Using json_pointer causes the warning of deprecated declarations

[zheng-yu-yang]

Description

The JSON pointer example produces compiler warning.

Reproduction steps

1. Pick the code from https://json.nlohmann.me/features/json_pointer/#value-access.
2. Modify the code a litle so that it can be compiled and run directly (attached below).
3. Compile the code with g++-11 and g++-13.
4. See the compiler warning of [-Wdeprecated-declarations].

Note:
clang++-14 and clang++-18 compiles the same code without the deprecated-declarations warning.

Expected vs. actual results

I want to access the json node with json_pointer without compiler warning. If I follow the warning by using to_string method, the meaning is totally different and causes runtime exception.

To be specific, the parameter p1 should be binded to

reference at(const json_pointer& ptr)

but its binded to

    template<class KeyType, detail::enable_if_t<
                 detail::is_usable_as_basic_json_key_type<basic_json_t, KeyType>::value, int> = 0>
    reference at(KeyType && key)

Minimal code example

#include "nlohmann/json.hpp"

using json = nlohmann::json;

int main() {
    // the JSON value from above
    auto j = json::parse(R"({
        "array": ["A", "B", "C"],
        "nested": {
            "one": 1,
            "two": 2,
            "three": [true, false]
        }
    })");

    auto p1 = json::json_pointer("/nested/three/1");
    auto p2 = json::json_pointer("/nested/four");

    // access values
    auto val2 = j.at(p1);    // false
    auto val3 = j.value(p2, 0); // 0
}

Error messages

given by g++-11 (similar is from g++-13):


