Question

Please consider the following:

// from main file

// first arguement gets enc_array
Rock rock (voice.getEncArray());
// getEncArray() gets a vector of vectors:
// std::vector<std::vector<unsigned int> > enc_array;

// in rock.hpp file, consider members
Rock( const std::vector<std::vector<unsigned int> > &);

std::vector<std::vector<unsigned int> * > remaining;

const std::vector<std::vector<unsigned int> > * population;

// in rock.cpp
Rock::Rock ( const vector<vector<unsigned int> > & v) :
  population (&v),
  ....

// in one of the class member functions
for ( vector<vector<unsigned int> >::const_iterator ci = population->begin(); ci != population->end(); ++ci ) {
    // for some indexes...
    remaining.push_back (& (*ci));      // <------  PROBLEM
}

gcc reports:

error: no matching function for call to 'std::vector<std::vector<unsigned int>*>::push_back(const std::vector<unsigned int>*)'

note: void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::vector<unsigned int>*; _Alloc = std::allocator<std::vector<unsigned int>*>; std::vector<_Tp, _Alloc>::value_type = std::vector<unsigned int>*] <near match>

note:   no known conversion for argument 1 from 'const std::vector<unsigned int>*' to 'std::vector<unsigned int>* const&'

I understand that I’m trying to push the addresses of vector<int> that were considered const to a non-const vector.
After remaining is filled-up, no other method will change its data, so in fact it should be const.
But I cannot declare remaining as a const vector, as it would give an error.

error: no matching function for call to 'std::vector<std::vector<unsigned int>*>::push_back(const std::vector<unsigned int>*) const'

note: candidate is:
note: void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::vector<unsigned int>*; _Alloc = std::allocator<std::vector<unsigned int>*>; std::vector<_Tp, _Alloc>::value_type = std::vector<unsigned int>*] <near match>

note:   no known conversion for argument 1 from 'const std::vector<unsigned int>*' to 'std::vector<unsigned int>* const&'

Do I really need to copy elements from population to remaining? Or is there anything else I can do in order not to have this overhead?

Answer 1

Try

std::vector<const std::vector<unsigned int> *> remaining;