nlohmann::json_pointer::operator<=>

// since C++20
class json_pointer {
    template<typename RefStringTypeRhs>
    std::strong_ordering operator<=>(const json_pointer<RefStringTypeRhs>& rhs) const noexcept; // *NOPAD*
};

3-way compares two JSON pointers by lexicographically comparing their sequences of reference tokens: corresponding reference tokens are compared with string_t's own operator<=>, and the first pair of tokens that differs determines the result. If all corresponding reference tokens compare equal, the JSON pointer with fewer reference tokens is ordered first.

Template parameters

RefStringTypeRhs : the string type of the right-hand side JSON pointer

Parameters

rhs (in) : JSON pointer to compare *this with

Return value

the std::strong_ordering of the 3-way comparison of *this and rhs

Exception safety

No-throw guarantee: this function never throws exceptions.

Complexity

Linear in the number of reference tokens.

Notes

Ordering enables use as an associative container key

Together with operator==, operator<=> makes json_pointer a LessThanComparable type, so it can be used as the key type of ordered associative containers such as std::map or std::set.

Before C++20

Without C++20's three-way comparison, json_pointer provides a non-member operator< instead, which orders JSON pointers the same way. JSON pointers can therefore be used as keys of ordered associative containers with any supported C++ standard.

Examples

Example

The example demonstrates 3-way comparing JSON pointers.

#include <compare>
#include <iostream>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

const char* to_string(const std::strong_ordering& so)
{
    if (std::is_lt(so))
    {
        return "less";
    }
    else if (std::is_gt(so))
    {
        return "greater";
    }
    return "equal";
}

int main()
{
    // different JSON pointers
    json::json_pointer ptr1("/a/b");
    json::json_pointer ptr2("/a/c");
    json::json_pointer ptr3("/a/b/c");
    json::json_pointer ptr4("/a/b");

    // 3-way compare JSON pointers
    std::cout << "\"" << ptr1 << "\" <=> \"" << ptr2 << "\": " << to_string(ptr1 <=> ptr2) << '\n' // *NOPAD*
              << "\"" << ptr1 << "\" <=> \"" << ptr3 << "\": " << to_string(ptr1 <=> ptr3) << '\n' // *NOPAD*
              << "\"" << ptr1 << "\" <=> \"" << ptr4 << "\": " << to_string(ptr1 <=> ptr4) << std::endl; // *NOPAD*
}

Output:

"/a/b" <=> "/a/c": less
"/a/b" <=> "/a/b/c": less
"/a/b" <=> "/a/b": equal

Version history

Added in version 3.11.2.

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