std::distance and const vs. non-const iterators

[jsmith]

Hmm, so

#include <iostream>
#include <iterator>
#include <vector>

struct Foo {
   typedef std::vector<char> Container;

   Foo() : vec( 10, 'x' ) {}
 
   void bar() {
       typedef Container::const_iterator ConstIter;
       for( ConstIter i = vec.begin(); i != vec.end(); ++i )
           std::cout << "Element #" << std::distance( vec.begin(), i )
                     << " is " << *i << std::endl;
   }

    Container vec;
};

int main() {
    Foo f;
    f.bar();
}

does not compile, giving the complaint


foo.cpp: In member function `void Foo::bar()':
foo.cpp:13: error: no matching function for call to `distance(
__gnu_cxx::__normal_iterator<char*, std::vector<char, std::allocator<char> >
>, Foo::bar()::ConstIter&)'



because the compiler wants to call the non-const version of vector<char>::begin() since the member function bar() is not const, but the other parameter to std::distance() is a const_iterator and std::distance() of course wants two iterators of the same type. So apparently either there is no implicit conversion function from iterator to const_iterator or the compiler can't figure to call it. Anyway...

So, assuming that I cannot make the member function const for reasons far too complicated to go into here, does anyone have a slick way to solve the problem that is not either 1) change the const_iterator to iterator, or 2) use an obnoxious const_cast?

[Gumbercules]

you could make another const_iterator variable to hold vec.begin(), but that's also inelegant. But sticking in a (ConstIter) isn't so bad. Don't need an actual const_cast.

[Bazzy]

what about this:

const Container &v = vec;
std::cout << "Element #" << std::distance( v.begin(), i )
    << " is " << *i << std::endl;

[jsmith]

Yah, I'm trying to avoid an extra local variable because that's just me :)

I hate being defeated by the language/compiler and having to make another local variable...

[Bazzy]

Why don't you use just the ConstIter constructor?
std::distance( ConstIter(vec.begin()), i )

[jsmith]

Aha... why did I not think of that???

That'll work... no more lines of code needed :)

Thanks.