问题
When sorting eg a vector of pairs :
vector<pair<int, double>> v;
sort(v.begin(), v.end());
You don't need to specify a sorting criterion to have a sorting based on the lexicographical order of the pairs since, when not elseway specified, a lexicographical comparison applies.
Is a similar behaviour standard for tuples as well ?
In VS2012 this compiles
vector<tuple<int, double, char>> tv;
sort(tv.begin(), tv.end());
but is it standard mandated to do so ?
回答1:
They do, see operator==,!=,<,<=,>,>=(std::tuple):
operator== operator!= operator< operator<= operator> operator>=lexicographically compares the values in the tuple
回答2:
According to standard [20.4.2.7 Relational operators]: Yes
Overloaded operator for tuple:
template<class... TTypes, class... UTypes>
constexpr bool operator<(const tuple<TTypes...>& t,
const tuple<UTypes...>& u);
Returns: The result of a lexicographical comparison between t and u.
The result is defined as:
(bool)(get<0>(t) < get<0>(u)) ||(!(bool)(get<0>(u) < get<0>(t)) && ttail < utail)
where rtail for some tuple r is a tuple containing all but the first element of r.
For any two zero-length tuples e and f, e < f returns false.
回答3:
In 20.4.2.7 Relational operators [tuple.rel]
template<class... TTypes, class... UTypes>
bool operator<(const tuple<TTypes...>& t, const tuple<UTypes...>& u);
Returns: The result of a lexicographical comparison between t and u. The result is defined as:
(bool)(get<0>(t) < get<0>(u)) || (!(bool)(get<0>(u) < get<0>(t)) && ttail < utail)
where rtail for some tuple r is a tuple containing all but the first element of r. For any two zero-length tuples e and f, e < f returns false.
So no, they don't have an implicit one, they have an explicit
来源：https://stackoverflow.com/questions/23827036/do-tuples-have-an-implicit-lexicographical-comparison