In file included from include/nlohmann/detail/exceptions.hpp:25,
                 from include/nlohmann/detail/conversions/from_json.hpp:26,
                 from include/nlohmann/adl_serializer.hpp:14,
                 from include/nlohmann/json.hpp:34,
                 from main.cpp:1:
include/nlohmann/detail/meta/type_traits.hpp: In instantiation of ‘struct nlohmann::json_abi_v3_12_0::detail::is_comparable<std::less<std::__cxx11::basic_string<char> >, std::__cxx11::basic_string<char>, nlohmann::json_abi_v3_12_0::json_pointer<std::__cxx11::basic_string<char> >&, void>’:
include/nlohmann/detail/meta/type_traits.hpp:581:31:   required by substitution of ‘template<class KeyType, typename std::enable_if<typename std::conditional<(typename std::conditional<(((nlohmann::json_abi_v3_12_0::detail::is_comparable<std::less<std::__cxx11::basic_string<char> >, std::__cxx11::basic_string<char>, KeyType, void>::value && (!(true && std::is_same<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type, std::__cxx11::basic_string<char> >::value))) && ((! true) || std::integral_constant<bool, false>::value)) && (! nlohmann::json_abi_v3_12_0::detail::is_specialization_of<nlohmann::json_abi_v3_12_0::json_pointer, typename std::remove_cv<typename std::remove_reference<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type>::type>::type>::value)), std::integral_constant<bool, true>, std::integral_constant<bool, false> >::type::value && (! nlohmann::json_abi_v3_12_0::detail::is_json_iterator_of<nlohmann::json_abi_v3_12_0::basic_json<>, typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type>::value)), std::integral_constant<bool, true>, std::integral_constant<bool, false> >::type::value, int>::type <anonymous> > const value_type& nlohmann::json_abi_v3_12_0::basic_json<>::at<KeyType, <anonymous> >(KeyType&&) const [with KeyType = nlohmann::json_abi_v3_12_0::json_pointer<std::__cxx11::basic_string<char> >&; typename std::enable_if<typename std::conditional<(typename std::conditional<(((nlohmann::json_abi_v3_12_0::detail::is_comparable<std::less<std::__cxx11::basic_string<char> >, std::__cxx11::basic_string<char>, KeyType, void>::value && (!(true && std::is_same<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type, std::__cxx11::basic_string<char> >::value))) && ((! true) || std::integral_constant<bool, false>::value)) && (! nlohmann::json_abi_v3_12_0::detail::is_specialization_of<nlohmann::json_abi_v3_12_0::json_pointer, typename std::remove_cv<typename std::remove_reference<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type>::type>::type>::value)), std::integral_constant<bool, true>, std::integral_constant<bool, false> >::type::value && (! nlohmann::json_abi_v3_12_0::detail::is_json_iterator_of<nlohmann::json_abi_v3_12_0::basic_json<>, typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type>::value)), std::integral_constant<bool, true>, std::integral_constant<bool, false> >::type::value, int>::type <anonymous> = <missing>]’
main.cpp:20:21:   required from here
include/nlohmann/detail/meta/type_traits.hpp:568:33: warning: ‘nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::operator nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::string_t() const [with RefStringType = std::__cxx11::basic_string<char>; nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::string_t = std::__cxx11::basic_string<char>]’ is deprecated: Since 3.11.0; use to_string() [-Wdeprecated-declarations]
  568 | decltype(std::declval<Compare>()(std::declval<A>(), std::declval<B>())),
      |          ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from include/nlohmann/json.hpp:50,
                 from main.cpp:1:
include/nlohmann/detail/json_pointer.hpp:81:5: note: declared here
   81 |     operator string_t() const
      |     ^~~~~~~~
In file included from include/nlohmann/detail/exceptions.hpp:25,
                 from include/nlohmann/detail/conversions/from_json.hpp:26,
                 from include/nlohmann/adl_serializer.hpp:14,
                 from include/nlohmann/json.hpp:34,
                 from main.cpp:1:
include/nlohmann/detail/meta/type_traits.hpp:569:33: warning: ‘nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::operator nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::string_t() const [with RefStringType = std::__cxx11::basic_string<char>; nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::string_t = std::__cxx11::basic_string<char>]’ is deprecated: Since 3.11.0; use to_string() [-Wdeprecated-declarations]
  569 | decltype(std::declval<Compare>()(std::declval<B>(), std::declval<A>()))
      |          ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from include/nlohmann/json.hpp:50,
                 from main.cpp:1:
include/nlohmann/detail/json_pointer.hpp:81:5: note: declared here
   81 |     operator string_t() const
      |     ^~~~~~~~
In file included from include/nlohmann/detail/exceptions.hpp:25,
                 from include/nlohmann/detail/conversions/from_json.hpp:26,
                 from include/nlohmann/adl_serializer.hpp:14,
                 from include/nlohmann/json.hpp:34,
                 from main.cpp:1:
include/nlohmann/detail/meta/type_traits.hpp:568:33: warning: ‘nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::operator nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::string_t() const [with RefStringType = std::__cxx11::basic_string<char>; nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::string_t = std::__cxx11::basic_string<char>]’ is deprecated: Since 3.11.0; use to_string() [-Wdeprecated-declarations]
  568 | decltype(std::declval<Compare>()(std::declval<A>(), std::declval<B>())),
      |          ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from include/nlohmann/json.hpp:50,
                 from main.cpp:1:
include/nlohmann/detail/json_pointer.hpp:81:5: note: declared here
   81 |     operator string_t() const
      |     ^~~~~~~~
In file included from include/nlohmann/detail/exceptions.hpp:25,
                 from include/nlohmann/detail/conversions/from_json.hpp:26,
                 from include/nlohmann/adl_serializer.hpp:14,
                 from include/nlohmann/json.hpp:34,
                 from main.cpp:1:
include/nlohmann/detail/meta/type_traits.hpp:569:33: warning: ‘nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::operator nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::string_t() const [with RefStringType = std::__cxx11::basic_string<char>; nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::string_t = std::__cxx11::basic_string<char>]’ is deprecated: Since 3.11.0; use to_string() [-Wdeprecated-declarations]
  569 | decltype(std::declval<Compare>()(std::declval<B>(), std::declval<A>()))
      |          ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from include/nlohmann/json.hpp:50,
