printf vs cout in C

Question

What is the difference between printf   and cout in C

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I m not a programmer  but I have been a human factors engineer  I feel a programming language should be easy to learn  understand and use  and this requires that it have a simple and consistent linguistic structure  Although all the languages is symbolic and thus  at its core  arbitrary  there are conventions and following them makes the language easier to learn and use   There are a vast number of functions in C   and other languages written as function parameter   a syntax that was originally used for functional relationships in mathematics in the pre-computer era  printf   follows this syntax and if the writers of C   wanted to create any logically different method for reading and writing files they could have simply created a different function using a similar syntax   In Python we of course can print using the also fairly standard object method syntax  i e  variablename print  since variables are objects  but in C   they are not   I m not fond of the cout syntax because the  lt  lt  operator does not follow any rules  It is a method or function  i e  it takes a parameter and does something to it  However it is written as though it were a mathematical comparison operator  This is a poor approach from a human factors standpoint

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cout lt  lt   Hello   printf   s    Hello       Both are used to print values  They have completely different syntax  C   has both  C only has printf

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From the C   FAQ       15 1  Why should I use  lt iostream gt   instead of the traditional  lt cstdio gt        Increase type safety  reduce errors  allow extensibility  and provide inheritability       printf   is arguably not broken  and scanf   is perhaps livable despite being error prone  however both are limited with respect to what C   I O can do  C   I O  using  lt  lt  and  gt  gt   is  relative to C  using printf   and scanf             More type-safe  With  lt iostream gt   the type of object being I O d is   known statically by the compiler  In   contrast   lt cstdio gt  uses     fields to   figure out the types dynamically    Less error prone  With  lt iostream gt   there are no redundant       tokens that have to be consistent   with the actual objects being I O d    Removing redundancy removes a class   of errors    Extensible  The C    lt iostream gt  mechanism allows new user-defined   types to be I O d without breaking   existing code  Imagine the chaos if   everyone was simultaneously adding   new incompatible     fields to   printf   and scanf       Inheritable  The C    lt iostream gt  mechanism is built from real classes   such as std  ostream and   std  istream  Unlike  lt cstdio gt  s   FILE   these are real classes and   hence inheritable  This means you can   have other user-defined things that   look and act like streams  yet that   do whatever strange and wonderful   things you want  You automatically   get to use the zillions of lines of   I O code written by users you don t   even know  and they don t need to   know about your  extended stream    class       On the other hand  printf is significantly faster  which may justify using it in preference to cout in very specific and limited cases  Always profile first   See  for example  http   programming-designs com 2009 02 c-speed-test-part-2-printf-vs-cout

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I d like to point out that if you want to play with threads in C    if you use cout you can get some interesting results   Consider this code    include  lt string gt   include  lt iostream gt   include  lt thread gt   using namespace std   void task int taskNum  string msg        for  int i   0  i  lt  5    i            cout  lt  lt       lt  lt  taskNum  lt  lt        lt  lt  msg  lt  lt  endl           int main         thread t1 task  1   AAA        thread t2 task  2   BBB        t1 join        t2 join        return 0        g     thread cpp -o thread out -ansi -pedantic -pthread -std c  0x   Now  the output comes all shuffled  It can yield different results too  try executing several times     12    ABABAB    12    ABABAB    12    ABABAB    12    ABABAB    12    ABABAB   You can use printf to get it right  or you can use mutex    1  AAA  2  BBB  1  AAA  2  BBB  1  AAA  2  BBB  1  AAA  2  BBB  1  AAA  2  BBB   Have fun

