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boost.png (6897 bytes)The Boost Format library

The <boost/format.hpp> format class provides printf-like formatting, in a type-safe manner which allows output of user-defined types.


A format object is constructed from a format-string, and is then given arguments through repeated calls to operator%.
Each of those arguments are then converted to strings, who are in turn combined into one string, according to the format-string.

cout << boost::format("writing %1%,  x=%2% : %3%-th try") % "toto" % 40.23 % 50; 
     // prints "writing toto,  x=40.230 : 50-th try"

How it works

  1. When you call format(s), where s is the format-string, it constructs an object, which parses the format string and look for all directives in it and prepares internal structures for the next step.
  2. Then, either immediately, as in
    cout << format("%2% %1%") % 36 % 77;
    or later on, as in
    format fmter("%2% %1%");
    fmter % 36; fmter % 77;
    you feed variables into the formatter.
    those variables are dumped into an internal stream, which state is set according to the given formatting options in the format-string -if there are any-, and the format object stores the string results for the last step.
  3. Once all arguments have been fed you can dump the format object to a stream, or get its string value by using the str() member function, or the free function str(const format& ) in namespace boost. The result string stays accessible in the format object until another argument is passed, at which time it is reinitialised.
    // fmter was previously created and fed arguments, it can print the result :
    cout << fmter ;  
    // You can take the string result :
    string s  = fmter.str();
    // possibly several times :
    s = fmter.str( );
    // You can also do all steps at once :
    cout << boost::format("%2% %1%") % 36 % 77; 
    // using the str free function :
    string s2 = str( format("%2% %1%") % 36 % 77 );
  4. Optionnally, after step 3, you can re-use a format object and restart at step2 : fmter % 18 % 39;
    to format new variables with the same format-string, saving the expensive processing involved at step 1.
All in all, the format class translates a format-string (with eventually printf-like directives) into operations on an internal stream, and finally returns the result of the formatting, as a string, or directly into an output stream.


using namespace std;
using boost::format;
using boost::io::group;

Sample Files

The program sample_formats.cpp demonstrates simple uses of format.

sample_new_features.cpp illustrates the few formatting features that were added to printf's syntax such as simple positional directives, centered alignment, and 'tabulations'.

sample_advanced.cpp demonstrates uses of advanced features, like reusing, and modifying, format objects, etc..

And sample_userType.cpp shows the behaviour of the format library on user-defined types.


boost::format( format-string ) % arg1 % arg2 % ... % argN

The format-string contains text in which special directives will be replaced by strings resulting from the formatting of the given arguments.
The legacy syntax in the C and C++ worlds is the one used by printf, and thus format can use directly printf format-strings, and produce the same result (in almost all cases. see Incompatibilities with printf for details)
This core syntax was extended, to allow new features, but also to adapt to the C++ streams context. Thus, format accepts several forms of directives in format-strings :

On top of the standard printf format specifications, new features were implemented, like centered alignment. See new format specification for details.

printf format specifications

The printf format specifications supported by Boost.format follows the Unix98 Open-group printf precise syntax, rather than the standard C printf, which does not support positional arguments. (Common flags have the same meaning in both, so it should not be a headache for anybody)
Note that it is an error to use positional format specifications (e.g. %3$+d) mixed with non-positional ones (e.g. %+d) in the same format string.
In the Open-group specification, referring to the same argument several times (e.g. "%1$d %1$d") has undefined behaviour. Boost.format's behaviour in such cases is to allow each argument to be reffered to any number of times. The only constraint is that it expects exactly P arguments, P being the maximum argument number used in the format string. (e.g., for "%1$d %10$d", P == 10 ).
Supplying more, or less, than P arguments raises an exception. (unless it was set otherwise, see exceptions)

A specification spec has the form:

    [ N$ ] [ flags ] [ width ] [ . precision ] [ argument-type ] conversion-specifier
Fields inside square brackets are optional. Each of those fields are explained one by one in the following list:

new format-specifications

Differences of behaviour vs printf

Suppose you have variables x1, x2 (built_in types, supported by C's printf),
and a format string s intended for use with a printf function this way :
printf(s, x1, x2);

In almost all cases, the result will be the same as with this command :
cout << format(s) % x1 % x2;

But because some printf format specifications don't translate well into stream formatting options, there are a few notable imperfections in the way Boost.format emulates printf.
In any case, the format class should quietly ignore the unsupported options, so that printf format-strings are always accepted by format and produce almost the same output as printf.

