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+<head>
+ <meta http-equiv="Content-Language" content="en-us">
+ <meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
+
+ <title>Type-safe 'printf-like' format class</title>
+</head>
+
+<body bgcolor="#FFFFFF" text="#000000">
+ <h1><img align="middle" alt="boost.png (6897 bytes)" height="86" src=
+ "../../../boost.png" width="277">Type-safe 'printf-like' <b>format
+ class</b></h1>
+
+ <h2>Choices made</h2>
+
+ <p>"Le pourquoi du comment" ( - "the why of the how")</p>
+ <hr>
+
+ <h3>The syntax of the format-string</h3>
+
+ <p>Format is a new library. One of its goal is to provide a replacement for
+ printf, that means format can parse a format-string designed for printf,
+ apply it to the given arguments, and produce the same result as printf
+ would have.<br>
+ With this constraint, there were roughly 3 possible choices for the syntax
+ of the format-string :</p>
+
+ <ol>
+ <li>Use the exact same syntax of printf. It's well known by many
+ experienced users, and fits almost all needs. But with C++ streams, the
+ type-conversion character, crucial to determine the end of a directive,
+ is only useful to set some associated formatting options, in a C++
+ streams context (%x for setting hexa, etc..) It would be better to make
+ this obligatory type-conversion character, with modified meaning,
+ optional.</li>
+
+ <li>extend printf syntax while maintaining compatibility, by using
+ characters and constructs not yet valid as printf syntax. e.g. : "%1%",
+ "%[1]", "%|1$d|", .. Using begin / end marks, all sort of extension can
+ be considered.</li>
+
+ <li>Provide a non-legacy mode, in parallel of the printf-compatible one,
+ that can be designed to fit other objectives without constraints of
+ compatibilty with the existing printf syntax.<br>
+ But Designing a replacement to printf's syntax, that would be clearly
+ better, and as much powerful, is yet another task than building a format
+ class. When such a syntax is designed, we should consider splitting
+ Boost.format into 2 separate libraries : one working hand in hand with
+ this new syntax, and another supporting the legacy syntax (possibly a
+ fast version, built with safety improvement above snprintf or the
+ like).</li>
+ </ol>In the absence of a full, clever, new syntax clearly better adapted to
+ C++ streams than printf, the second approach was chosen. Boost.format uses
+ printf's syntax, with extensions (tabulations, centered alignements) that
+ can be expressed using extensions to this syntax.<br>
+ And alternate compatible notations are provided to address the weaknesses
+ of printf's :
+
+ <ul>
+ <li><i>"%<b>N</b>%"</i> as a simpler positional, typeless and optionless
+ notation.</li>
+
+ <li><i>%|spec|</i> as a way to encapsulate printf directive in movre
+ visually evident structures, at the same time making printf's
+ 'type-conversion character' optional.</li>
+ </ul>
+ <hr>
+
+ <h3>Why are arguments passed through an operator rather than a function
+ call ?</h3><br>
+ The inconvenience of the operator approach (for some people) is that it
+ might be confusing. It's a usual warning that too much of overloading
+ operators gets people real confused.<br>
+ Since the use of format objects will be in specific contexts ( most often
+ right after a "cout << ") and look like a formatting string followed
+ by arguments indeed :
+
+ <blockquote>
+ <pre>
+format(" %s at %s with %s\n") % x % y % z;
+</pre>
+ </blockquote>we can hope it wont confuse people that much.
+
+ <p>An other fear about operators, is precedence problems. What if I someday
+ write <b>format("%s") % x+y</b><br>
+ instead of <i>format("%s") % (x+y)</i> ??<br>
+ It will make a mistake at compile-time, so the error will be immediately
+ detected.<br>
+ indeed, this line calls <i>tmp = operator%( format("%s"), x)</i><br>
+ and then <i>operator+(tmp, y)</i><br>
+ tmp will be a format object, for which no implicit conversion is defined,
+ and thus the call to operator+ will fail. (except if you define such an
+ operator, of course). So you can safely assume precedence mistakes will be
+ noticed at compilation.</p>
+
+ <p><br>
+ On the other hand, the function approach has a true inconvenience. It needs
+ to define lots of template function like :</p>
+
+ <blockquote>
+ <pre>
+template <class T1, class T2, .., class TN>
+string format(string s, const T1& x1, .... , const T1& xN);
+
+</pre>
+ </blockquote>and even if we define those for N up to 500, that is still a
+ limitation, that C's printf does not have.<br>
+ Also, since format somehow emulates printf in some cases, but is far from
+ being fully equivalent to printf, it's best to use a radically different
+ appearance, and using operator calls succeeds very well in that !