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With primitives  it probably doesn t matter entirely which one you use  I say where it gets usefulness is when you want to output complex objects   For example  if you have a class    include  lt iostream gt   include  lt cstdlib gt   using namespace std   class Something   public          Something int x  int y  int z    a x   b y   c z              int a          int b          int c           friend ostream amp  operator lt  lt  ostream amp   const Something amp        ostream amp  operator lt  lt  ostream amp  o  const Something amp  s            o  lt  lt  s a  lt  lt        lt  lt  s b  lt  lt        lt  lt  s c          return o     int main void            Something s 3  2  1               output with printf         printf   i   i   i n   s a  s b  s c               output with cout         cout  lt  lt  s  lt  lt  endl           return 0      Now the above might not seem all that great  but let s suppose you have to output this in multiple places in your code  Not only that  let s say you add a field  int d   With cout  you only have to change it in once place  However  with printf  you d have to change it in possibly a lot of places and not only that  you have to remind yourself which ones to output    With that said  with cout  you can reduce a lot of times spent with maintenance of your code and not only that if you re-use the object  Something  in a new application  you don t really have to worry about output

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For me  the real differences which would make me go for  cout  rather than  printf  are   1   lt  lt  operator can be overloaded for my classes   2  Output stream for cout can be easily changed to a file      copy paste      include  lt iostream gt   include  lt fstream gt  using namespace std   int main          cout  lt  lt   This is sent to prompt   lt  lt  endl      ofstream file      file open   test txt        streambuf  sbuf   cout rdbuf        cout rdbuf file rdbuf         cout  lt  lt   This is sent to file   lt  lt  endl      cout rdbuf sbuf       cout  lt  lt   This is also sent to prompt   lt  lt  endl      return 0      3  I find cout more readable  especially when we have many parameters   One problem with cout is the formatting options  Formatting the data  precision  justificaton  etc   in printf is easier

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Of course you can write  something  a bit better to keep maintenance    include  lt iostream gt   include  lt cstdlib gt   using namespace std   class Something       public          Something int x  int y  int z    a x   b y   c z              int a          int b          int c           friend ostream amp  operator lt  lt  ostream amp   const Something amp             void print   const   printf   i   i   i n   a  b  c         ostream amp  operator lt  lt  ostream amp  o  const Something amp  s        o  lt  lt  s a  lt  lt        lt  lt  s b  lt  lt        lt  lt  s c      return o     int main void        Something s 3  2  1           Output with printf     s print       Simple as well  isn t it          Output with cout     cout  lt  lt  s  lt  lt  endl       return 0      And a bit extended test of cout vs  printf  added a test of  double   if anyone wants to do more testing  Visual nbsp Studio nbsp 2008  release version of the executable     include  lt stdio h gt   include  lt iostream gt   include  lt ctime gt   class TimedSection       char const  d name        timespec d start      clock t d start       public          TimedSection char const  name                d name name                          clock gettime CLOCK REALTIME   amp d start               d start   clock                       TimedSection                 clock t end                clock gettime CLOCK REALTIME   amp end               end   clock                double duration     1e3    end tv sec - d start tv sec                                  1e-6    end tv nsec - d start tv nsec                                                                   double   end - d start    CLOCKS PER SEC               std  cerr  lt  lt  d name  lt  lt    t   lt  lt  std  fixed  lt  lt  duration   1000 0  lt  lt    ms n                  int main         const int iters   1000000      char const  text    01234567890123456789                 TimedSection s  cout with only endl            for  int i   0  i  lt  iters    i              std  cout  lt  lt  std  endl                      TimedSection s  cout with only    n             for  int i   0  i  lt  iters    i              std  cout  lt  lt    n                       TimedSection s  printf with only    n             for  int i   0  i  lt  iters    i              printf   n                        TimedSection s  cout with string constant and endl            for  int i   0  i  lt  iters    i              std  cout  lt  lt   01234567890123456789   lt  lt  std  endl                      TimedSection s  cout with string constant and    n             for  int i   0  i  lt  iters    i              std  cout  lt  lt   01234567890123456789 n                       TimedSection s  printf with string constant and    n             for  int i   0  i  lt  iters    i              printf  01234567890123456789 n                        TimedSection s  cout with some stuff and endl            for  int i   0  i  lt  iters    i              std  cout  lt  lt  text  lt  lt   01234567890123456789   lt  lt  i  lt  lt  std  endl                      TimedSection s  cout with some stuff and    n             for  int i   0  i  lt  iters    i              std  cout  lt  lt  text  lt  lt   01234567890123456789   lt  lt  i  lt  lt    n                       TimedSection s  printf with some stuff and    n             for  int i   0  i  lt  iters    i              printf   s01234567890123456789 i n   text  i                       TimedSection s  cout with formatted double  width  amp  precision once             std  cout  lt  lt  std  fixed  lt  lt  std  scientific  lt  lt  std  right  lt  lt  std  showpoint          std  cout width 8           for  int i   0  i  lt  iters    i              std  cout  lt  lt  text  lt  lt  8 315  lt  lt  i  lt  lt    n                       TimedSection s  cout with formatted double  width  amp  precision on each call             std  cout  lt  lt  std  fixed  lt  lt  std  scientific  lt  lt  std  right  lt  lt  std  showpoint           for  int i   0  i  lt  iters    i                std  cout width 8                 std  cout precision 3                 std  cout  lt  lt  text  lt  lt  8 315  lt  lt  i  lt  lt    n                                     TimedSection s  printf with formatted double            for  int i   0  i  lt  iters    i              printf   8 3f i n   8 315  i             The result is   cout with only endl    6453 000000 ms cout with only   n     125 000000 ms printf with only   n     156 000000 ms cout with string constant and endl    6937 000000 ms cout with string constant and   n     1391 000000 ms printf with string constant and   n     3391 000000 ms cout with some stuff and endl    9672 000000 ms cout with some stuff and   n     7296 000000 ms printf with some stuff and   n     12235 000000 ms cout with formatted double  width  amp  precision once     7906 000000 ms cout with formatted double  width  amp  precision on each call     9141 000000 ms printf with formatted double    3312 000000 ms

