error: no match for 'operator[]' in... <near match> - c++

Question

This fail to compile in gcc 4.1.2 / RedHat 5 :
#include <string>
#include <vector>
#include <map>
class Toto {
public:
typedef std::string SegmentName;
};
class Titi {
public:
typedef Toto::SegmentName SegmentName; // import this type in our name space
typedef std::vector<SegmentName> SegmentNameList;
SegmentNameList segmentNames_;
typedef std::map<SegmentName, int> SegmentTypeContainer;
SegmentTypeContainer segmentTypes_;
int getNthSegmentType(unsigned int i) const {
int result = -1;
if(i < segmentNames_.size())
{
SegmentName name = segmentNames_[i];
result = segmentTypes_[ name ];
}
return result;
}
};
The error is :
error: no match for 'operator[]' in '(...)segmentTypes_[name]'
/usr/lib/gcc/x86_64-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/stl_map.h:340:
note: candidates are: _Tp& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const _Key&)
[with _Key = std::basic_string<char, std::char_traits<char>, std::allocator<char> >, _Tp = int, _Compare = std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, _Alloc = std::allocator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, int> >]
Why ? The map is rather straightforward. I guess this has to do with the typedefs, but what is wrong ?
[edit] Even if I remove all the typedefs and use std::string everywhere, the problem persists... Am I misusing maps ?

Answer 1

std::map::operator[] is non-const and you're trying to use it from a const method.
You could achieve this using std::map::find, which returns a const_iterator:
SegmentTypeContainer::const_iterator iter = segmentTypes_.find(name);
If you're using C++11, you could also use std::map::at, which will throw an exception if the key is not found in the map:
result = segmentTypes_.at(name);

Answer 2

std::map::operator[] is not a const method, but you are calling it from a const method of your class. The reason for this is that it adds an element if the key is not present.
You can use C++11 at():
result = segmentTypes_.at(name); // throws exception if key not present.
or use std::map::find.
SegmentTypeContainer::const_iterator it = segmentTypes_.find(name);
if (it != segmentTypes_.end())
{
// OK, element with key name is present
result = it->second;
}