std::map 查找无法正常工作
std::map.find() 的目的是，如果有的话，将 map::iterator 返回到它找到的元素，如果没有，则返回到 end() 。我得到 BadPtr 返回给我。完全相同的构造在代码的其他部分中运行良好。这是什么？
class OntologyContainer {
map<string, OntologyClass*> data;
OntologyClass* last_added;
public:
class iterator : public std::iterator<bidirectional_iterator_tag, OntologyClass> {
map<string, OntologyClass*>::iterator itr;
public:
iterator(map<string, OntologyClass*>::iterator it) : itr(it) { }
...
}
iterator begin() {
return iterator(data.begin());
}
iterator end() {
return iterator(data.end());
}
iterator Find(const string & to_find) {
map<string, OntologyClass*>::iterator it = data.find(to_find);
// this is where it fails
return iterator(it);
}
map::iterator 被包装是为了使运算符 * 和 ->分别返回OntologyClass对象和指针：
OntologyClass& operator* () {
return *(itr->second);
}
OntologyClass* operator->() {
return itr->second;
}
std::map.find() is intended to return an map::iterator to an element it found if there is some or to the end() if not. I get BadPtr returned to me. The exactly same construct works fine in the other part of code. What is this?
class OntologyContainer {
map<string, OntologyClass*> data;
OntologyClass* last_added;
public:
class iterator : public std::iterator<bidirectional_iterator_tag, OntologyClass> {
map<string, OntologyClass*>::iterator itr;
public:
iterator(map<string, OntologyClass*>::iterator it) : itr(it) { }
...
}
iterator begin() {
return iterator(data.begin());
}
iterator end() {
return iterator(data.end());
}
iterator Find(const string & to_find) {
map<string, OntologyClass*>::iterator it = data.find(to_find);
// this is where it fails
return iterator(it);
}
map::iterator is wrapped for the sake of making operators * and -> returning OntologyClass objects and pointers respectively:
OntologyClass& operator* () {
return *(itr->second);
}
OntologyClass* operator->() {
return itr->second;
}
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这可能与您继承自
std::iterator的事实有关，使您的迭代器 value_type 为OntologyClass，而不是 指向OntologyClass的指针，这是您的地图迭代器使用的。您如何实现解引用运算符？It might be something to do with the fact that you inherit from
std::iterator<bidirectional_iterator_tag, OntologyClass>, making your iterator value_type to beOntologyClass, rather than a pointer toOntologyClass, which is what your map iterator uses. How are you implementing the dereference operator?如果你可以使用 boost 我建议使用 boost::transform_iterator 。
这个堆栈溢出答案有一个很好的例子来说明如何做到这一点：
迭代器适配器仅迭代映射中的值？< /a>
if you can use boost I suggest using boost::transform_iterator.
This stack overflow answer has a decent example on how to do that:
iterator adapter to iterate just the values in a map?