api/json_pointer/operator_spaceship.md

# <small>nlohmann::json_pointer::</small>operator<=>

```cpp
// since C++20
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

!!! note "Ordering enables use as an associative container key"

    Together with [`operator==`](operator_eq.md), `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`.

!!! note "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.

    ```cpp
    #include <compare>
    #include <iostream>
    #include <nlohmann/json.hpp>
    
    using 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
    
    ```

## See also

- [`operator==`](operator_eq.md) - compare: equal
- [`operator!=`](operator_ne.md) - compare: not equal

## Version history

- Added in version 3.11.2.