| <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> |
| |
| <html> |
| <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> |