input logging stream

input_logger.hh

// input logging
// inspired from https://en.cppreference.com/w/cpp/io/basic_streambuf/underflow
#include <iostream>
#include <sstream>

class buffer_logger : public std::streambuf {
    std::streambuf* src;
    char ch; // single-byte buffer

    std::string input;

protected:
    int underflow() {
        traits_type::int_type i = src->sbumpc();
        if (traits_type::not_eof(i)) {
            ch = traits_type::to_char_type(i);
            setg(&ch, &ch, &ch+1); // make one read position available
    
            // 
            input += ch;
            if( ch == '\n') {
                std::cerr << "=>" << input;
                input = "";
            }
        }
        else {
                std::cerr << "*>" << input << std::endl;
                input = "";
        }

        return i;
    }
    
public:
    buffer_logger(std::streambuf* buf) : src(buf) {
        setg(&ch, &ch+1, &ch+1); // buffer is initially full
    }
};

class input_logger
{
    std::istream & in;
    std::streambuf* const sbuf;
    buffer_logger blog;

public:
    input_logger(std::istream& in): 
        in( in),
        sbuf(in.rdbuf()),
        blog( sbuf)
    {
        in.rdbuf(&blog);
    }

    ~input_logger() { in.rdbuf(sbuf); }
};



// Return content of stream as a string
// http://wordaligned.org/articles/cpp-streambufs
std::string load_stream(std::istream& src)
{
    std::ostringstream buf;
    buf << src.rdbuf();
    return buf.str();
}

usage.cc

#include "input_logger.hh"

int main() {
    // assume an input stream exist (work as well with cin)
    char a[] = "This is the input\nhello\n";
    std::istringstream in(std::string(std::begin(a), std::end(a)));

    // Then tie in stream to log stream, wich will output every input to cerr
    input_logger log( in);

    // use in stream normally
    std::string first, second;
    getline( in, first);            // will log =>This is the input
    in >> second; in.ignore();      // will log =>hello

    // and it works as expected
    std::cerr << "first is '" << first << "'\n";
    std::cerr << "second is '" << second << "'\n";
}