C++ variadic template iterate vector and compare elements - c++

Question

I have a variadic class template
template <size_t ...T>
struct Foo
{
std::vector<size_t> t;
bool IsEqual()
{
//??
}
};
which I want to use like:
Foo<1,2,3,4> foo;
foo.data = {1,2,3,4};
foo.IsEqual();
How can I implement IsEqual to iterate and compare every element of the vector and return false / true if the elements are in the same order as the template parameters?

Answer 1

Use the index sequence trick:
bool IsEqual()
{
return t.size() == sizeof...(T) &&
IsEqual(std::make_index_sequence<sizeof...(T)>{});
}
with:
template <size_t... Is>
bool IsEqual(std::index_sequence<Is...> ) {
bool valid = true;
using expander = int[];
expander{0,
(valid = valid && t[Is] == T,
0)...
};
return valid;
}
Could even do this in one function by taking advantage of the fact that every value computation and side effect in an initializer-clause is sequenced before the next one by doing this in one go:
bool IsEqual()
{
if (t.size() == sizeof...(T)) {
auto it = t.begin();
bool valid = true;
using expander = int[];
expander{0,
(valid = valid && *it++ == T,
0)...
};
return valid;
}
else {
return false;
}
}

Answer 2

Simply unpack template arguments.
template <size_t ...T>
struct Foo
{
std::vector<size_t> t;
bool IsEqualTemplate(size_t index)
{
return true;
}
template <typename FIRSTARG, typename ...OTHERARGS>
bool IsEqualTemplate(size_t index, FIRSTARG firstArg, OTHERARGS... otherArgs)
{
return t[index] == firstArg && IsEqualTemplate(index + 1, otherArgs...);
}
bool IsEqual()
{
return t.size() == sizeof...(T) ? IsEqualTemplate(0, T...) : false;
}
};