json_pointer - a code example from doc's does not compile

[jrockinl]

Description

Gnu C++ does not compile the code example below. GCC 11.4.0, with -std=c++11 option
Example taken from https://json.nlohmann.me/features/json_pointer/#introduction
json::json_pointer p = "/nested/one";

main.cpp code found in "Minimal code example" field.

The _json_pointer literal is the only line that compiles correctly.

note: Clang compiler (clang++ v14.0.0) basically reports the same errors.

Reproduction steps

g++ main.cpp --std=c++11

Expected vs. actual results

JSON Pointer creation¶
JSON Pointers can be created from a string:
json::json_pointer p = "/nested/one";

Minimal code example

#include <string>
#include "json.hpp"

using json = nlohmann::json;

int main()
{
    std::string ptrStr = "/nested";
    ptrStr += "/one";

    json::json_pointer ptr1 = "/nested/one";
    auto               ptr2 = "/nested/one"_json_pointer;
    json::json_pointer ptr3 = ptrStr;
}

Error messages

GNU:
main.cpp: In function ‘int main()’:
main.cpp:11:31: error: conversion from ‘const char [12]’ to non-scalar type ‘nlohmann::json_abi_v3_11_3::basic_json<>::json_pointer’ {aka ‘nlohmann::json_abi_v3_11_3::json_pointer<std::__cxx11::basic_string<char> >’} requested
   11 |     json::json_pointer ptr1 = "/nested/one";
      |                               ^~~~~~~~~~~~~
main.cpp:13:31: error: conversion from ‘std::string’ {aka ‘std::__cxx11::basic_string<char>’} to non-scalar type ‘nlohmann::json_abi_v3_11_3::basic_json<>::json_pointer’ {aka ‘nlohmann::json_abi_v3_11_3::json_pointer<std::__cxx11::basic_string<char> >’} requested
   13 |     json::json_pointer ptr3 = ptrStr;
      |                               ^~~~~~


Clang:
main.cpp:11:24: error: no viable conversion from 'const char[12]' to 'json::json_pointer' (aka 'json_pointer<std::basic_string<char>>')
    json::json_pointer ptr1 = "/nested/one";
                       ^      ~~~~~~~~~~~~~
./json.hpp:13841:7: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'const char[12]' to 'const nlohmann::json_pointer<std::basic_string<char>> &' for 1st argument
class json_pointer
      ^
./json.hpp:13841:7: note: candidate constructor (the implicit move constructor) not viable: no known conversion from 'const char[12]' to 'nlohmann::json_pointer<std::basic_string<char>> &&' for 1st argument
./json.hpp:13868:14: note: explicit constructor is not a candidate
    explicit json_pointer(const string_t& s = "")
             ^
main.cpp:13:24: error: no viable conversion from 'std::string' (aka 'basic_string<char>') to 'json::json_pointer' (aka 'json_pointer<std::basic_string<char>>')
    json::json_pointer ptr3 = ptrStr;
                       ^      ~~~~~~
./json.hpp:13841:7: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'std::string' (aka 'basic_string<char>') to 'const nlohmann::json_pointer<std::basic_string<char>> &' for 1st argument
class json_pointer
      ^
./json.hpp:13841:7: note: candidate constructor (the implicit move constructor) not viable: no known conversion from 'std::string' (aka 'basic_string<char>') to 'nlohmann::json_pointer<std::basic_string<char>> &&' for 1st argument
./json.hpp:13868:14: note: explicit constructor is not a candidate
    explicit json_pointer(const string_t& s = "")
             ^
2 errors generated.

Compiler and operating system

Ubuntu 22.04

Library version

3.11.1 and 3.11.3

Validation

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

[dimitre]

I'm having a similar issue using json_pointers which we use extensively on openFrameworks projectGenerator

[json.exception.parse_error.109] parse error: array index 'objects' is not a number

errors like this in code that used to work well before 3.11.3

[kfwu1999]

The compilation errors occur because the constructor of json::json_pointer is marked as explicit.
Removing the explicit keyword allows the code to compile and pass the tests, but i think it might violate the design intentions.

So, I think the better approach is to adjust the example code to use direct initialization.

Minimal code example (Modified)

#include <string>
#include "json.hpp"

using json = nlohmann::json;

int main()
{
    std::string ptrStr = "/nested";
    ptrStr += "/one";

    json::json_pointer ptr1("/nested/one");
    auto               ptr2 = "/nested/one"_json_pointer;
    json::json_pointer ptr3(ptrStr);
}

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[nlohmann]

I can confirm the issue.