C++ Tips and Tricks

**Sebastiani** · 12-19-2009

Noticing this thread, I thought it would be interesting to start one on C++ techniques. Maybe a mod could make it a sticky, as well?

Here's the first installment: debugging macros. Very handy when you just want to see the name and value of a variable or expression. For convenience, they can be turned off by defining NDEBUG.

Code:


#ifndef BUG_HPP
#define BUG_HPP

/*	
	A simple set of macros for printing variable/expression text and values.
	SBUG( stream, expression ) writes output to any std::ostream-like object.
	BUG( expression ) writes output to std::cout.
	If NDEBUG is defined, macros expand to nothing.
*/

#include <ostream> // for std::cout
#include <cctype> // for isdigit

namespace BUG_IMPL_ {

template < typename Stream, typename Type >
void print_dispatch( Stream& stream, char const* expression, Type const& value )
{
	stream << "*** [" << __FILE__ << ", line #" << __LINE__ << "] ***\n";
/*
	We'll flush the stream twice, just in case the outputting of 'value' should crash, 
	throw an exception, etc - at least we'll know what file/line we were at before the fact
*/
	stream.flush( );
/*
	Check if the expression was some sort of literal
*/
	int
		ch = expression[ 0 ];
	if( !isalpha( ch ) && ch != '_' )
	{
	/*
		If already quoted, output as is
	*/
		if( ch == '"' || ch == '\'' )
			stream << " " << expression << "\n";
		else
			stream << " '" << expression << "'\n";
	}		
	else
		stream << " " << expression << " = '" << value << "'\n";
	stream.flush( );	
}

} // namespace BUG_IMPL_

#ifndef NDEBUG
#define SBUG( stream, expression ) BUG_IMPL_::print_dispatch( stream, #expression, expression );
#define BUG( expression ) SBUG( std::cout, expression )
#else
#define SBUG( stream, expression )
#define BUG( expression )
#endif // !NDEBUG

#endif // BUG_HPP

Example usage:

Code:



#include <iostream>
#include <cmath>

int main( void )
{
    BUG( "entering main" );
    int
        a = 200;
    float
        b = 0.5;
    char const*
        c = "ten";
    BUG( a );
    BUG( b );
    BUG( sqrt( a * b ) );
    BUG( c );
    BUG( "exiting main" );
}

Output:

*** [test.cpp, line #62] ***
"entering main"
*** [test.cpp, line #69] ***
a = '200'
*** [test.cpp, line #70] ***
b = '0.5'
*** [test.cpp, line #71] ***
sqrt( a * b ) = '10'
*** [test.cpp, line #72] ***
c = 'ten'
*** [test.cpp, line #73] ***
"exiting main"

And don't forget, they work with user-defined types, too:

Code:



#include <ostream>
#include <string>

using namespace
    std;

struct name
{
    name( string const& first, string const& last )
    : first( first ), last( last )
    {    }
    
    friend ostream& operator << ( ostream& stream, name const& name )
    {
        return stream << name.last << ", " << name.first;
    }
    
    string 
        first, 
        last;
};    
    
int main( void )
{
    name
        someone( "Sebastian", "Garth" );
    BUG( someone );
}

Output:

*** [test.cpp, line #103] ***
someone = 'Garth, Sebastian'

Cheers!

**jeffcobb** · 12-19-2009

I would like to suggest having the ability to push this stuff out through a socket for remote debugging purposes......

**Sebastiani** · 12-19-2009

Originally Posted by jeffcobb

I would like to suggest having the ability to push this stuff out through a socket for remote debugging purposes......

That's essentially what the SBUG macro is for. To get it to work, you could derive your socket writer directly from the standard output streams, for example. A fairly good overview can be found here. A better approach might be to use the boost::iostreams library, though. Much more flexible, in the long run. Another option (and note that I added templated stream parameter to the print routine) is to use customized class - it would need to support the '<<' operator, of course, as well as a 'flush' member function, so that may not be the most practical approach, in general.

**Sebastiani** · 12-19-2009

Just thought of an easy way to do it without having to resort to using inheritance (eg: the last option I mentioned):

Code:



#include <sstream>
#include <iostream>
#include <cmath>

using namespace
	std;

/*
	The hypothetical connection class
*/
class remote_connection
{	
	public:
/*
	Ideally, we would return the number of bytes written. For simplicity's sake, 
	we'll just assume that the class throws an exception on error.
*/	
	void write( char const* data, size_t length )
	{
		// ...send the data. We'll just write to the console for now...
		cout.write( data, length );
		cout.flush( );
	}
	
	// ...the rest of the implementation...
};

/*
	'Writer' just needs to implement a 'write' member function. 
	Everything else is handled by the adapter.
*/
template < typename Writer >
class bug_writer_adapter
{
	public:
	
	bug_writer_adapter( Writer& writer )
	: writer( writer )
	{	}
	
	template < typename Type >
	friend bug_writer_adapter& operator << 
	( 
		bug_writer_adapter& stream, 
		Type const& value
	)
	{
		stringstream
			helper;
		helper << value;
		stream.data += helper.str( );
		return stream;
	}
	
	void flush( void )
	{
		if( !data.empty( ) )
		{
			writer.write( &data[ 0 ], data.size( ) );
			data.clear( );
		}
	}
	
	protected:

	Writer
		writer;
	string
		data;
};

int main( void )
{
	remote_connection
		test;
	bug_writer_adapter< remote_connection >
		brca( test );
	SBUG( brca, "entering main" );
	int
		a = 200;
	float
		b = 0.5;
	char const*
		c = "ten";
	SBUG( brca, a );
	SBUG( brca, b );
	SBUG( brca, sqrt( a * b ) );
	SBUG( brca, c );
	SBUG( brca, "exiting main" );
}

So the underlying remote connection class doesn't have to be modified, obviously - the adapter translates all the debugging output to text and then passes the formatted chunk of bytes to it. All and all, it's a pretty simple setup.

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