                 from main.cpp:1:
include/nlohmann/detail/json_pointer.hpp:81:5: note: declared here
   81 |     operator string_t() const
      |     ^~~~~~~~
In file included from include/nlohmann/detail/exceptions.hpp:25,
                 from include/nlohmann/detail/conversions/from_json.hpp:26,
                 from include/nlohmann/adl_serializer.hpp:14,
                 from include/nlohmann/json.hpp:34,
                 from main.cpp:1:
include/nlohmann/detail/meta/type_traits.hpp: In instantiation of ‘struct nlohmann::json_abi_v3_12_0::detail::is_comparable<std::less<std::__cxx11::basic_string<char> >, const std::__cxx11::basic_string<char>&, nlohmann::json_abi_v3_12_0::json_pointer<std::__cxx11::basic_string<char> >&, void>’:
include/nlohmann/json.hpp:2328:20:   required by substitution of ‘template<class ValueType, class KeyType, class ReturnType, typename std::enable_if<((((nlohmann::json_abi_v3_12_0::detail::is_transparent<std::less<std::__cxx11::basic_string<char> > >::value && (! nlohmann::json_abi_v3_12_0::detail::is_specialization_of<nlohmann::json_abi_v3_12_0::json_pointer, typename std::remove_cv<typename std::remove_reference<_Arg>::type>::type>::value)) && nlohmann::json_abi_v3_12_0::detail::is_comparable<std::less<std::__cxx11::basic_string<char> >, const std::__cxx11::basic_string<char>&, KeyType, void>::value) && nlohmann::json_abi_v3_12_0::detail::is_getable<nlohmann::json_abi_v3_12_0::basic_json<>, ReturnType>::value) && (! std::is_same<nlohmann::json_abi_v3_12_0::detail::value_t, typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type>::value)), int>::type <anonymous> > ReturnType nlohmann::json_abi_v3_12_0::basic_json<>::value<ValueType, KeyType, ReturnType, <anonymous> >(KeyType&&, ValueType&&) const [with ValueType = int; KeyType = nlohmann::json_abi_v3_12_0::json_pointer<std::__cxx11::basic_string<char> >&; ReturnType = int; typename std::enable_if<((((nlohmann::json_abi_v3_12_0::detail::is_transparent<std::less<std::__cxx11::basic_string<char> > >::value && (! nlohmann::json_abi_v3_12_0::detail::is_specialization_of<nlohmann::json_abi_v3_12_0::json_pointer, typename std::remove_cv<typename std::remove_reference<_Arg>::type>::type>::value)) && nlohmann::json_abi_v3_12_0::detail::is_comparable<std::less<std::__cxx11::basic_string<char> >, const std::__cxx11::basic_string<char>&, KeyType, void>::value) && nlohmann::json_abi_v3_12_0::detail::is_getable<nlohmann::json_abi_v3_12_0::basic_json<>, ReturnType>::value) && (! std::is_same<nlohmann::json_abi_v3_12_0::detail::value_t, typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type>::value)), int>::type <anonymous> = <missing>]’
main.cpp:21:24:   required from here
include/nlohmann/detail/meta/type_traits.hpp:568:33: warning: ‘nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::operator nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::string_t() const [with RefStringType = std::__cxx11::basic_string<char>; nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::string_t = std::__cxx11::basic_string<char>]’ is deprecated: Since 3.11.0; use to_string() [-Wdeprecated-declarations]
  568 | decltype(std::declval<Compare>()(std::declval<A>(), std::declval<B>())),
      |          ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from include/nlohmann/json.hpp:50,
                 from main.cpp:1:
include/nlohmann/detail/json_pointer.hpp:81:5: note: declared here
   81 |     operator string_t() const
      |     ^~~~~~~~
In file included from include/nlohmann/detail/exceptions.hpp:25,
                 from include/nlohmann/detail/conversions/from_json.hpp:26,
                 from include/nlohmann/adl_serializer.hpp:14,
                 from include/nlohmann/json.hpp:34,
                 from main.cpp:1:
include/nlohmann/detail/meta/type_traits.hpp:569:33: warning: ‘nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::operator nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::string_t() const [with RefStringType = std::__cxx11::basic_string<char>; nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::string_t = std::__cxx11::basic_string<char>]’ is deprecated: Since 3.11.0; use to_string() [-Wdeprecated-declarations]
  569 | decltype(std::declval<Compare>()(std::declval<B>(), std::declval<A>()))
      |          ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from include/nlohmann/json.hpp:50,
                 from main.cpp:1:
include/nlohmann/detail/json_pointer.hpp:81:5: note: declared here
   81 |     operator string_t() const
      |     ^~~~~~~~
In file included from include/nlohmann/detail/exceptions.hpp:25,
                 from include/nlohmann/detail/conversions/from_json.hpp:26,
                 from include/nlohmann/adl_serializer.hpp:14,
                 from include/nlohmann/json.hpp:34,
                 from main.cpp:1:
include/nlohmann/detail/meta/type_traits.hpp:568:33: warning: ‘nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::operator nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::string_t() const [with RefStringType = std::__cxx11::basic_string<char>; nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::string_t = std::__cxx11::basic_string<char>]’ is deprecated: Since 3.11.0; use to_string() [-Wdeprecated-declarations]
  568 | decltype(std::declval<Compare>()(std::declval<A>(), std::declval<B>())),
      |          ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from include/nlohmann/json.hpp:50,
                 from main.cpp:1:
include/nlohmann/detail/json_pointer.hpp:81:5: note: declared here
   81 |     operator string_t() const
      |     ^~~~~~~~
In file included from include/nlohmann/detail/exceptions.hpp:25,
                 from include/nlohmann/detail/conversions/from_json.hpp:26,
                 from include/nlohmann/adl_serializer.hpp:14,
                 from include/nlohmann/json.hpp:34,
                 from main.cpp:1:
include/nlohmann/detail/meta/type_traits.hpp:569:33: warning: ‘nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::operator nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::string_t() const [with RefStringType = std::__cxx11::basic_string<char>; nlohmann::json_abi_v3_12_0::json_pointer<RefStringType>::string_t = std::__cxx11::basic_string<char>]’ is deprecated: Since 3.11.0; use to_string() [-Wdeprecated-declarations]
  569 | decltype(std::declval<Compare>()(std::declval<B>(), std::declval<A>()))
      |          ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from include/nlohmann/json.hpp:50,
                 from main.cpp:1:
include/nlohmann/detail/json_pointer.hpp:81:5: note: declared here
   81 |     operator string_t() const
      |     ^~~~~~~~
main.cpp: In function ‘int main()’:
main.cpp:21:10: warning: unused variable ‘val3’ [-Wunused-variable]
   21 |     auto val3 = j.value(p2, 0); // 0
      |          ^~~~

