Hooks into any writes to the given ostream.

callback_streambuf.h

#include <functional>
#include <streambuf>
#include <iostream>

/**
 * \brief Hooks into any writes to the given ostream.
 * \tparam Duplicate If duplicate is true then it will pass through to the original stream.
 */
template<bool Duplicate>
class callback_streambuf final : public std::streambuf
{
public:
    callback_streambuf(std::ostream& stream, std::function<void(const char_type*, std::streamsize)> callback)
		: callback(callback)
        , parent (stream.rdbuf(this))
	    , stream(stream)
    {
    }

    virtual ~callback_streambuf()
    {
        stream.rdbuf(parent);
    }

protected:
    std::streamsize xsputn(char_type const* s, std::streamsize count)
	{
        callback(s, count);
        if constexpr (Duplicate)
            parent->sputn(s, count);
        return count;
    }

private:
    std::function<void(const char_type*, std::streamsize)> callback;
    std::streambuf* parent;
    std::ostream& stream;
};