User · Answer

And I quote      In high level terms  the main differences are type safety  cstdio   doesn t have it   performance  most iostreams implementations are   slower than the cstdio ones  and extensibility  iostreams allows   custom output targets and seamless output of user defined types

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I m surprised that everyone in this question claims that std  cout is way better than printf  even if the question just asked for differences  Now  there is a difference - std  cout is C    and printf is C  however  you can use it in C    just like almost anything else from C   Now  I ll be honest here  both printf and std  cout have their advantages   Real differences  Extensibility  std  cout is extensible  I know that people will say that printf is extensible too  but such extension is not mentioned in the C standard  so you would have to use non-standard features - but not even common non-standard feature exists   and such extensions are one letter  so it s easy to conflict with an already-existing format    Unlike printf  std  cout depends completely on operator overloading  so there is no issue with custom formats - all you do is define a subroutine taking std  ostream as the first argument and your type as second  As such  there are no namespace problems - as long you have a class  which isn t limited to one character   you can have working std  ostream overloading for it   However  I doubt that many people would want to extend ostream  to be honest  I rarely saw such extensions  even if they are easy to make   However  it s here if you need it   Syntax  As it could be easily noticed  both printf and std  cout use different syntax  printf uses standard function syntax using pattern string and variable-length argument lists  Actually  printf is a reason why C has them - printf formats are too complex to be usable without them  However  std  cout uses a different API - the operator  lt  lt  API that returns itself   Generally  that means the C version will be shorter  but in most cases it won t matter  The difference is noticeable when you print many arguments  If you have to write something like Error 2  File not found   assuming error number  and its description is placeholder  the code would look like this  Both examples work identically  well  sort of  std  endl actually flushes the buffer    printf  Error  d   s  n   id  errors id    std  cout  lt  lt   Error    lt  lt  id  lt  lt        lt  lt  errors id   lt  lt       lt  lt  std  endl    While this doesn t appear too crazy  it s just two times longer   things get more crazy when you actually format arguments  instead of just printing them  For example  printing of something like 0x0424 is just crazy  This is caused by std  cout mixing state and actual values  I never saw a language where something like std  setfill would be a type  other than C    of course   printf clearly separates arguments and actual type  I really would prefer to maintain the printf version of it  even if it looks kind of cryptic  compared to iostream version of it  as it contains too much noise    printf  0x 04x n   0x424   std  cout  lt  lt   0x   lt  lt  std  hex  lt  lt  std  setfill  0    lt  lt  std  setw 4   lt  lt  0x424  lt  lt  std  endl    Translation  This is where the real advantage of printf lies  The printf format string is well    a string  That makes it really easy to translate  compared to operator  lt  lt  abuse of iostream  Assuming that the gettext   function translates  and you want to show Error 2  File not found   the code to get translation of the previously shown format string would look like this   printf gettext  Error  d   s  n    id  errors id      Now  let s assume that we translate to Fictionish  where the error number is after the description  The translated string would look like  2 s oru  1 d  n  Now  how to do it in C    Well  I have no idea  I guess you can make fake iostream which constructs printf that you can pass to gettext  or something  for purposes of translation  Of course    is not C standard  but it s so common that it s safe to use in my opinion   Not having to remember look-up specific integer type syntax  C has lots of integer types  and so does C    std  cout handles all types for you  while printf requires specific syntax depending on an integer type  there are non-integer types  but the only non-integer type you will use in practice with printf is const char    C string  can be obtained using to c method of std  string    For instance  to print size t  you need to use  zd  while int64 t will require using   PRId64   The tables are available at http   en cppreference com w cpp io c fprintf and http   en cppreference com w cpp types integer   You can t print the NUL byte   0  Because printf uses C strings as opposed