+
+ <p><br>
+ Anyhow, if we actually chose the formal function call templates system, it
+ would only be able to print Classes T for which there is an</p>
+
+ <blockquote>
+ <pre>
+operator<< ( stream, const T&)
+</pre>
+ </blockquote>Because allowing both const and non const produces a
+ combinatorics explosion - if we go up to 10 arguments, we need 2^10
+ functions.<br>
+ (providing overloads on T& / const T& is at the frontier of defects
+ of the C++ standard, and thus is far from guaranteed to be supported. But
+ right now several compilers support those overloads)<br>
+ There is a lot of chances that a class which only provides the non-const
+ equivalent is badly designed, but yet it is another unjustified restriction
+ to the user.<br>
+ Also, some manipulators are functions, and can not be passed as const
+ references. The function call approach thus does not support manipulators
+ well.
+
+ <p>In conclusion, using a dedicated binary operator is the simplest, most
+ robust, and least restrictive mechanism to pass arguments when you can't
+ know the number of arguments at compile-time.</p>
+ <hr>
+
+ <h3>Why operator% rather than a member function 'with(..)'
+ ??</h3>technically,
+
+ <blockquote>
+ <pre>
+format(fstr) % x1 % x2 % x3;
+</pre>
+ </blockquote>has the same structure as
+
+ <blockquote>
+ <pre>
+format(fstr).with( x1 ).with( x2 ).with( x3 );
+</pre>
+ </blockquote>which does not have any precedence problem. The only drawback,
+ is it's harder for the eye to catch what is done in this line, than when we
+ are using operators. calling .with(..), it looks just like any other line
+ of code. So it may be a better solution, depending on tastes. The extra
+ characters, and overall cluttered aspect of the line of code using
+ 'with(..)' were enough for me to opt for a true operator.
+ <hr>
+
+ <h3>Why operator% rather than usual formatting operator<< ??</h3>
+
+ <ul>
+ <li>because passing arguments to a format object is *not* the same as
+ sending variables, sequentially, into a stream, and because a format
+ object is not a stream, nor a manipulator.<br>
+ We use an operator to pass arguments. format will use them as a
+ function would, it simply takes arguments one by one.<br>
+ format objects can not provide stream-like behaviour. When you try to
+ implement a format object that acts like a manipulator, returning a
+ stream, you make the user beleive it is completely like a
+ stream-manipulator. And sooner or later, the user is deceived by this
+ point of view.<br>
+ The most obvious example of that difference in behaviour is
+
+ <blockquote>
+ <pre>
+cout << format("%s %s ") << x;
+cout << y ; // uh-oh, format is not really a stream manipulator
+</pre>
+ </blockquote>
+ </li>
+
+ <li>precedence of % is higher than that of <<. It can be viewd as a
+ problem, because + and - thus needs to be grouped inside parentheses,
+ while it is not necessary with '<<'. But if the user forgets, the
+ mistake is catched at compilation, and hopefully he won't forget
+ again.<br>
+ On the other hand, the higher precedence makes format's behaviour very
+ straight-forward.
