Custom class ostringstream output error

问题

I don't know why this is erroring, but I'm just trying to add something "akin" to endl so that I can throw what's in an ostringstream to our debugger. I have the following:

class debug_stream_info
{
public:
    debug_stream_info(int errorLine, char *errorFile, int level)
        :m_errorLine(errorLine), m_errorFile(errorFile), m_logLevel(level)
    {
    }

    friend std::basic_ostringstream<char>& operator<<(std::basic_ostringstream<char>& os, debug_stream_info& debug_info);

private:
    int m_errorLine;
    std::string m_errorFile;
    int m_logLevel;

};

std::basic_ostringstream<char>& operator<<(std::basic_ostringstream<char>& os, debug_stream_info& debug_info)
{
    // Write the stream's contents to cpu_debug
    // Deleted custom logging function.  No errors here though

    // Clear the stream for re-use.
    os.str("");
    os.seekp(0);

    return os;
}

int main(int argc, char** argv)
{
    std::ostringstream myout;
    myout << "hey there" << " and some more " << "Numbers!!: " << 435 << 54.2 << " that's good for numbers" << debug_stream_info(__LINE__, __FILE__, LOG_LEVEL);

    return 0;
}

The error I'm getting is: error C2679: binary '<<' : no operator found which takes a right-hand operand of type 'debug_stream_info' (or there is no acceptable conversion) for the line in main. This is on VS2008.

I'm including sstream, iostream, etc, and have the namespaces set up right. I'm getting no other errors. I even tried replacing all occurrances of basic_ostream with just ostringstream and there was no difference (I'll be having a w_char version later, but I wanted the simple case to work first). I made the object on the line above and then passed a fully-constructed object on the line, and the error was exactly the same. I've changed the signature of the second argument to and from const with no change as well.

Any ideas on what I'm doing wrong here?

Edit: since EVERY response seems to want to put it there, I can NOT use std::ostream because I want this to work ONLY for std::ostringstream (and std::basic_ostringstream) and not for any type of output stream. Besides, the function wouldn't compile with ostream anyways, since I'm using the os.str() method, which isn't in ostream, only the sub-classes.

回答1:

The real problem with your code is that you've overloaded std::ostringstream rather than std::ostream. So your code would work if you write this:

debug_stream_info info(/** blah blah**/);

std::ostringstream oss;
oss << info ; //OK

However this will not work:

oss << 1 << info; //ERROR

This is compilation error because the expression oss<<1 returns an object of type std::ostream& which doesn't have overload which takes debug_stream_info as second argument. That means if you use cast as:

static_cast<std::ostringstream&>(oss << 1) << info; //OK

then that should work again.

So the solution is to overload std::ostream, instead of std::basic_ostringstream.

Also, the second parameter should be const & . This is also a problem with your code.

So write this:

std::ostream& operator<<(std::ostream&, debug_stream_info const &);
                                                        //^^^^^^^ note this

The second parameter should be const & so that you could write temporary objects to the stream.

回答2:

debug_stream_info(__LINE__, __FILE__, LOG_LEVEL); is creating unnamed object which is not returning anything hence error

#include <iostream>
#include <cstdio>
#include <sstream>
using namespace std;

class debug_stream_info
{
public:
    debug_stream_info(int errorLine, char *errorFile, int level)
        :m_errorLine(errorLine), m_errorFile(errorFile), m_logLevel(level)
    {
    }

    friend std::basic_ostringstream<char>& operator<<(std::basic_ostringstream<char>& os, debug_stream_info& debug_info);
    std::ostringstream& fun(std::ostringstream& os)
    {
        os<<"Ashish"<<endl;
        return os;
    }
private:
    int m_errorLine;
    std::string m_errorFile;
    int m_logLevel;

};

std::basic_ostringstream<char>& operator<<(std::basic_ostringstream<char>& os, debug_stream_info& debug_info)
{
    // Write the stream's contents to cpu_debug
    // Deleted custom logging function.  No errors here though

    // Clear the stream for re-use.
//    os.str("");
//    os.seekp(0);

    return os;
}

int main(int argc, char** argv)
{

    std::ostringstream myout, test;
        myout << "hey there" << " and some more " << "Numbers!!: " << 435 << 54.2 << " that's good for numbers"
         << debug_stream_info(1, "/home/ashish/test", 1).fun(test);
    return 0;
}

回答3:

Nawaz has explained very clearly why you're getting the error. The usual solution in this case is to define your own stream type, unrelated to std::istream. Something along the lines of:

class DebugStream
{
    std::ostringstring* collector;

public:
    template <typename T>
    DebugStream& operator<<( T const& value )
    {
        if ( collector != NULL ) {
            *collector << value;
        }
        return *this;
    }
};

There are infinite variations on this; in your case, you could add a non-template member function for your type; more likely, you'd add a constructor which took the same arguments:

DebugStream( int lineNumber, std::string const& filename, int logLevel )
    : collector( isActive( logLevel ) ? new std::ostringstream : NULL )
{
    //  Initial insertion of lineNumber, filename, timestamp...
}

You can also add a destructor which atomically flushes the collected data to a file (or sends an email, or writes it to the system log, or whatever). (Be very careful about this. You don't want an exception to escape from the destructor, even if the logging fails.)

Finally, you might want to use a custom streambuf, rather than stringstream. Say one that keeps the allocated buffer from one instance to the next. And if you do this, rather than newing the stream each time, you might pick up an instance from a table, indexed by the log level (and initialized from a configuration file).

来源：https://stackoverflow.com/questions/10386702/custom-class-ostringstream-output-error

标签

c++

stringstream