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Remove README.compose because String::compose is not used in GParted
String::ucompose is used (note the added "u").
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Curtis Gedak
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Documentation for the compose library
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-------------------------------------
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The file compose.hpp contains the source for a small C++ library for
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composition of strings from arbitrary objects convertible to strings
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(such as integers, floats etc.). This is the documentation for that
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library. The utility of the library is thought to be greatest for
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programmers needing translation of their programs, but in fact it can
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be used for its mere convenience.
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The file ucompose.hpp has a front-end that uses Glib::ustrings which
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come from the C++ GUI library Gtkmm (see www.gtkmm.org). Use
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ucompose.hpp and String::ucompose if you are using Gtkmm - else your
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ustrings will end up corrupted someday.
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The initiating problem - a prelude
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----------------------------------
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The basic problem this library solves is that of creating strings such
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as "Fact is that 10 gobbles are worth 100$ of troubles." where the two
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numbers, 10 and 100, are to be determined at runtime and may vary. C++
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doesn't have a good solution for this problem - unless you use
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something like itoa() (which isn't really portable and doesn't work
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for arbitrary objects, say GobbliGobs, that define their own
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conversion routines by overloading "ostream & operator<<(ostream &,
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Object)"), the best option you have is to aggregate an ostringstream:
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std::ostringstream os;
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os << gobble_no;
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std::string gobble_no_as_string = os.str();
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But even Bjarne Stroustrup thinks this is embarrassing for ordinary
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integer-to-string conversion. The Boost family of libraries (see
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www.boost.org) defines a lexical_cast<T>() which one may use to
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conveniently compose a complex string (in fact it uses a stringstream
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internally):
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#include <boost/lexical_cast.hpp>
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std::string s = "Fact is that " + boost::lexical_cast<std::string>(gobble_no)
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+ " gobbles are worth "
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+ boost::lexical_cast<std::string>(gobble_price)
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+ "$ of troubles.";
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However, this is prone to subtle errors when written (one may easily
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forget a '+' or a space on one side of a converted object, both
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annoyingly requiring recompilation and attention), hard to read, and a
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real internationalisation killer. Imagine translating the strings
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"Fact is that "
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" gobbles are worth "
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"$ of troubles."
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not quite sure whether the strings actually form an entity and with
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only fragmented clues of what is inserted between them. What is worse,
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most languages do not order the different parts of sentences the same
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way English does. And by chopping up the string, we have more or less
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enforced a particular order (or at least made the changing of it most
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difficult for the translator). Also, one now has to mark up three
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strings in the program to get one string translated - another source
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of errors.
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Furthermore, the lexical cast of Boost doesn't allow us to control
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even simple formatting requests, such as increased (or, often more
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importantly, decreased) precision of floating point numbers.
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Interlude: The Solution that isn't really a Solution
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----------------------------------------------------
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So, one might think, isn't this string composition problem already
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solved? Does our inheritage from C not already provide us with the
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solution, the end to all these troubles? Upon thinking this, one
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probably has the dreaded printf, "print formatted", family in mind.
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Except from the type insecurity resulting from usage of such
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functions, a problem already partly solved by modern compiler
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warnings, suffice to say that they mix badly with ordinary C++ style
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of programming. For good reason, they don't return proper
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std::string's and usually even require one to worry about memory
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management. Needless worries.
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Finale: An end to the problem
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-----------------------------
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So what we need really need is something like
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std::string s = compose("Fact is that %1 gobbles are worth %2$ of troubles.",
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gobble_no, gobble_price);
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This is what this library gives.
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Semantics in details (...and there was much rejoice)
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----------------------------------------------------
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Actually, what the library really gives is 15 overloaded template
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functions like
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template <typename T1, typename T2>
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inline std::string compose(const std::string &fmt,
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const T1 &o1, const T2 &o2);
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declared in the namespace String - each allowing one extra parameter
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to join the composition - and some implementation details in the
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namespace StringPrivate (notably a class that defines a generic,
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unlimited parameter substitution mechanism).
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The first parameter is the format string containing percent
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specifications (%1, %2 etc.) and the following generically typed
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parameters are the objects to be inserted in the string. The 15
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functions allow up to 15 parameters to be inserted. For instance:
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String::compose("%1 times %2 equals %3", 1.5, 2, 3);
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// "1.5 times 2 equals 3"
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A percent specification consists of a percent sign followed by an
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integer. The format string is parsed at first so any possible new
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percent specifications that may be constructed halfway through the
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composition by the inserted strings, aren't interpreted as such:
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String::compose("1st: %1 2nd: %2", "%2", 1234);
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// "1st: %2 2nd: 1234"
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Of course, this format allows one to easily swap the specification
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strings:
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String::compose("No. %2 is better than no. %1", 1, 2);
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// "No. 2 is better than no. 1"
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One may silently leave out a specification string:
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String::compose("%1, %3", "Hey", "hi", "ho");
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// "Hey, ho"
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Or even an object (all percent specifications are always erased):
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String::compose("%1 %2: '%3'", "Twin", "geeks");
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// "Twin geeks: ''"
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And specifications may be repeated:
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String::compose("I am feeling so %1, %1, %1!", "happy");
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// "I am feeling so happy, happy, happy!"
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This gives potential translators of the strings considerable freedom.
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Conversion of objects, the art of
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----------------------------------
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Internally the template functions use an object that stores a
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stringstream for converting arguments. Thus adding an extra parameter
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to the compose function is very much like appending an extra "<< arg"
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to a stream output statement.
