问题
I was browsing cppreference and saw that vector's comparison operations have been removed in C++20, and the spaceship operator (<=>) has been introduced. The same thing can be seen for many other standard library containers like set and map.
How do I do the comparisons in the new standard? Also, will C++20 start giving errors on older code?
回答1:
If you continue to browse on the reference site a little, you might come to the section on default comparisons, which simply states that:
In brief, a class that defines
operator<=>automatically gets compiler-generated operators<,<=,>, and>=.
So, if the "spaceship" operator exists for a class, the compiler will auto-generate the remaining comparison operators using the result of the <=> operator.
Note that the == operator is not generated (even though it should be possible), but std::vector keeps an overload of operator==.
As for:
will C++ 20 start giving errors on older codes ?
No, it will not.
When you build with a C++20 compiler, the standard library used with it should also be made for C++20 and thus implement the <=> operator, which will then be used as explained above.
However, if you use a C++20 compiler to build with an older standard library, that older standard library will still have the older comparison operators implemented.
来源：https://stackoverflow.com/questions/60999163/why-have-comparison-operators-been-removed-from-standard-library-containers-in-c