Losing fields when filling C++ std::map - c++

Question

I have a problem in using the std::map correctly. The class Example is a class with an ID, a label, a vector of keypoints and a descriptor matrix. The class Examples is a map for retrieving an example given its ID. The examples are read from files on disk, stored in the map, then used later.
Even if it is conceptually very simple, I am not able to fill the map properly.
I have the following class:
class Example
{
public:
std::string id;
std::string label;
std::vector<cv::KeyPoint> keypoints;
cv::Mat descriptors;
Example(std::string id_, std::string label_)
: id(id_), label(label_)
{
// ... nothing ...
}
string to_string() const
{
stringstream ss;
ss << "#" << id
<< " (" << label << ")"
<< " - #keypoints " << keypoints.size()
<< ", descr " << descriptors.rows << " x " << descriptors.cols;
return ss.str();
} // to_string
}; // class Example
ostream& operator <<(ostream & out, const Example &ex)
{
out << ex.to_string();
return out;
} // operator <<
And this one:
// OLD: class Examples : public std::map<std::string, Example*> {
class Examples {
// New line after Martini's comment
std::map<std::string, Example*> _map;
[...]
void fill() {
// create an example
Example *example = new Example(id, label);
// inputstream in
// Read all the keypoints
cv::KeyPoint p;
for(int i=0; ... ) {
in.read(reinterpret_cast<char *>(&p), sizeof(cv::KeyPoint));
example->keypoints.push_back(p); // push_back copies p
} // for
// ... misc code
cv::Mat descr(n_keypoints, d_size, cv_type, cv::Scalar(1));
// ... read Mat from inputstream in, then assign it to the example
example->descriptors = descr;
// SEE THE OUTPUT BELOW
std::cout << "INSERT THIS: " << (*example) << std::endl;
_map.insert(std::pair<string,Example*>(id, example));
std::cout << "READ THIS: " << *(get_example(id)) << std::endl;
// ... other code
} // fill
// Code modified after Martini's comment.
Example* get_example(const std::string &id) const {
std::map<std::string, Example*>::const_iterator it = _map.find(id);
if( it == _map.end()) {
// ... manage error
// ... then exit
} // if
return it->second;
} // get_example
} // class Examples
The output from the insert/get lines is:
INSERT THIS: #122225084 (label) - #keypoints 711, descr 711 x 128
READ THIS: #122225084 (label) - #keypoints 0, descr 0 x 0
In the insert I had a pointer to an example with 711 keypoints and a 711x128 descriptor matrix. If I read the example using its ID right after the insert, I get a pointer to an example with 0 keypoints and an empty matrix.
What am I doing wrong?

Answer 1

Looking into your code one possible explanation is that you already have element in the map with the same key. To diagnose that first of all print value of pointer before you add object and after that (something like this):
std::cout << "INSERT THIS: " << (void *)example << " " << (*example) << std::endl;
_map.insert(std::pair<string,Example*>(id, example));
std::cout << "READ THIS: " << (void *)get_example(id) << " " << *(get_example(id)) << std::endl;
Next or another way is to check result of insert:
if( !_map.insert(std::pair<string,Example*>(id, example)).second )
std::cout << "ERROR: example:" << id << " is already there";
If you want to override element unconditionally you can use oprator[]:
_map[ id ] = example;
If there are really duplicates you will get memory leak (you are getting it anyway) so I would strongly recommend to use smart pointer to store data in your map.