conversion from std::map<std::basic_string<char>,std::pair<int,int(*)(const std::vector::Mat

Question

I have defined new type:

typedef int(* func) (const std::vector<cv::Mat>&, cv::Mat&);

Then I did class member:

std::map< std::string, std::pair<int,func> > functions;

In function, in first line:

pair<funcId,func> functionSet::getRandomFunction() const
{
    map<string, pair<int,func>>::iterator it = functions.begin();
    std::advance(it, functions.size());

    string name = it->first;
    func function = it->second.second;
    int argumentsNumber = it->second.first;
    funcId id = make_pair(argumentsNumber,name);

    pair<funcId,func> p = make_pair(id,function);

    return p;
}

I got this error:

error: conversion from 'std::map, std::pair&, cv::Mat&)> > ::const_iterator {aka std::_Rb_tree_const_iterator, std::pair&, cv::Mat&)> > >}' to non-scalar type 'std::map, std::pair&, cv::Mat&)> >::iterator {aka std::_Rb_tree_iterator, std::pair&, cv::Mat&)> > >}' requested map>::iterator it = functions.begin();

Answer 1

Your method marked as const, and so this has type const functionSet*. Change you first line:

map<string, pair<int,func>>::const_iterator it = functions.begin();

or if your compiler supports C++11 standard:

auto it = functions.cbegin();

Answer 2

Look at the output again:

conversion from
'std::map, std::pair&, cv::Mat&)> >::const_iterator {...}'
to non-scalar type
'std::map, std::pair&, cv::Mat&)> >::iterator {...}'

So it seems that functions is a const map, its begin() returns a const iterator, and it cannot be converted into a normal iterator.

The reason it is constant because because getRandomFunction is a constant member function and therefore it can see the class members as constants.

As you do not modify the map, simply using const_iterator should solve the problem:

map<string, pair<int,func>>::const_iterator it = functions.begin();