ADL issue in input_adapter

[mika-fischer]

json::parse does not work with containers which only provide begin() and end() member functions. This worked in 3.7.0.

Example https://godbolt.org/z/NtXn4M:

#if 1
#include "https://raw.githubusercontent.com/nlohmann/json/v3.8.0/single_include/nlohmann/json.hpp"
#else
#include "https://raw.githubusercontent.com/nlohmann/json/v3.7.0/single_include/nlohmann/json.hpp"
#endif

struct container {
  std::string data;              
  auto begin() const { return data.begin(); }
  auto end()   const { return data.end();   }
};

auto foo(container& c) { return nlohmann::json::parse(c); }

gives a (long) compile error.

This is probably because of the unqualified begin and end in the trailing return type here (https://github.com/nlohmann/json/blob/v3.8.0/single_include/nlohmann/json.hpp#L4796-L4805) without using std::begin; using std::end;:

template<typename ContainerType>
 auto input_adapter(const ContainerType& container) -> decltype(input_adapter(begin(container), end(container)))
 {
     // Enable ADL
     using std::begin;
     using std::end;
  
     return input_adapter(begin(container), end(container));
 }

The old code had std::begin and std::end there, which is also wrong, for different reasons. Unfortunately, I don't know how to fix it...

[gregmarr]

You could fix this by adding these functions in the same namespace as container

auto begin(container &c) {
    return c.begin();
}
auto end(container &c) {
    return c.end();
}

but I think it would be better to not require this.

@Francois-air does this function need the trailing return type, or can it be removed? The function only has a single return statement, so the auto and return should be sufficient.

[mika-fischer]

You could fix this by adding these functions in the same namespace as container

auto begin(container &c) {
    return c.begin();
}
auto end(container &c) {
    return c.end();
}

Sure, I'm aware of that. The bug is that this should not be required of the user, as you say.

@Francois-air does this function need the trailing return type, or can it be removed? The function only has a single return statement, so the auto and return should be sufficient.

I would assume that it's because of C++11 compatibility...

By "I don't know how to fix this" I meant: I don't know how to correctly specify the return type with C++11, so that it works in all cases...

[mika-fischer]

Probably something like this is the way to go: https://stackoverflow.com/a/35036343

[gregmarr]

Ugh, I was thinking that C++11 supported automatic return type deduction from a single return and C++14 expanded it to multiple returns, but it was actually 14 that added the automatic return type deduction and 17 that expanded it to multiple returns. :(

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.