I'm reading David Abrahams's C++ Template Metaprogramming, on ch 5.6 Sequence Equality he says:
It's important, particularly when handling computed results, not to fall into the trap of relying on sequence type identity. For example, you should not expect the following assertion to pass:
BOOST_STATIC_ASSERT(( // error boost::is_same< mpl::pop_back<mpl::vector<int, short> >::type , mpl::vector<int> >::value ));For most purposes, the two types being compared above will act the same, and most of the time you'll never notice a difference. That said, the result of using mpl::pop_back on a specialization of mpl::vector will not be another specialization of mpl::vector!
As you saw in our exploration of dimensional analysis in Chapter 3, a function template that can only be called with two identical types is likely not to work as expected if those types are sequences. The same goes for a class template partial specialization that matches only when two type arguments are identical.
The correct way to check for sequence equality is always to use the equal algorithm, as follows:
BOOST_STATIC_ASSERT(( // OK mpl::equal< mpl::pop_back<mpl::vector<int, short> >::type , mpl::vector<int> >::value ));
I don't understand here: "That said, the result of using mpl::pop_back on a specialization of mpl::vector will not be another specialization of mpl::vector", then what is the result of mpl::popback here?
Boost manual on pop_back says the Return type is "Back Extensible Sequence", isn't mpl::vector a back extensible sequence?
