How to pass variable number of arguments to printf sprintf

Question

I have a class that holds an  error  function that will format some text   I want to accept a variable number of arguments and then format them using printf   Example   class MyClass   public      void Error const char  format             The Error method should take in the parameters  call printf sprintf to format it and then do something with it   I don t want to write all the formatting myself so it makes sense to try and figure out how to use the existing formatting

User · Answer

You are looking for variadic functions  printf   and sprintf   are variadic functions - they can accept a variable number of arguments   This entails basically these steps    The first parameter must give some indication of the number of parameters that follow  So in printf    the  format  parameter gives this indication - if you have 5 format specifiers  then it will look for 5 more arguments  for a total of 6 arguments   The first argument could be an integer  eg  myfunction 3  a  b  c   where  3  signifies  3  arguments  Then loop through and retrieve each successive argument  using the va start   etc  functions     There are plenty of tutorials on how to do this - good luck

User · Answer

have a look at vsnprintf as this will do what ya want http   www cplusplus com reference clibrary cstdio vsprintf   you will have to init the va list arg array first  then call it   Example from that link     vsprintf example      include  lt stdio h gt   include  lt stdarg h gt   void Error  char   format           char buffer 256     va list args    va start  args  format     vsnprintf  buffer  255  format  args         do something with the error    va end  args

User · Answer

I should have read more on existing questions in stack overflow   C   Passing Variable Number of Arguments is a similar question   Mike F has the following explanation      There s no way of calling  eg  printf   without knowing how many arguments   you re passing to it  unless you want   to get into naughty and non-portable   tricks       The generally used solution is to   always provide an alternate form of   vararg functions  so printf has   vprintf which takes a va list in place   of the      The     versions are just   wrappers around the va list versions    This is exactly what I was looking for   I performed a test implementation like this   void Error const char  format             char dest 1024   16       va list argptr      va start argptr  format       vsprintf dest  format  argptr       va end argptr       printf dest

User · Answer

void Error const char  format             va list argptr      va start argptr  format       vfprintf stderr  format  argptr       va end argptr       If you want to manipulate the string before you display it and really do need it stored in a buffer first  use vsnprintf instead of vsprintf  vsnprintf will prevent an accidental buffer overflow error

User · Answer

Have a look at the example http   www cplusplus com reference clibrary cstdarg va arg   they pass the number of arguments to the method but you can ommit that and modify the code appropriately  see the example

User · Answer

Simple example below   Note you should pass in a larger buffer  and test to see if the buffer was large enough or not  void Log LPCWSTR pFormat              va list pArg      va start pArg  pFormat       char buf 1000       int len    vsntprintf buf  1000  pFormat  pArg       va end pArg         do something with buf

User · Answer

Using functions with the ellipses is not very safe  If performance is not critical for log function consider using operator overloading as in boost  format  You could write something like this    include  lt sstream   include  lt boost format hpp   include  lt iostream  using namespace std   class formatted log t   public      formatted log t const char  msg     fmt msg          formatted log t     cout  lt  lt  fmt  lt  lt  endl         template  lt typename T      formatted log t  operator   T value            fmt   value          return  this         protected      boost  format                fmt      formatted log t log const char  msg    return formatted log t  msg          use int main          log  hello  s in  d-th time      world    10000000      return 0      The following sample demonstrates possible errors with ellipses   int x   SOME VALUE  double y   SOME MORE VALUE  printf   some var    f  other one  f   y  x       no errors at compile time  but error at runtime  compiler do not know types you wanted log   some var    f  other one  f      y   x     no errors   f only for compatibility  you could write  1  instead

[c++] How to pass variable number of arguments to printf/sprintf

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