explicit constructor with default does not compile

[bpmckinnon]

From include/nlohmann/json.hpp

The change from ValueType ret; to ValueType ret{}; is causing a compiler error for the construction of a type with an explicit default constructor of the form.

explicit Foo(const alloc_context& allocContext = alloc_context())

In vs2019 it gives the following errors, using c++17
error C2512: '###': no appropriate default constructor available
note: Constructor for class '###' is declared 'explicit'

This is happening in a from_json call to my custom container type

Worked in 3.9.1, does not work in 3.10.2

[nlohmann]

Do you have a complete example?

[bpmckinnon]

This will repro the issue. It seems to be related to when a vector of a class contains a class that has an explicit constructor. I can probably work around this... but removing the {} from ValueType ret{} also fixes the problem, but I'm sure it's needed in some cases.

#include "nlohmann/json.hpp"

class FooAlloc
{};

class Foo
{
public:
    explicit Foo(const FooAlloc& = FooAlloc()) : value(false) {}

    bool value;
};

class FooBar
{
public:
    Foo foo;
};

inline void from_json(const nlohmann::json& j, FooBar& fb) { j.at("value").get_to(fb.foo.value); }

void test()
{
    nlohmann::json j;
    std::vector<FooBar> foo;
    j.at("foo").get_to(foo);
}

[nlohmann]

I can confirm the issue with Xcode on macOS, so it's not an MSVC issue.

[nlohmann]

Interestingly, the {} is not mentioned in the documentation:

json/include/nlohmann/json.hpp

Lines 3035 to 3040 in 4b1cb9e

	The function is equivalent to executing
	@code {.cpp}
	ValueType ret;
	JSONSerializer<ValueType>::from_json(*this, ret);
	return ret;
	@endcode

The change was made in #2561 with 1101f0e, apparently to silence a warning from Clang-Tidy. I was not aware that this could break code like this.

I will add your example as regression test and revert the change and check if the CI passes.

[nlohmann]

In fact, this is the Clang-Tidy warning that led to the (breaking) change:

error: uninitialized record type: 'ret' [cppcoreguidelines-pro-type-member-init,hicpp-member-init,-warnings-as-errors]
        ValueType ret;
        ^
                     {}
´´`

[bpmckinnon]

Can you use
auto ret = ValueType(); ?

[nlohmann]

PR #3079 is ready for review. Feedback greatly appreciated!