(b,e);
return 0;
}
After the insert
, my_set
contains a single value, -858993460
or 0xcccccccc
. insert
, my_set
包含单个值-858993460
或0xcccccccc
。 Why? 为什么? If I print
*b
in a loop, all the values are printed as expected. 如果我循环打印
*b
,则所有值将按预期打印。
Ok, solved it myself suddenly. 好吧,我自己突然解决了。 The problem is the typedef
result_type
(which is not mentioned in the documentation, but unless I have it, the code does not compile ( result_type
is not a member of map_keys<K,V>
)). 问题是typedef
result_type
(在文档中未提及,但是除非我拥有它,否则代码不会编译( result_type
不是map_keys<K,V>
的成员))。 If I instead typedef it as 如果我改用typedef作为
typedef K result_type;
It works. 有用。 I suppose as it was written earlier, it returned the address of something?
我想像它先前写的那样,它返回了某个地址?
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