Redirect Stream to spdlog Logger

StreamLogger.hpp

#include <streambuf>
#include <ostream>

#include "spdlog/spdlog.h"

template<class Element = char, class Trait = std::char_traits<Element>>
class StreamLogger : public std::basic_streambuf<Element, Trait>
{
public:
    StreamLogger(std::ostream& stream, std::shared_ptr<spdlog::logger> logger)
        : m_stream{ stream }, m_logger{ std::move(logger) }
    {
        // Redirect provided stream
        m_buffer = m_stream.rdbuf(this);
    };

    ~StreamLogger()
    {
        try
        {
            // Restore provided stream
            m_stream.rdbuf(m_buffer);

        }
        catch(const std::ios_base::failure& exception)
        {
            const auto message = std::string{ "StreamLogger was unable to restore the provided stream: " };
            m_logger->error(message + exception.what());
        }
    }

    StreamLogger(const StreamLogger& other) = delete;
    StreamLogger& operator=(const StreamLogger& other) = delete;

    StreamLogger(StreamLogger&& other) noexcept = default;
    StreamLogger& operator=(StreamLogger&& other) noexcept = default;

protected:
    std::streamsize xsputn(const Element* elements, const std::streamsize count) override
    {
        m_string_buffer.append(elements, static_cast<std::string::size_type>(count));
        return count;
    }

    typename Trait::int_type overflow(typename Trait::int_type final_character) override
    {
        m_logger->error(m_string_buffer);
        m_string_buffer.clear();
        return Trait::not_eof(final_character);
    }

    std::basic_ostream<Element, Trait>& m_stream;
    std::streambuf* m_buffer;
    std::basic_string<Element, Trait> m_string_buffer;
    std::shared_ptr<spdlog::logger> m_logger;
};

/*
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This software is available under 2 licenses -- choose whichever you prefer.
------------------------------------------------------------------------------

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binary, for any purpose, commercial or non-commercial, and by any
means.

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of this software dedicate any and all copyright interest in the
software to the public domain. We make this dedication for the benefit
of the public at large and to the detriment of our heirs and
successors. We intend this dedication to be an overt act of
relinquishment in perpetuity of all present and future rights to this
software under copyright law.

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EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
OTHER DEALINGS IN THE SOFTWARE.
------------------------------------------------------------------------------

MIT License

Copyright (c) 2020-2023 Lukas Dürrenberger

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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------------------------------------------------------------------------------
*/

Thanks for the snippet. Quite useful! Just notice that this is not move safe. The first destroyed StreamLogger will reset the std::ostream to its original state.

Oh, someone found my gist 😄

Any suggestion for a fix?

After trying some work arounds, these are are the few things that may fix things:

• Make constructor private
• Delete move constructor/assignment
• Add a factory function that returns a non- discardable unique pointer
• std::basic_streambuf<Element, Trait> didn’t want to do anything for me. I had to write everything without character and traits templates, maybe has something to do with my standard library, I still don’t understand why 🤷🏻‍♂️.
• Add an open source license ;)

Make constructor private

How do you construct an instance then?

Delete move constructor/assignment

I guess that makes sense, if it's not movable anyways.

Add a factory function that returns a non- discardable unique pointer

What's a non-discardable unique pointer? 😄

std::basic_streambuf<Element, Trait> didn’t want to do anything for me

Maybe something with spdlog changed. The class is already three years old.

Add an open source license ;)

Done 🙂

What's a non-discardable unique pointer? 😄

I mean this function signature: [[nodiscard]] std::unique_ptr<StreamLogger<…>> StreamLogger<…>::make(…). The pointer must not be discarded because there is no observable side-effects if the object is immediately discarded. This is so that people don’t forget to assign it to an object on creation.

How do you construct an instance then?

The factory function make(…) still has access to private parts for of its class, for example, the constructor.

Maybe something with spdlog changed.

No really, my tests also didn’t work with std::stringing and std::cout when I used the templated version.

Add an open source license ;)

Done 🙂

Thank you :)