to C   strings  it cannot print NUL byte without specific tricks  In certain cases it s possible to use  c with   0  as an argument  although that s clearly a hack   Differences nobody cares about  Performance  Update  It turns out that iostream is so slow that it s usually slower than your hard drive  if you redirect your program to file   Disabling synchronization with stdio may help  if you need to output lots of data  If the performance is a real concern  as opposed to writing several lines to STDOUT   just use printf   Everyone thinks that they care about performance  but nobody bothers to measure it  My answer is that I O is bottleneck anyway  no matter if you use printf or iostream  I think that printf could be faster from a quick look into assembly  compiled with clang using the -O3 compiler option   Assuming my error example  printf example does way fewer calls than the cout example  This is int main with printf   main                                       main   BB 0          push     lr          ldr     r0   LCPI0 0         ldr     r2   LCPI0 1         mov     r1   2         bl      printf         mov     r0   0         pop      lr          mov     pc  lr          align  2   BB 1    You can easily notice that two strings  and 2  number  are pushed as printf arguments  That s about it  there is nothing else  For comparison  this is iostream compiled to assembly  No  there is no inlining  every single operator  lt  lt  call means another call with another set of arguments   main                                       main   BB 0          push     r4  r5  lr          ldr     r4   LCPI0 0         ldr     r1   LCPI0 1         mov     r2   6         mov     r3   0         mov     r0  r4         bl       ZSt16  ostream insertIcSt11char traitsIcEERSt13basic ostreamIT T0 ES6 PKS3 l         mov     r0  r4         mov     r1   2         bl       ZNSolsEi         ldr     r1   LCPI0 2         mov     r2   2         mov     r3   0         mov     r4  r0         bl       ZSt16  ostream insertIcSt11char traitsIcEERSt13basic ostreamIT T0 ES6 PKS3 l         ldr     r1   LCPI0 3         mov     r0  r4         mov     r2   14         mov     r3   0         bl       ZSt16  ostream insertIcSt11char traitsIcEERSt13basic ostreamIT T0 ES6 PKS3 l         ldr     r1   LCPI0 4         mov     r0  r4         mov     r2   1         mov     r3   0         bl       ZSt16  ostream insertIcSt11char traitsIcEERSt13basic ostreamIT T0 ES6 PKS3 l         ldr     r0   r4          sub     r0  r0   24         ldr     r0   r0          add     r0  r0  r4         ldr     r5   r0   240          cmp     r5   0         beq      LBB0 5   BB 1                                      ZSt13  check facetISt5ctypeIcEERKT PS3  exit         ldrb    r0   r5   28          cmp     r0   0         beq      LBB0 3   BB 2          ldrb    r0   r5   39          b        LBB0 4  LBB0 3          mov     r0  r5         bl       ZNKSt5ctypeIcE13 M widen initEv         ldr     r0   r5          mov     r1   10         ldr     r2   r0   24          mov     r0  r5         mov     lr  pc         mov     pc  r2  LBB0 4                                     ZNKSt5ctypeIcE5widenEc exit         lsl     r0  r0   24         asr     r1  r0   24         mov     r0  r4         bl       ZNSo3putEc         bl       ZNSo5flushEv         mov     r0   0         pop      r4  r5  lr          mov     pc  lr  LBB0 5          bl       ZSt16  throw bad castv          align  2   BB 6    However  to be honest  this means nothing  as I O is the bottleneck anyway  I just wanted to show that iostream is not faster because it s  type safe   Most C implementations implement printf formats using computed goto  so the printf is as fast as it can be  even without compiler being aware of printf  not that they aren t - some compilers can optimize printf in certain cases - constant string ending with  n is usually optimized to puts    Inheritance  I don t know why you would want to inherit ostream  but I don t care  It s possible with FILE too   class MyFile   public FILE      Type safety  True  variable length argument lists have no safety  but that doesn t matter  as popular C compilers can detect problems with printf format string if you enable warnings  In fact  Clang can do that without enabling warnings     cat safety c   include  lt stdio h gt   int main void        printf  String   s n   42       return 0       clang safety c  safety c 4 28  warning  format specifies type  char    but the argument has type  int   -Wformat      printf  String   s n   42                                                      d 1 warning generated    gcc -Wall safety c safety c  In function    main     safety c 4 5  warning  format     s    expects argument of type    char       but argument 2 has type    int     -Wformat        printf  String   s n   42