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This implies seamless support for manipulators and that user-supplied
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conversions are supported for free. To add output support for one of
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your own classes, simply add the appropriate operator<< overload, just
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as you would for std::cout:
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ostream &operator<<(ostream &, const MyType &my_obj)
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{
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// ...
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}
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The manipulator support imply that constructions like the following
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are possible:
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#include <iomanip>
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// ...
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double r = 1.0 / 6;
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String::compose("1/6 app. equals %1, %2, and %3", r,
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std::setprecision(10), r,
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std::setprecision(3), r);
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// "1/6 app. equals 0.166667, 0.1666666667, and 0.167"
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Note that within the same call of compose the stringstream is not
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cleared, so the same rules governs whether the settings are remembered
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as for ordinary ostreams (look up the rules in your favourite C++
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standard iostream documentation):
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String::compose("1/6 app. equals %1, %2, and %3", r,
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std::setprecision(10), r, r);
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// "1/6 app. equals 0.166667, 0.1666666667, and 0.1666666667"
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Each call of compose constructs a new stringstream so settings are not
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preserved across these.
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Internally, the manipulator detection works by examining the output
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from the stringstream - if it is empty, then the parameter is not
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considered real output, and the specification number is not
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incremented. This behaviour is needed to give the above semantics, but
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it also implies that if your insert the empty string, "", the compose
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object will not consider it output; as a consequence the following
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example does not work as expected:
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String::compose("I'm a%1 alien at the age of %2.",
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happiness > 10 ? " happy" : "", 99800);
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// "I'm a happy alien at the age of 99800."
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// "I'm a99800 alien at the age of ."
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But read below for why you really should not do this anyway, and for
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the simple remedy.
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Escaping the parse
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------------------
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All double percent signs are replaced with a single sign; if the
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double signs appears in front of an integer, the resulting single
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percent sign followed by the integer isn't considered a specification.
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Thus, to get a percent specification:
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String::compose("This is a %1: %%1", "specification");
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// "This is a specification: %1"
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Single percent signs are left untouched in case they aren't followed
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by a number:
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String::compose("%1% done", 0.98 * 100);
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// "98% done"
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To get two percent signs in a row:
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String::compose("%1 %2 signs: %%%%", 2, '%');
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// "2 % signs: %%"
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A final advice against cleverness
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---------------------------------
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Do not let the techniques this library offers tempt you into trying
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overly clever string compositions, or the translators of your program
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will curse you in eternity! For instance:
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String::compose("A%1 man", man.is_angry() ? "n angry" : " content");
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// "An angry man"
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// "A happy man"
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The inherent problem with this approach is that it hardcodes the way
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the sentence is constructed; obviously this is only guaranteed to work
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for English. It also makes the code harder to read. Instead, fork the
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code on a slightly higher level and repeat the whole string:
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std::string description;
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if (man.is_angry())
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description = String::compose("An angry man");
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else
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description = String::compose("A content man");
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Now the code is also much easier to read. This (slightly contrived,
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but realistic, I have seen it in real code) example is of course quite
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easy to spot, but in generel one needs to be very careful as soon as
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one substitute strings into ordinary text. One more example:
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String::compose("This is a %1", thing->got_wheels() ? "car" : "house");
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Innocent looking, but absolutely wrong. Most languages will need to
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change the part "a " of the string too (e.g. in Danish it is "en bil"
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(a car) but "et hus" (a house)).
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Completely decoupled words should of course not cause any problems:
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String::compose("The XML error was caused by the element '%1'",
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element.name);
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In fact, factoring out the common part of such strings (e.g. putting
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it in a xml_error function) may be much of a convenience for the
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translators.
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Often, you would want to use constructions such as the above ones when
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some part of the string sometime needs to be in plural form:
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String::compose("There is a total of %1 %2.", n, n == 1 ? "hen" : "hens");
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But do not do this - for some languages, a two-way branch isn't
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enough. Instead the translation system should have some means of
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solving this (for gettext, look up the function 'ngettext').
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Practically speaking, the art of improvement
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--------------------------------------------
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The latest, super-duper-improved (hah!) version of the library should
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always be available together with a small test example from
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http://www.cs.aau.dk/~olau/compose/
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Note that the license of the library is GNU Lesser General Public
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License. For the uninitiated, this basically means that you may use
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the library freely in any way you want, but if you change any of the
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code of it and distribute it in binary form, you _must_ also release
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the source for the modified library under the LGPL. Thus you must
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share your improvements with the rest of the world, just like I shared
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my code with you. No big deal. If you have any comments, suggestions
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for improvements, critics, (good) jokes, portability fixes,
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interesting uses, or perhaps just a word of wisdom, let me know.
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Ole Laursen <olau@hardworking.dk>, 2003.
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A short historical anecdote - postlude
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--------------------------------------
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One might think that the name of the namespace "String::" is pretty
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lame; and, indeed, I agree. However, the library was initially born
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because the Gtkmm family of libraries doesn't help with composition of
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strings and e.g. integers in a way so that the placement of the
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integers is decoupled from the conversion to strings; a procedure that
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is frequently badly needed in a GUI-intensitive program.
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So when I got the idea of using a stringstream together with a
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template member function, I promptly implemented the Composition class
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(defined in the StringPrivate namespace). But, alas, the functionality
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isn't conceived to be important enough for Gtkmm users to be put into
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Glibmm (a support library for Gtkmm that contains various helpers), so
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instead I'm placing it here. Unfortunately, this left me without a
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proper namespace (formerly "Glib::") to wrap the functionality in -
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thus the current "String::".
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My apologies. Maybe the situation will change one day. Suggestions are
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welcome.
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