Here is the full list of such differences : Also, note that the special 'n' type-specification (used to tell printf to save in a variable the number of characters output by the formatting) has no effect in format.
Thus format strings containing this type-specification should produce the same converted string by printf or format. It will not cause differences in the formatted strings between printf and format.
To get the number of characters in the formatted string using Boost.Format, you can use the size() member function :
format formatter("%+5d");
cout << formatter % x;
unsigned int n = formatter.size();

User-defined types output

All flags which are translated into modification to the stream state act recursively within user-defined types. ( the flags remain active, and so does the desired format option, for each of the '<<' operations that might be called by the user-defined class)

e.g., with a Rational class, we would have something like :
Rational ratio(16,9);
cerr << format("%#x \n")  % ratio;  // -> "0x10/0x9 \n"

It's a different story for other formatting options. For example, setting width applies to the final output produced by the object, not to each of its internal outputs, and that's fortunate :

cerr << format("%-8d")  % ratio;  // -> "16/9    "      and not    "16      /9       "
cerr << format("%=8d")  % ratio;  // -> "  16/9  "      and not    "   16   /    9   "

But so does the 0 and ' ' options (contrarily to '+' which is directly translated to the stream state by showpos. But no such flags exist for the zero and space printf options)
and that is less natural :

cerr << format("%+08d \n")  % ratio;  // -> "+00016/9"
cerr << format("% 08d \n")  % ratio;  // -> "000 16/9"
It is possible to obtain a better behaviour by carefully designing the Rational's operator<< to handle the stream's width, alignment and showpos paramaters by itself. This is demonstrated in sample_userType.cpp.

Manipulators, and internal stream state

The internal stream state of format is saved before and restored after output of an argument; therefore, the modifiers are not sticky and affect only the argument they are applied to.
The default state for streams, as stated by the standard, is : precision 6, width 0, right alignment, and decimal flag set.

The state of the internal format stream can be changed by manipulators passed along with the argument; via the group function, like that :

cout << format("%1% %2% %1%\n") % group(hex, showbase, 40) % 50; // prints "0x28 50 0x28\n"

When passing N items inside a 'group' Boost.format needs to process manipulators diferently from regular argument, and thus using group is subject to the following constraints :

  1. the object to be printed must be passed as the last item in the group
  2. the first N-1 items are treated as manipulators, and if they do produce output, it is discarded

Such manipulators are passed to the streams right before the following argument, at every occurrence. Note that formatting options specified within the format string are overridden by stream state modifiers passed this way. For instance in the following code, the hex manipulator has priority over the d type-specification in the format-string which would set decimal output :

cout << format("%1$d %2% %1%\n") % group(hex, showbase, 40) % 50; 
// prints "0x28 50 0x28\n"



Boost.format enforces a number of rules on the usage of format objects. The format-string must obeys the syntax described above, the user must supply exactly the right number of arguments before outputting to the final destination, and if using modify_item or bind_arg, items and arguments index must not be out of range.
When format detects that one of these rules is not satisfied, it raises a corresponding exception, so that the mistakes don't go unnoticed and unhandled.
But the user can change this behaviour to fit his needs, and select which types of errors may raise exceptions using the following functions :

unsigned char exceptions(unsigned char newexcept); // query and set
unsigned char exceptions() const;                  // just query

The user can compute the argument newexcept by combining the following atoms using binary arithmetic :

For instance, if you don't want Boost.format to detect bad number of arguments, you can define a specific wrapper function for building format objects with the right exceptions settings :

boost::format  my_fmt(const std::string & f_string) {
    using namespace boost::io;
    format fmter(f_string);
    fmter.exceptions( all_error_bits ^ ( too_many_args_bit | too_few_args_bit )  );
    return fmter;

It is then allowed to give more arguments than needed (they are simply ignored) :

cout << my_fmt(" %1% %2% \n") % 1 % 2 % 3 % 4 % 5;

And if we ask for the result before all arguments are supplied, the corresponding part of the result is simply empty

cout << my_fmt(" _%2%_ _%1%_ \n") % 1 ;
// prints      " __ _1_ \n"

A Note about performance

The performance of boost::format for formatting a few builtin type arguments with reordering can be compared to that of Posix-printf, and of the equivalent stream manual operations to give a measure of the overhead incurred. The result may greatly depend on the compiler, standard library implementation, and the precise choice of format-string and arguments.

