There is performance inefficiency found by coverity tool json2.1.1/include/nlohmann/json.hpp

[qinglucich]

4063 @SInCE version 1.0.0
4064 */
4065 template<class ValueType, typename std::enable_if<
4066 std::is_convertible<basic_json_t, ValueType>::value, int>::type = 0>

CID 10413 (#1 of 1): Big parameter passed by value (PASS_BY_VALUE)
pass_by_value: Passing parameter default_value of type cerebras::file_metadata (size 168 bytes) by value.
4067 ValueType value(const typename object_t::key_type& key, ValueType default_value) const
4068 {

[nlohmann]

Thanks for reporting. Here is the line in context: https://github.com/nlohmann/json/blob/v2.1.1/src/json.hpp#L4066

[nlohmann]

This is also relevant for 3.0.0.

[nlohmann]

Fixed. Thanks for reporting!

[murderotica]

Wouldn't using universal reference instead of const& be better (in case of an r-value a move will be possible instead of always copy)?

This
ValueType value(const json_pointer& ptr, const ValueType& default_value)
replaced with this

ValueType value(const json_pointer& ptr, ValueType&& default_value) { //... JSON_CATCH (out_of_range&) { return std::forward<ValueType>(default_value); } }

reference example https://godbolt.org/g/Yq2euX

[theodelrieu]

There is quite a lot of room for zero-copy/rvalue references support in the library indeed.

Note that the return type should be changed to auto (...) -> decltype(std::forward<ValueType>(default_value)) to avoid copies.

[nlohmann]

@murderotica, @theodelrieu Can you create a PR for these optimizations?

[theodelrieu]

I'll add that to my TODO list.