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TL DR  Always do your own research  in regard of generated machine code size  performance  readability and coding time before trusting random comments online  including this one   I m no expert  I just happened to overhear two co-workers talking about how we should avoid using C   in embedded systems because of performance issues  Well  interesting enough  I did a benchmark based on a real project task   In said task  we had to write some config to RAM  Something like      coffee hot   sugar none   milk breast   mac AA BB CC DD EE FF   Here s my benchmark programs  Yes  I know OP asked about printf    not fprintf    Try to capture the essence and by the way  OP s link points to fprintf   anyway    C program   char coffee 10   sugar 10   milk 10   unsigned char mac 6       Initialize those things here      FILE   f   fopen  a txt    wt     fprintf f   coffee  s nsugar  s nmilk  s nmac  02X  02X  02X  02X  02X  02X n   coffee  sugar  milk  mac 0   mac 1  mac 2  mac 3  mac 4  mac 5     fclose f     C   program     Everything else is identical except   std  ofstream f  a txt   std  ios  out    f  lt  lt   coffee    lt  lt  coffee  lt  lt    n   f  lt  lt   sugar    lt  lt  sugar  lt  lt    n   f  lt  lt   milk    lt  lt  milk  lt  lt    n   f  lt  lt   mac    lt  lt   int mac 0   lt  lt           lt  lt   int mac 1   lt  lt           lt  lt   int mac 2   lt  lt           lt  lt   int mac 3   lt  lt           lt  lt   int mac 4   lt  lt           lt  lt   int mac 5   lt  lt  endl  f close      I did my best to polish them before I looped them both 100 000 times  Here are the results   C program   real    0m 8 01s user    0m 2 37s sys     0m 5 58s   C   program   real    0m 6 07s user    0m 3 18s sys     0m 2 84s   Object file size   C   - 2 092 bytes C   - 3 272 bytes   Conclusion  On my very specific platform  with a very specific processor  running a very specific version of Linux kernel  to run a program which is compiled with a very specific version of GCC  in order to accomplish a very specific task  I would say the C   approach is more suitable because it runs significantly faster and provide much better readability  On the other hand  C offers small footprint  in my opinion  means nearly nothing because program size is not of our concern   Remeber  YMMV