Common stream implementations eventually call functions of the printf family for the actual formatting of numbers. In general, printf will be noticeably faster than the direct stream operations due to the reordering overhead (allocations to store the pieces of string, stream initialisation at each item formatting, ..). The direct stream operations would be faster than boost::format - one can expect a ratio ranging from 2 to 5 or more.

When iterated formattings are a performance bottleneck, performance can be slightly increased by parsing the format string into a format object, and copying it at each formatting, in the following way:

    const boost::format fmter(fstring);
    dest << boost::format(fmter) % arg1 % arg2 % arg3 ;

As an example of performance results, the author measured the time of execution of iterated formattings with 4 different methods:

  1. posix printf
  2. manual stream output (to a dummy nullStream stream sending the bytes into oblivion)
  3. boost::format copied from a const object as shown above
  4. the straight boost::format usage

the test was compiled with g++-3.3.3 and the following timings were measured (in seconds, and ratios):

string     fstring="%3$0#6x %1$20.10E %2$g %3$0+5d \n";
double     arg1=45.23;
double     arg2=12.34;
int        arg3=23;

- release mode :
printf                 : 2.13
nullStream             : 3.43,  = 1.61033 * printf
boost::format copied   : 6.77,  = 3.1784  * printf ,  = 1.97376 * nullStream
boost::format straight :10.67,  = 5.00939 * printf ,  = 3.11079 * nullStream

- debug mode :
printf                 : 2.12
nullStream             : 3.69,  = 1.74057 * printf
boost::format copied   :10.02,  = 4.72642 * printf ,  = 2.71545 * nullStream
boost::format straight :17.03,  = 8.03302 * printf ,  = 4.61518 * nullStream

Class Interface Extract

namespace boost {

template<class charT, class Traits=std::char_traits<charT> > 
class basic_format 
  typedef std::basic_string<charT, Traits> string_type;
  typedef typename string_type::size_type     size_type;
  basic_format(const charT* str);
  basic_format(const charT* str, const std::locale & loc);
  basic_format(const string_type& s);
  basic_format(const string_type& s, const std::locale & loc);
  basic_format& operator= (const basic_format& x);

  void clear(); // reset buffers
  basic_format& parse(const string_type&); // clears and parse a new format string

  string_type str() const;
  size_type size() const;

  // pass arguments through those operators :
  template<class T>  basic_format&   operator%(T& x);  
  template<class T>  basic_format&   operator%(const T& x);

  // dump buffers to ostream :
  friend std::basic_ostream<charT, Traits>& 
  operator<< <> ( std::basic_ostream<charT, Traits>& , basic_format& ); 

   // Choosing which errors will throw exceptions :
   unsigned char exceptions() const;
   unsigned char exceptions(unsigned char newexcept);

// ............  this is just an extract .......
}; // basic_format

typedef basic_format<char >          format;
typedef basic_format<wchar_t >      wformat;

// free function for ease of use :
template<class charT, class Traits> 
std::basic_string<charT,Traits>  str(const basic_format<charT,Traits>& f) {
      return f.str();

} // namespace boost


This class's goal is to bring a better, C++, type-safe and type-extendable printf equivalent to be used with streams.

Precisely, format was designed to provide the following features :

In the process of the design, many issues were faced, and some choices were made, that might not be intuitively right. But in each case they were taken for some reasons.


The author of Boost format is Samuel Krempp.   He used ideas from Rüdiger Loos' format.hpp and Karl Nelson's formatting classes.

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Revised 23 October, 2017

Copyright © 2002 Samuel Krempp

Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at