How do I concatenate multiple C strings on one line

Question

C  has a syntax feature where you can concatenate many data types together on 1 line   string s   new String    s     Hello world      myInt   niceToSeeYouString  s    someChar1   interestingDecimal   someChar2    What would be the equivalent in C    As far as I can see  you d have to do it all on separate lines as it doesn t support multiple strings variables with the   operator  This is OK  but doesn t look as neat   string s  s     Hello world       nice to see you       or not      The above code produces an error

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You may use this header for this regard  https   github com theypsilon concat  using namespace concat   assert concat 1 2 3 4 5      12345      Under the hood you will be using a std  ostringstream

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Something like this works for me  namespace detail       void concat impl std  ostream amp        do nothing           template lt typename T  typename    Args gt      void concat impl std  ostream amp  os  const T amp  t  Args amp  amp     args                os  lt  lt  t          concat impl os  std  forward lt Args gt  args                  namespace detail     template lt typename    Args gt  std  string concat Args amp  amp     args        std  ostringstream os      detail  concat impl os  std  forward lt Args gt  args           return os str             std  string s  Hello World      s   concat s  myInt  niceToSeeYouString  myChar  myFoo

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If you are willing to use c  11 you can utilize user-defined string literals and define two function templates that overload the plus operator for a std  string object and any other object  The only pitfall is not to overload the plus operators of std  string  otherwise the compiler doesn t know which operator to use  You can do this by using the template std  enable if from type traits  After that strings behave just like in Java or C   See my example implementation for details   Main code   include  lt iostream gt   include  c sharp strings hpp   using namespace std   int main         int i   0      float f   0 4      double d   1 3e-2      string s      s     Hello world        nice to see you       i                      47          f         d      cout  lt  lt  s  lt  lt  endl      return 0      File c sharp strings hpp  Include this header file in all all places where you want to have these strings    ifndef C SHARP STRING H INCLUDED  define C SHARP STRING H INCLUDED   include  lt type traits gt   include  lt string gt   inline std  string operator      const char a    long unsigned int i        return std  string a      template lt typename T gt  inline typename std  enable if lt  std  is same lt std  string  T gt   value  amp  amp                           std  is same lt char  T gt   value  amp  amp                           std  is same lt const char   T gt   value  std  string gt   type operator   std  string s  T i        return s   std  to string i      template lt typename T gt  inline typename std  enable if lt  std  is same lt std  string  T gt   value  amp  amp                           std  is same lt char  T gt   value  amp  amp                           std  is same lt const char   T gt   value  std  string gt   type operator   T i  std  string s        return std  to string i    s      endif    C SHARP STRING H INCLUDED

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s     Hello world       nice to see you       or not      Those character array literals are not C   std  strings - you need to convert them   s    string  Hello world       string  nice to see you       string  or not       To convert ints  or any other streamable type  you can use a boost lexical cast or provide your own function   template  lt typename T gt  string Str  const T  amp  t        ostringstream os     os  lt  lt  t     return os str        You can now say things like   string s   string  The meaning is      Str  42

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Here s the one-liner solution    include  lt iostream gt   include  lt string gt   int main       std  string s   std  string  Hi       there      friends     std  cout  lt  lt  s  lt  lt  std  endl     std  string r   std  string  Magic number       std  to string 13           std  cout  lt  lt  r  lt  lt  std  endl     return 0      Although it s a tiny bit ugly  I think it s about as clean as you cat get in C       We are casting the first argument to a std  string and then using the  left to right  evaluation order of operator  to ensure that its left operand is always a std  string   In this manner  we concatenate the std  string on the left with the const char   operand on the right and return another std  string  cascading the effect   Note  there are a few options for the right operand  including const char    std  string  and char   It s up to you to decide whether the magic number is 13 or 6227020800

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This works for me    include  lt iostream gt   using namespace std    define CONCAT2 a b      string a  string b   define CONCAT3 a b c    string a  string b  string c   define CONCAT4 a b c d  string a  string b  string c  string d    define HOMEDIR  c   example   int main          const char  filename    myfile        string path   CONCAT4 HOMEDIR      filename   txt         cout  lt  lt  path      return 0      Output   c  example myfile txt

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To offer a solution that is more one-line-ish  A function concat can be implemented to reduce the  classic  stringstream based solution to a single statement  It is based on variadic templates and perfect forwarding     Usage   std  string s   concat someObject    Hello     42    I concatenate   anyStreamableType       Implementation   void addToStream std  ostringstream amp        template lt typename T  typename    Args gt  void addToStream std  ostringstream amp  a stream  T amp  amp  a value  Args amp  amp     a args        a stream  lt  lt  std  forward lt T gt  a value       addToStream a stream  std  forward lt Args gt  a args          template lt typename    Args gt  std  string concat Args amp  amp     a args        std  ostringstream s      addToStream s  std  forward lt Args gt  a args           return s str

