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//assuming v has at least 1 element
if ( std::equal(v.begin() + 1, v.end(), v.begin()) )
{
//all equal
}
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if ( std::adjacent_find( myvector.begin(), myvector.end(), std::not_equal_to<>() ) == myvector.end() )
{
std::cout << "All elements are equal each other" << std::endl;
}