C equivalent of StringBuffer StringBuilder

Question

Is there a C   Standard Template Library class that provides efficient string concatenation functionality  similar to C  s StringBuilder or Java s StringBuffer

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The Rope container may be worth if have to insert delete string into the random place of destination string or for a long char sequences  Here is an example from SGI s implementation   crope r 1000000   x                crope is rope lt char gt   wrope is rope lt wchar t gt                                     Builds a rope containing a million  x s                                     Takes much less than a MB  since the                                    different pieces are shared  crope r2   r    abc    r           concatenation  takes on the order of 100s                                    of machine instructions  fast crope r3   r2 substr 1000000  3            yields  abc   fast  crope r4   r2 substr 1000000  1000000      also fast  reverse r2 mutable begin    r2 mutable end                                        correct  but slow  may take a                                    minute or more

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std  string is the C   equivalent  It s mutable

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A convenient string builder for c    Like many people answered before  std  stringstream is the method of choice  It works good and has a lot of conversion and formatting options  IMO it has one pretty inconvenient flaw though  You can not use it as a one liner or as an expression  You always have to write   std  stringstream ss  ss  lt  lt   my data    lt  lt  42  std  string myString  ss str        which is pretty annoying  especially when you want to initialize strings in the constructor   The reason is  that a  std  stringstream has no conversion operator to std  string and b  the operator  lt  lt     s of the stringstream don t return a stringstream reference  but a std  ostream reference instead - which can not be further computed as a string stream   The solution is to override std  stringstream and to give it better matching operators   namespace NsStringBuilder   template lt typename T gt  class basic stringstream   public std  basic stringstream lt T gt    public      basic stringstream           operator const std  basic string lt T gt     const                                  return std  basic stringstream lt T gt   str                              basic stringstream lt T gt  amp  operator lt  lt     bool  val                                std  basic stringstream lt T gt   operator  lt  lt    val   return  this        basic stringstream lt T gt  amp  operator lt  lt     char  val                                std  basic stringstream lt T gt   operator  lt  lt    val   return  this        basic stringstream lt T gt  amp  operator lt  lt     signed char  val                         std  basic stringstream lt T gt   operator  lt  lt    val   return  this        basic stringstream lt T gt  amp  operator lt  lt     unsigned char  val                       std  basic stringstream lt T gt   operator  lt  lt    val   return  this        basic stringstream lt T gt  amp  operator lt  lt     short  val                               std  basic stringstream lt T gt   operator  lt  lt    val   return  this        basic stringstream lt T gt  amp  operator lt  lt     unsigned short  val                      std  basic stringstream lt T gt   operator  lt  lt    val   return  this        basic stringstream lt T gt  amp  operator lt  lt     int  val                                 std  basic stringstream lt T gt   operator  lt  lt    val   return  this        basic stringstream lt T gt  amp  operator lt  lt     unsigned int  val                        std  basic stringstream lt T gt   operator  lt  lt    val   return  this        basic stringstream lt T gt  amp  operator lt  lt     long  val                                std  basic stringstream lt T gt   operator  lt  lt    val   return  this        basic stringstream lt T gt  amp  operator lt  lt     unsigned long  val                       std  basic stringstream lt T gt   operator  lt  lt    val   return  this        basic stringstream lt T gt  amp  operator lt  lt     long long  val                           std  basic stringstream lt T gt   operator  lt  lt    val   return  this        basic stringstream lt T gt  amp  operator lt  lt     unsigned long long  val                  std  basic stringstream lt T gt   operator  lt  lt    val   return  this        basic stringstream lt T gt  amp  operator lt  lt     float  val                               std  basic stringstream lt T gt   operator  lt  lt    val   return  this        basic stringstream lt T gt  amp  operator lt  lt     double  val                              std  basic stringstream lt T gt   operator  lt  lt    val   return  this        basic stringstream lt T gt  amp  operator lt  lt     long double  val                         std  basic stringstream lt T gt   operator  lt  lt    val   return  this        basic stringstream lt T gt  amp  operator lt  lt     void   val                               std  basic stringstream lt T gt   operator  lt  lt    val   return  this        basic stringstream lt T gt  amp  operator lt  lt     std  streambuf   val                     std  basic stringstream lt T gt   operator  lt  lt    val   return  this        basic stringstream lt T gt  amp  operator lt  lt     std  ostream amp     val  std  ostream amp       std  basic stringstream lt T gt   operator  lt  lt    val   return  this        basic stringstream lt T gt  amp  operator lt  lt     std  ios amp     val  std  ios amp               std  basic stringstream lt T gt   operator  lt  lt    val   return  this        basic stringstream lt T gt  amp  operator lt  lt     std  ios base amp     val  std  ios base amp     std  basic stringstream lt T gt   operator  lt  lt    val   return  this        basic stringstream lt T gt  amp  operator lt  lt     const T   val                            return static cast lt basic stringstream lt T gt  amp  gt  std  operator  lt  lt    this  val          basic stringstream lt T gt  amp  operator lt  lt     const std  basic string lt T gt  amp   val         return static cast lt basic stringstream lt T gt  amp  gt  std  operator  lt  lt    this  val c str            typedef basic stringstream lt char gt         stringstream  typedef basic stringstream lt wchar t gt      wstringstream      With this  you can write things like  std  string myString  NsStringBuilder  stringstream    lt  lt   my data    lt  lt  42     even in the constructor    I have to confess I didn t measure the performance  since I have not used it in an environment which makes heavy use of string building yet  but I assume it won t be much worse than std  stringstream  since everything is done via references  except the conversion to string  but thats a copy operation in std  stringstream as well