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include  lt sstream gt   include  lt string gt   std  stringstream ss  ss  lt  lt   Hello  world     lt  lt  myInt  lt  lt  niceToSeeYouString  std  string s   ss str      Take a look at this Guru Of The Week article from Herb Sutter  The String Formatters of Manor Farm

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auto s   string  one   append  two   append  three

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Based on above solutions I made a class var string for my project to make life easy  Examples   var string x  abc  d  s   123   def    std  string y    std  string x  const char  z   x c str      The class itself    include  lt stdlib h gt   include  lt stdarg h gt   class var string   public      var string const char  cmd                     va list args          va start args  cmd           vsnprintf buffer  sizeof buffer  - 1  cmd  args               var string           operator std  string                 return std  string buffer              operator char                  return buffer             const char  c str                 return buffer             int system                 return   system buffer         private      char buffer 4096        Still wondering if there will be something better in C

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Maybe you like my  Streamer  solution to really do it in one line    include  lt iostream gt   include  lt sstream gt  using namespace std   class Streamer    class for one line string generation   public       Streamer amp  clear      clear content               ss str         set to empty string         ss clear       clear error flags         return  this             template  lt typename T gt      friend Streamer amp  operator lt  lt  Streamer amp  streamer T str      add to streamer      string str      get current string       return ss str      private      stringstream ss      template  lt typename T gt  Streamer amp  operator lt  lt  Streamer amp  streamer T str    streamer ss lt  lt str return streamer    Streamer streamer     make this a global variable   class MyTestClass    just a test class   public      MyTestClass     data 0 12345        friend ostream amp  operator lt  lt  ostream amp  os const MyTestClass amp  myClass   private      double data      ostream amp  operator lt  lt  ostream amp  os const MyTestClass amp  myClass     print test class   return os lt  lt myClass data     int main         int i 0      string s1  streamer clear   lt  lt  foo  lt  lt  bar  lt  lt  test   str                            test strings     string s2  streamer clear   lt  lt  i   lt  lt i   lt  lt     lt  lt i   lt  lt     lt  lt i   lt  lt     lt  lt 0 666  str       test numbers     string s3  streamer clear   lt  lt  test class   lt  lt MyTestClass    str                    test with test class     cout lt  lt  s1     lt  lt s1 lt  lt     lt  lt endl      cout lt  lt  s2     lt  lt s2 lt  lt     lt  lt endl      cout lt  lt  s3     lt  lt s3 lt  lt     lt  lt endl

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Using C  14 user defined literals and std  to string the code becomes easier   using namespace std  literals  string literals  std  string str  str     Hello World   s    nice to see you   s    or not s  str     Hello World   s   std  to string my int    other string    Note that concatenating string literals can be done at compile time  Just remove the     str     Hello World     nice to see you     or not

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Have you tried to avoid the     instead use var   var       it worked for me    include  lt iostream h gt     for string  string myName       int  age   30  myName   myName    Vincent     Thorpe    30         2019

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Stringstream with a simple preproccessor macro using a lambda function seems nice    include  lt sstream gt   define make string args     std  stringstream ss  ss  lt  lt  args  return ss        and then  auto str   make string  hello   lt  lt    there   lt  lt  10  lt  lt

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In 5 years nobody has mentioned  append    include  lt string gt   std  string s  s append  Hello world      s append  nice to see you      s append  or not

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boost  format    or std  stringstream  std  stringstream msg  msg  lt  lt   Hello world     lt  lt  myInt   lt  lt  niceToSeeYouString  msg str       returns std  string object

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Your code can be written as1   s    Hello world    nice to see you    or not        but I doubt that s what you re looking for   In your case  you are probably looking for streams   std  stringstream ss  ss  lt  lt   Hello world     lt  lt  42  lt  lt   nice to see you    std  string s   ss str        1  can be written as    This only works for string literals   The concatenation is done by the compiler

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In C  20 you ll be able to do   auto s   std  format            Hello world     myInt  niceToSeeYouString     Until then you could do the same with the  fmt  library   auto s   fmt  format            Hello world     myInt  niceToSeeYouString     Disclaimer  I m the author of  fmt

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The actual problem was that concatenating string literals with   fails in C        string s    s     Hello world       nice to see you       or not      The above code produces an error    In C    also in C   you concatenate string literals by just placing them right next to each other   string s0    Hello world     nice to see you     or not    string s1    Hello world      same    nice to see you      result    or not    string s2         Hello world      line breaks in source code as well as         nice to see you      comments don t matter         or not      This makes sense  if you generate code in macros    define TRACE arg  cout  lt  lt   arg      lt  lt   arg   lt  lt  endl       a simple macro that can be used like this  int a   5  TRACE a  a    7  TRACE a  TRACE a 7  TRACE 17 11     live demo       or  if you insist in using the   for string literals  as already suggested by underscore d    string s   string  Hello world      nice to see you     or not      Another solution combines a string and a const char  for each concatenation step  string s  s     Hello world    s     nice to see you    s     or not

