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      This section describes some common pitfalls with BOOST_FOREACH.
    
      Since BOOST_FOREACH is a macro, it must have exactly two
      arguments, with exactly one comma separating them. That's not always convenient,
      especially when the type of the loop variable is a template. Consider trying
      to iterate over a std::map:
    
std::map<int,int> m; // ERROR! Too many arguments to BOOST_FOREACH macro. BOOST_FOREACH(std::pair<int,int> p, m) // ...
One way to fix this is with a typedef.
std::map<int,int> m; typedef std::pair<int,int> pair_t; BOOST_FOREACH(pair_t p, m) // ...
Another way to fix it is to predeclare the loop variable:
std::map<int,int> m; std::pair<int,int> p; BOOST_FOREACH(p, m) // ...
      Under the covers, BOOST_FOREACH uses iterators to traverse
      the element sequence. Before the loop is executed, the end iterator is cached
      in a local variable. This is called hoisting, and it is
      an important optimization. It assumes, however, that the end iterator of the
      sequence is stable. It usually is, but if we modify the sequence by adding
      or removing elements while we are iterating over it, we may end up hoisting
      ourselves on our own petard.
    
Consider the following code:
std::vector<int> vect(4, 4); BOOST_FOREACH(int i, vect) { vect.push_back(i + 1); }
This code will compile, but it has undefined behavior. That is because it is logically equivalent to the following:
std::vector<int> vect(4, 4); for(std::vector<int>::iterator it1 = vect.begin(), it2 = vect.end(); it1 != it2; ++it1) { int i = *it1; vect.push_back(i + 1); // Oops! This invalidates it1 and it2! }
      The call to vect.push_back()
      will cause all iterators into vect
      to become invalid, including it1
      and it2. The next iteration
      through the loop will cause the invalid iterators to be used. That's bad news.
    
      The moral of the story is to think twice before adding and removing elements
      from the sequence over which you are iterating. If doing so could cause iterators
      to become invalid, don't do it. Use a regular for
      loop instead.