Local copy given by operator[] or at()

[mbelhadi]

Description

Hello,

I have strange behaviour when I try to get a value from a json object.

Reproduction steps

Just trying de get a reference using operator[] or .at()

Expected vs. actual results

I expected the actual reference of a value when I do this

std::string const &value = obj["key"];
// or
std::string const &value = obj.at("key");

Minimal code example

// ...
json obj = {{"key", "value"}};
const std::string &a = obj["key"]; // new local copy of value
const std::string &b = obj.at("key"); // new local copy of value
const std::string &c = obj.at("key").get_ref<std::string const &>(); // the real ref
std::cout << a.c_str() != c.c_str() << std::endl; // false
std::cout << b.c_str() != c.c_str() << std::endl; // false

Error messages

No response

Compiler and operating system

gcc (Ubuntu 9.4.0-1ubuntu1~20.04.1)

Library version

develop commit 8fcdbf2

Validation

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

[falbrechtskirchinger]

This is as designed and you already figured out that you need to use get_ref().

operator[] and at() return a basic_json reference which implicitly converts to a copy of and not a reference to the internal value.

[mbelhadi]

Why do you not send always a reference , and the final variable defines the copy behaviour as STL does.

std::string const &a = obj["key"]; // ref to const ref
std::string &b = obj["key"]; // ref to ref
std::string c = obj["key"]; // ref to copy

The get or get_ref have a heavy syntax, may be there is a technical limitation for doing this.

Thanks for your answer, closing the issue.

[gregmarr]

FYi, operator[] and at() do return references, it's the implicit conversions to std::string afterwards that convert to a copy.