问题
According to cppreference, std::type_info::operator!= gets removed with C++20, however, std::type_info::operator== apparently remains.
What's the reasoning behind? I might agree on comparing for inequality being meaningless, but then comparing for equality would be just as meaningless as well, wouldn't it?
Similarly, operator!= of many other standard library types, including containers such as std::unordered_map::operator!= and std::unordered_set::operator!= will be removed in C++20 according to cppreference.
Having to write if(!(id1 == id2)) doesn't make any code any clearer compared to if(id1 != id2), in contrast, just the opposite...
回答1:
In C++20 the way that the relational operators work was changed, notably with the introduction of the spaceship <=> operator. In particular, If you only provide operator==, then a != b is rewritten to !(a == b).
From [over.match.oper]/3.4:
The rewritten candidate set is determined as follows:
- For the relational ([expr.rel]) operators, the rewritten candidates include all non-rewritten candidates for the expression x <=> y.
- For the relational ([expr.rel]) and three-way comparison ([expr.spaceship]) operators, the rewritten candidates also include a synthesized candidate, with the order of the two parameters reversed, for each non-rewritten candidate for the expression y <=> x.
- For the != operator ([expr.eq]), the rewritten candidates include all non-rewritten candidates for the expression x == y.
- For the equality operators, the rewritten candidates also include a synthesized candidate, with the order of the two parameters reversed, for each non-rewritten candidate for the expression y == x.
- For all other operators, the rewritten candidate set is empty.
And [over.match.oper]/9:
If a rewritten operator== candidate is selected by overload resolution for an operator @, its return type shall be cv bool, and x @ y is interpreted as:
- if @ is != and the selected candidate is a synthesized candidate with reversed order of parameters, !(y == x),
- otherwise, if @ is !=, !(x == y),
- otherwise (when @ is ==), y == x,
in each case using the selected rewritten operator== candidate.
As such, an explicit overload for operator!= is no longer necessary. The removal of the operator has not changed comparison semantics.
All containers have had their operator!= removed, as far as I can tell (check e.g. the vector synopsis). The only exceptions are the container adaptors std::queue and std::stack: my guess is that it is to preserve backwards compatibility when used with third-party containers, in case the equality operators are not symmetric.
回答2:
We don't need a library provided operator!= anymore. Providing operator== allows the compiler to do some juggling and evaluate a != b in terms of a == b, all on its own.
[over.match.oper]
3 For a unary operator @ with an operand of a type whose cv-unqualified version is T1, and for a binary operator @ with a left operand of a type whose cv-unqualified version is T1 and a right operand of a type whose cv-unqualified version is T2, four sets of candidate functions, designated member candidates, non-member candidates, built-in candidates, and rewritten candidates, are constructed as follows:
3.4.3 For the != operator ([expr.eq]), the rewritten candidates include all non-rewritten candidates for the expression x == y.
std::type_info and many more library types had their operator!= removed as part of P1614 - The Mothership has Landed.
来源：https://stackoverflow.com/questions/58319928/why-is-operator-removed-in-c20-for-many-standard-library-types