dump() method doesn't work with a custom allocator

[michsco]

Even if #161 isn't fixed, the dump() method still assumes the default allocator is used. Consider the following code:

nlohmann::basic_json<std::map, std::vector, StringWithMyAllocator, bool, int64_t, uint64_t, double, MyAllocator> j;

// add a string that is stored as std::string
j["name"] = "Niels";

// explicit conversion to string
StringWithMyAllocator s = j.dump();    // {\"happy\":true,\"pi\":3.141}

This results in a compiler error:
'return': cannot convert from 'std::basic_string<char,std::char_traits,std::allocator>' to 'std::basic_string<char,std::char_traits,MyAllocator>'

The issue is due to the stringstream that is used without specifying the allocator for the stream. As a result, dump fails to compile due to the call to ss.str().

string_t dump(const int indent = -1) const
{
    std::stringstream ss;

    if (indent >= 0)
    {
        dump(ss, true, static_cast<unsigned int>(indent));
    }
    else
    {
        dump(ss, false, 0);
    }

    return ss.str();
}

[nlohmann]

I actually never tried to use custom allocators in the string type. Your example shows that there is an apparent disconnect between the (in theory) adjustable basic_json::string_t and the (implicitly assumed) std::string returned by str().

The quick "fix" would be to replace the return type of dump from basic_json::string_t to std::string, because the type used to store strings inside a JSON value does not necessarily need to be the same type of a serialization.

Alternatively, the user-defined allocator inside basic_json::string_t would need to be passed down into the string stream.

[nlohmann]

I created a branch feature/issue268 (see a834338) which fixes the compilation error. Please let me know whether this works for you.

[nlohmann]

Thanks for checking back, and I am looking forward to see (the definitely needed) fixes!

[michsco]

I created a fork of the project and a new branch feature/custom-allocator that contains initial changes. There are still a lot of assumptions built into the code using the default allocator for std::vector, std::map, and std::string.