User · Answer

I wanted to add something new because of the following   At a first attemp I failed to beat  std  ostringstream  s operator lt  lt   efficiency  but with more attemps I was able to make a StringBuilder that is faster in some cases   Everytime I append a string I just store a reference to it somewhere and increase the counter of the total size    The real way I finally implemented it  Horror   is to use a opaque buffer std  vector  lt  char        1 byte header  2 bits to tell if following data is  moved string  string or byte    6 bits to tell lenght of byte     for byte       I store directly bytes of short strings  for sequential memory access    for moved strings  strings appended with std  move    The pointer to a std  string object  we have ownership  set a flag in the class if there are unused reserved bytes there   for strings   The pointer to a std  string object  no ownership    There s also one small optimization  if last inserted string was mov d in  it checks for free reserved but unused bytes and store further bytes in there instead of using the opaque buffer  this is to save some memory  it actually make it slightly slower  maybe depend also on the CPU  and it is rare to see strings with extra reserved space anyway   This was finally slightly faster than std  ostringstream but it has few downsides    I assumed fixed lenght char types  so 1 2 or 4 bytes  not good for UTF8   I m not saying it will not work for UTF8  Just I don t checked it for laziness  I used bad coding practise  opaque buffer  easy to make it not portable  I believe mine is portable by the way  Lacks all features of ostringstream If some referenced string is deleted before mergin all the strings  undefined behaviour    conclusion  use std  ostringstream  It already fix the biggest bottleneck while ganing few   points in speed with mine implementation is not worth the downsides

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NOTE this answer has received some attention recently  I am not advocating this as a solution  it is a solution I have seen in the past  before the STL   It is an interesting approach and should only be applied over std  string or std  stringstream if after profiling your code you discover this makes an improvement   I normally use either std  string or std  stringstream   I have never had any problems with these   I would normally reserve some room first if I know the rough size of the string in advance   I have seen other people make their own optimized string builder in the distant past    class StringBuilder   private      std  string main      std  string scratch       const std  string  size type ScratchSize   1024      or some other arbitrary number  public      StringBuilder  amp  append const std  string  amp  str            scratch append str           if  scratch size    gt  ScratchSize                main append scratch               scratch resize 0                     return  this             const std  string  amp  str             if  scratch size    gt  0                main append scratch               scratch resize 0                     return main             It uses two strings one for the majority of the string and the other as a scratch area for concatenating short strings   It optimise s appends by batching the short append operations in one small string then appending this to the main string  thus reducing the number of reallocations required on the main string as it gets larger   I have not required this trick with std  string or std  stringstream   I think it was used with a third party string library before std  string  it was that long ago   If you adopt a strategy like this profile your application first

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The std  string append function isn t a good option because it doesn t accept many forms of data   A more useful alternative is to use std  stringstream  like so    include  lt sstream gt          std  stringstream ss     put arbitrary formatted data into the stream ss  lt  lt  4 5  lt  lt        lt  lt  4  lt  lt    whatever      convert the stream buffer into a string std  string str   ss str

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You can use  append   for simply concatenating strings   std  string s    string1   s append  string2      I think you might even be able to do   std  string s    string1   s     string2     As for the formatting operations of C  s StringBuilder  I believe snprintf  or sprintf if you want to risk writing buggy code  -    into a character array and convert back to a string is about the only option

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Since std  string in C   is mutable you can use that  It has a    operator and an append function   If you need to append numerical data use the std  to string functions   If you want even more flexibility in the form of being able to serialise any object to a string then use the std  stringstream class  But you ll need to implement your own streaming operator functions for it to work with your own custom classes

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std  string s    doesn t work with const char   what stuff like  string to add  appear to be   so definitely using stringstream is the closest to what is required - you just use  lt  lt  instead of

User · Answer

The C   way would be to use std  stringstream or just plain string concatenations  C   strings are mutable so the performance considerations of concatenation are less of a concern   with regards to formatting  you can do all the same formatting on a stream  but in a different way  similar to cout  or you can use a strongly typed functor which encapsulates this and provides a String Format like interface e g  boost  format

[c++] C++ equivalent of StringBuffer/StringBuilder?

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