Accessing strings (for example in keys or values) without having the lib create a copy of it.

[aguaviva]

I am ok with just having a "const char *" of the string.

I have tried several times an I haven;t had any luck, all I get are copies.

[nlohmann]

Can you provide an example? I currently do not know what you mean exactly.

• Please describe the steps to reproduce the issue. Can you provide a small but working code example?

• What is the expected behavior?

• And what is the actual behavior instead?

[alfaunits]

I feel ya, aguaviva.
I had the use of the same a few times.

If we ask for, say, json_var["name"] we would get a new JSON object, converted to a string.
Being able to instead get a string object, pointing to actual memory inside the json_var, where the vale of the element "name" would reduce memiry need.
Talking about very large JSONs of course.

[nickaein]

How about using get_ptr() to get a pointer to the stored object?

[alfaunits]

Would that not create a copy of an std::string when we try to use it as a string?

[nickaein]

Correct. The following code is one way to test it (live example):

Note: Per documentation, writing data to the returned pointer is discouraged. Take following code just as a demonstration.

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

int main() {

  nlohmann::json j;
  j["key"] = "hello";

  auto p1 = j["key"].get_ptr<std::string *>();
  auto p2 = j["key"].get_ptr<std::string *>();

  std::cout << "Addresses: " << p1 << "," << p2 << std::endl;

  std::cout << "Original:  " << j["key"].get<std::string>() << "," << *p1 << ","
            << *p2 << std::endl;

  *p1 += "1";
  std::cout << "Change p1: " << j["key"].get<std::string>() << "," << *p1 << ","
            << *p2 << std::endl;

  *p2 += "2";
  std::cout << "Change p2: " << j["key"].get<std::string>() << "," << *p1 << ","
            << *p2 << std::endl;

  return 0;
}

Typical output:

Addresses: 0xe37b20,0xe37b20
Original:  hello,hello,hello
Change p1: hello1,hello1,hello1
Change p2: hello12,hello12,hello12

[alfaunits]

Hmm, that works for me! Thanks Not sure about the OP, their needs might be different.

…

[aguaviva]

@nlohmann any chance that we could have const string &myvar = json["myvar"] 'to not return a copy as the default behaviour? This would speed up things a lot :)

[nlohmann]

json::operator[] returns a (const) reference of type json&. From there, you can use get, get_ref or get_ptr to access the value for various conversions.

[aguaviva]

why not make an operator overload to get json::operator[] return a std::string?

[nlohmann]

For several reasons:

• The type of the string is templated. It is std::string by default, but it could be another type. That's why the get_ptr and get_ref functions are do delicate as they allow you to dig into the JSON type which may lead to problems.
• What happens if you write std::string& mystring = myjson["foo"], but myjson["foo"] is an integer?

[aguaviva]

Alright, thanks for the explanation and for such a good lib :)

…

On Sun, Feb 9, 2020 at 10:15 AM Niels Lohmann ***@***.***> wrote: For several reasons: - The type of the string is templated. It is std::string by default, but it could be another type. That's why the get_ptr and get_ref functions are do delicate as they allow you to dig into the JSON type which may lead to problems. - What happens if you write std::string& mystring = myjson["foo"], but myjson["foo"] is an integer? — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#1916?email_source=notifications&email_token=ABHVYEZWJUNTAOB4OFA42P3RB7CRRA5CNFSM4KNEETO2YY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOELGG3WY#issuecomment-583822811>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ABHVYE3AWEX7X4KMHXFZBBLRB7CRRANCNFSM4KNEETOQ> .