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- <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"
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- <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>
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