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In c11   void printMessage std  string amp  amp  message        std  cout  lt  lt  message  lt  lt  std  endl      return message      this allow you to create function call like this   printMessage  message number       std  to string id      will print   message number   10

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you can also  extend  the string class and choose the operator you prefer    lt  lt    amp      etc       Here is the code using operator lt  lt  to show there is no conflict with streams  note  if you uncomment s1 reserve 30   there is only 3 new   operator requests  1 for s1  1 for s2  1 for reserve   you can t reserve at constructor time unfortunately   without reserve  s1 has to request more memory as it grows  so it depends on your compiler implementation grow factor  mine seems to be 1 5  5 new   calls in this example   namespace perso   class string public std  string   public      string    std  string          template lt typename T gt      string const T v   std  string v          template lt typename T gt      string amp  operator lt  lt  const T s            this  s          return  this              using namespace std   int main         using string   perso  string      string s1  s2  she         s1 reserve 30       s1  lt  lt   no    lt  lt   sunshine when    lt  lt  s2  lt  lt        lt  lt   s   lt  lt    gone       cout  lt  lt   Aint t   lt  lt  s1  lt  lt          lt  lt   endl       return 0

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If you write out the     it looks almost the same as C   string s  Some initial data      int i   5  s   s    Hello world       nice to see you      to string i      n

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You would have to define operator    for every data type you would want to concenate to the string  yet since operator lt  lt  is defined for most types  you should use std  stringstream   Damn  beat by 50 seconds

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As others said  the main problem with the OP code is that the operator   does not concatenate const char    it works with std  string  though   Here s another solution that uses C  11 lambdas and for each and allows to provide a separator to separate the strings    include  lt vector gt   include  lt algorithm gt   include  lt iterator gt   include  lt sstream gt   string join const string amp  separator              const vector lt string gt  amp  strings        if  strings empty            return          if  strings size      1          return strings 0        stringstream ss      ss  lt  lt  strings 0        auto aggregate     amp ss   amp separator  const string amp  s    ss  lt  lt  separator  lt  lt  s         for each begin strings    1  end strings   aggregate        return ss str        Usage   std  vector lt std  string gt  strings    a    b    c     std  string joinedStrings   join       strings     It seems to scale well  linearly   at least after a quick test on my computer  here s a quick test I ve written    include  lt vector gt   include  lt algorithm gt   include  lt iostream gt   include  lt iterator gt   include  lt sstream gt   include  lt chrono gt   using namespace std   string join const string amp  separator              const vector lt string gt  amp  strings        if  strings empty            return          if  strings size      1          return strings 0        stringstream ss      ss  lt  lt  strings 0        auto aggregate     amp ss   amp separator  const string amp  s    ss  lt  lt  separator  lt  lt  s         for each begin strings    1  end strings   aggregate        return ss str       int main         const int reps   1000      const string sep             auto generator        return  abcde          vector lt string gt  strings10 10       generate begin strings10   end strings10   generator        vector lt string gt  strings100 100       generate begin strings100   end strings100   generator        vector lt string gt  strings1000 1000       generate begin strings1000   end strings1000   generator        vector lt string gt  strings10000 10000       generate begin strings10000   end strings10000   generator        auto t1   chrono  system clock  now        for int i   0  i lt reps    i                join sep  strings10              auto t2   chrono  system clock  now        for int i   0  i lt reps    i                join sep  strings100              auto t3   chrono  system clock  now        for int i   0  i lt reps    i                join sep  strings1000              auto t4   chrono  system clock  now        for int i   0  i lt reps    i                join sep  strings10000              auto t5   chrono  system clock  now         auto d1   chrono  duration cast lt chrono  milliseconds gt  t2 - t1       auto d2   chrono  duration cast lt chrono  milliseconds gt  t3 - t2       auto d3   chrono  duration cast lt chrono  milliseconds gt  t4 - t3       auto d4   chrono  duration cast lt chrono  milliseconds gt  t5 - t4        cout  lt  lt   join 10         lt  lt  d1 count    lt  lt  endl      cout  lt  lt   join 100        lt  lt  d2 count    lt  lt  endl      cout  lt  lt   join 1000       lt  lt  d3 count    lt  lt  endl      cout  lt  lt   join 10000      lt  lt  d4 count    lt  lt  endl      Results  milliseconds    join 10      2 join 100     10 join 1000    91 join 10000   898

[c++] How do I concatenate multiple C++ strings on one line?

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