Compiler and operating system

gcc version 11.4.0 (Ubuntu 11.4.0-1ubuntu122.04.2) / gcc version 13.1.0 (Ubuntu 13.1.0-8ubuntu122.04)

Library version

nlohmann json 3.12.0

Validation

• The bug also occurs if the latest version from the develop branch is used.
• I can successfully compile and run the unit tests.

[gregmarr]

I tried this on Compiler Explorer with 3.11.1 and the develop branch HEAD, no error with either 11.4 or 13.4.
https://www.godbolt.org/z/MhaMrfj55
If you can reproduce this with the develop branch head, please show that using compiler explorer.

[zheng-yu-yang]

I tried this on Compiler Explorer with 3.11.1 and the develop branch HEAD, no error with either 11.4 or 13.4. https://www.godbolt.org/z/MhaMrfj55 If you can reproduce this with the develop branch head, please show that using compiler explorer.

Thanks for the reply. Would you try 3.12.0 which is used in my project where the issue was observed?

[gregmarr]

I tried this on Compiler Explorer with 3.11.1 and the develop branch HEAD, no error with either 11.4 or 13.4. https://www.godbolt.org/z/MhaMrfj55 If you can reproduce this with the develop branch head, please show that using compiler explorer.

Thanks for the reply. Would you try 3.12.0 which is used in my project where the issue was observed?

The 3.12.0 version isn't available as a library there, but I used the raw link https://raw.githubusercontent.com/nlohmann/json/refs/tags/v3.12.0/single_include/nlohmann/json.hpp instead and that does happen, so it appears that this was a 3.12.0 issue that has already been fixed.