+
+ <blockquote>
+ <pre>
+cout << format("%s %s ") % x % y << endl;
+</pre>
+ </blockquote>is treated exaclt like :
+
+ <blockquote>
+ <pre>
+cout << ( format("%s %s ") % x % y ) << endl;
+</pre>
+ </blockquote>So using %, the life of a format object does not interfere
+ with the surrounding stream context. This is the simplest possible
+ behaviour, and thus the user is able to continue using the stream after
+ the format object.<br>
+ <br>
+ With operator<<, things are much more problematic in this
+ situation. This line :
+
+ <blockquote>
+ <pre>
+cout << format("%s %s ") << x << y << endl;
+</pre>
+ </blockquote>is understood as :
+
+ <blockquote>
+ <pre>
+( ( ( cout << format("%s %s ") ) << x ) << y ) << endl;
+</pre>
+ </blockquote>Several alternative implementations chose
+ operator<<, and there is only one way to make it work :<br>
+ the first call to
+
+ <blockquote>
+ <pre>
+operator<<( ostream&, format const&)
+</pre>
+ </blockquote>returns a proxy, encapsulating both the final destination
+ (cout) and the format-string information<br>
+ Passing arguments to format, or to the final destination after
+ completion of the format are indistinguishable. This is a problem.
+
+ <p>I examined several possible implementations, and none is completely
+ satsifying.<br>
+ E.g. : In order to catch users mistake, it makes sense to raise
+ exceptions when the user passes too many arguments. But in this
+ context, supplementary arguments are most certainly aimed at the final
+ destination. There are several choices here :</p>
+
+ <ul>
+ <li>You can give-up detection of arity excess, and have the proxy's
+ template member operator<<( const T&) simply forward all
+ supplementary arguments to cout.</li>
+
+ <li>Require the user to close the format arguments with a special
+ manipulator, 'endf', in this way :
+
+ <blockquote>
+ <pre>
+cout << format("%s %s ") << x << y << endf << endl;
+</pre>
+ </blockquote>You can define endf to be a function that returns the
+ final destination stored inside the proxy. Then it's okay, after
+ endf the user is calling << on cout again.
+ </li>
+
+ <li>An intermediate solution, is to adress the most frequent use,
+ where the user simply wants to output one more manipulator item to
+ cout (a std::flush, or endl, ..)
+
+ <blockquote>
+ <pre>
+cout << format("%s %s \n") << x << y << flush ;
+</pre>
+ </blockquote>Then, the solution is to overload the operator<<
+ for manipulators. This way You don't need endf, but outputting a
+ non-manipulator item right after the format arguments is a mistake.
+ </li>
+ </ul><br>
+ The most complete solution is the one with the endf manipualtor. With
+ operator%, there is no need for this end-format function, plus you
+ instantly see which arguments are going into the format object, and
+ which are going to the stream.
+ </li>
+
+ <li>Esthetically : '%' is the same letter as used inside the
+ format-string. That is quite nice to have the same letter used for
+ passing each argument. '<<' is 2 letters, '%' is one. '%' is also
+ smaller in size. It overall improves visualisation (we see what goes with
+ what) :
+
+ <blockquote>
+ <pre>
+cout << format("%s %s %s") %x %y %z << "And avg is" << format("%s\n") %avg;
+</pre>
+ </blockquote>compared to :
+
+ <blockquote>
+ <pre>
+cout << format("%s %s %s") << x << y << z << endf <<"And avg is" << format("%s\n") << avg;
+</pre>
+ </blockquote>"<<" misleadingly puts the arguments at the same
+ level as any object passed to the stream.
+ </li>
+
+ <li>python also uses % for formatting, so you see it's not so "unheard
+ of" ;-)</li>
+ </ul>
+ <hr>
+
+ <h3>Why operator% rather than operator(), or operator[] ??</h3>
+
+ <p>operator() has the merit of being the natural way to send an argument
+ into a function. And some think that operator[] 's meaning apply well to
+ the usage in format.<br>
+ They're as good as operator% technically, but quite ugly. (that's a matter
+ of taste)<br>
+ And deepd down, using operator% for passing arguments that were referred to
+ by "%" in the format string seems much more natural to me than using those
+ operators.</p>
+ <hr>
+
+ <p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src=
+ "../../../doc/images/valid-html401.png" alt="Valid HTML 4.01 Transitional"
+ height="31" width="88"></a></p>
+
+ <p>Revised
+ <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->02 December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38510" --></p>
+
+ <p><i>Copyright © 2001 Samuel Krempp</i></p>
+
+ <p><i>Distributed under the Boost Software License, Version 1.0. (See
+ accompanying file <a href="../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
+ copy at <a href=
+ "http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
+</body>
+</html>