C ++是否有标准方法对两个std :: sets，vector等进行三态比较？

Question

Is there an equivalent to std::string::compare for other std types, such as std::set? I'm looking for a built-in algorithm that can do the comparison just once and give me something like -1, 0, 1 for less than, equal, greater than, respectively.

Answer 1

No there is no single algorithm which will do that. But you can compose such a function with the use of std::mismatch .

template<typename T1, typename T2>
int compare(T1 const& A, T2 const& B)
{
    // find the first element which differs
    auto mismatchPoint =
        std::mismatch(std::begin(A), std::end(A), std::begin(B), std::end(B));

    if (mismatchPoint.first == std::end(A)) {

        // no elements differ
        if (mismatchPoint.second == std::end(B))
            return 0;

        // A ends before B, so A < B
        return -1;
    } else if (mismatchPoint.second == std::end(B) {
        // B ends before A, so B < A
        return 1;
    } else {

        // compare the first different element
        if (*mismatchPoint.first < *mismatchPoint.second)
            return -1;
        else
            return 1;
    }
}

Answer 2

You might write (using only operator < ):

template <typename T>
int compare(const T& lhs, const T& rhs)
{
    if (lhs < rhs) return -1;
    else if (rhs < lhs) return 1;
    else return 0;
}

Answer 3

There is no generic three way comparison function in the standard library, so you will need to implement it yourself. It's trivial to implement it using the less-than operator.

There are also no standard algorithms that use a "three-way comparison" function (except those inherited from C that cannot be used with containers anyway).

There is a proposal to add a new operator to the standard that would implement such comparison.

Answer 4

C++2a introduces spaceship operator ( <=> ) which does mostly that. (See http://open-std.org/JTC1/SC22/WG21/docs/papers/2017/p0515r0.pdf )

Before that, you have to rely on other ways as proposed answers.