Question

I have the following code:

struct STFRandomTreeFunction
{
typedef double (*function_ptr)(const STFDataPoint& data, boost::unordered_map& preloaded_images);
};

struct STFRandomTreeFunctor
{
private:
    boost::unordered_map<std::string, cv::Mat> *image_cache;
    STFRandomTreeFunction::function_ptr function;
    std::string function_id;

public:
    STFRandomTreeFunctor(boost::unordered_map<std::string, cv::Mat>& cache, STFRandomTreeFunction::function_ptr function, std::string function_id)
    :image_cache(&cache), function(function), function_id(function_id){}

    std::string get_function_id(){
        return function_id;
    }

    double operator()(const TrainingDataPoint& data_point){
        return function(data_point, *image_cache);
    }
};

unordered_map<string, STFRandomTreeFunctor> lut;

double a(const STFDataPoint& b, unordered_map<string, Mat>& c){
    return 5;
}

int main(int argc, char* argv[]) {

    unordered_map<string, Mat> cache;
    lut["a"] = STFRandomTreeFunctor(cache, a, "a");
}

when I try to build I get the following error: boost/unordered/detail/allocate.hpp:262:1: error: no matching function for call to ‘STFRandomTreeFunctor::STFRandomTreeFunctor()’

But I have no idea why boost is trying to call STFRandomTreeFunctor(), is this because when we create the empty map, it tries to create a STFRandomTreeFunctor? If so, how can I work around this?

Thanks

Answer 1

The call

lut["a"] = x;

will first create an empty entry in themap (cfr. operator[]: “Effects:
If the container does not already contain an elements with a key equivalent to k, inserts the value std::pair<key_type const, mapped_type>(k, mapped_type())“). To construct mapped_type() it needs a default constructor.

If you want to avoid that, use the insert function:

bool inserted=lut.insert( std::make_pair("a",x) ).second;
assert(inserted); // unless it's ok to not overwrite already inserted keys

But this way, you’ll need a copy constructor (or a move-constructor), since the newly created pair will be copied into the map.