Question

I hit some behaviour I can’t seem to wrap my head around. Say I have a class defined as follows (a simple example for illustration):

class some_class
{
   public:
        some_class()
        {
            x_["0"] = "0";
            x_["1"] = "1";

            y_[0] = x_.find("0");
            y_[1] = x_.find("1");
        }

        int some_function(int i)
        {
            if(std::find(y_.begin(), y_.end(), x_.find("1")) != y_.end())
            {
                std::cout << "Found!" << std::endl;

                return 1001;
            }

            return -1001;
        }

   private:
       std::unordered_map<string, string> x_;
       std::array<unordered_map<string, string>::iterator, 2> y_;
};

The code compiles with Visual Studio 2012 RC in debug mode (others modes not tested), but during call to some_function the program fails with a following assertion message

… microsoft visual studio 11.0\vc\include\list Line: 289

Expression: list iterators incompatible…

The calling code looks like this

auto vector<int> as;
as.push_back(1);   

auto vector<int> bs;   
auto some = some_class();

std::transform(as.begin(), as.end(), std::inserter(bs, bs.begin()), [=](int i) { return  some.some_function(i); });

Question:

What would be the problem with this kind of an arrangement? I get the code run fine if I declare x_ and y_ in some_function instead of as member variables. The class is being called/used in std::transform like occasions, if that should factored in to the situation.

^{As a tangential aside, it looks like the following declaration will be rejected by the compiler, should it be rejected?}

std::unordered_map<string, string> x_;
std::array<decltype(x_.begin()), 2> y_;

The error message is

error C2228: left of ‘.begin’ must have class/struct/union

Answer 1

Your situation.

#include <unordered_map>
#include <array>
#include <iostream>
#include <string>

class some_class
{
   public:
        some_class()
        {
            x_["0"] = "0";
            x_["1"] = "1";

            y_[0] = x_.find("0");
            y_[1] = x_.find("1");
        }

        int some_function()
        {
            if(std::find(y_.begin(), y_.end(), x_.find("1")) != y_.end())
            {
                std::cout << "Found!" << std::endl;

                return 1001;
            }

            return -1001;
        }

   private:
       std::unordered_map<std::string, std::string> x_;
       std::array<std::unordered_map<std::string, std::string>::iterator, 2> y_;
};

void function(some_class cl)
{
    cl.some_function();
}

int main()
{
    some_class c;
    function(c);
}

Your iterators points on elements of another map, that stored in this after copy-ctor, and in find there is comparison.

    bool operator==(const _Myiter& _Right) const
        {   // test for iterator equality
 #if _ITERATOR_DEBUG_LEVEL == 2
        if (this->_Getcont() == 0
            || this->_Getcont() != _Right._Getcont())
            {   // report error
            _DEBUG_ERROR("list iterators incompatible");
            _SCL_SECURE_INVALID_ARGUMENT;
            }

for decltype –

std::unordered_map<string, string> x_;
std::array<decltype(x_.begin()), 2> y_;

incorrect in class-block, since there is no object. This will works if x_ will be static.