Get index of current element in C++ range-based for-loop

Assuming str is a std::string or other object with contiguous storage:

std::cin >> str;
for (char& c : str)
    if (c == 'b') v.push_back(&c - &str[0]);

Maybe it's enough to have a variable i?

unsigned i = 0;
for ( char c : str ) {
  if ( c == 'b' ) vector.push_back(i);
  ++i;
}

That way you don't have to change the range-based loop.

The range loop will not give you the index. It is meant to abstract away such concepts, and just let you iterate through the collection.