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People often claim that printf is much faster  This is largely a myth  I just tested it  with the following results   cout with only endl                     1461 310252 ms cout with only   n                       343 080217 ms printf with only   n                      90 295948 ms cout with string constant and endl      1892 975381 ms cout with string constant and   n        416 123446 ms printf with string constant and   n      472 073070 ms cout with some stuff and endl           3496 489748 ms cout with some stuff and   n            2638 272046 ms printf with some stuff and   n          2520 318314 ms   Conclusion  if you want only newlines  use printf  otherwise  cout is almost as fast  or even faster  More details can be found on my blog   To be clear  I m not trying to say that iostreams are always better than printf  I m just trying to say that you should make an informed decision based on real data  not a wild guess based on some common  misleading assumption   Update  Here s the full code I used for testing  Compiled with g   without any additional options  apart from -lrt for the timing     include  lt stdio h gt   include  lt iostream gt   include  lt ctime gt   class TimedSection       char const  d name      timespec d start      public          TimedSection char const  name                d name name                        clock gettime CLOCK REALTIME   amp d start                      TimedSection                 timespec end              clock gettime CLOCK REALTIME   amp end               double duration   1e3    end tv sec - d start tv sec                                  1e-6    end tv nsec - d start tv nsec               std  cerr  lt  lt  d name  lt  lt    t   lt  lt  std  fixed  lt  lt  duration  lt  lt    ms n                  int main         const int iters   10000000      char const  text    01234567890123456789                 TimedSection s  cout with only endl            for  int i   0  i  lt  iters    i              std  cout  lt  lt  std  endl                      TimedSection s  cout with only    n             for  int i   0  i  lt  iters    i              std  cout  lt  lt    n                       TimedSection s  printf with only    n             for  int i   0  i  lt  iters    i              printf   n                        TimedSection s  cout with string constant and endl            for  int i   0  i  lt  iters    i              std  cout  lt  lt   01234567890123456789   lt  lt  std  endl                      TimedSection s  cout with string constant and    n             for  int i   0  i  lt  iters    i              std  cout  lt  lt   01234567890123456789 n                       TimedSection s  printf with string constant and    n             for  int i   0  i  lt  iters    i              printf  01234567890123456789 n                        TimedSection s  cout with some stuff and endl            for  int i   0  i  lt  iters    i              std  cout  lt  lt  text  lt  lt   01234567890123456789   lt  lt  i  lt  lt  std  endl                      TimedSection s  cout with some stuff and    n             for  int i   0  i  lt  iters    i              std  cout  lt  lt  text  lt  lt   01234567890123456789   lt  lt  i  lt  lt    n                       TimedSection s  printf with some stuff and    n             for  int i   0  i  lt  iters    i              printf   s01234567890123456789 i n   text  i

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More differences   printf  returns an integer value  equal to the number of characters printed  and  cout  does not return anything  And   cout  lt  lt   y      lt  lt  7  is not atomic   printf   s    d    y   7   is atomic   cout performs typechecking  printf doesn t   There s no iostream equivalent of    d

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One is a function that prints to stdout  The other is an object that provides several member functions and overloads of operator lt  lt  that print to stdout  There are many more differences that I could enumerate  but I m not sure what you are after

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printf is a function whereas cout is a variable

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Two points not otherwise mentioned here that I find significant    1  cout carries a lot of baggage if you re not already using the STL  It adds over twice as much code to your object file as printf  This is also true for string  and this is the major reason I tend to use my own string library    2  cout uses overloaded  lt  lt  operators  which I find unfortunate  This can add confusion if you re also using the  lt  lt  operator for its intended purpose  shift left   I personally don t like to overload operators for purposes tangential to their intended use    Bottom line  I ll use cout  and string  if I m already using the STL  Otherwise  I tend to avoid it

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I would like say that extensibility lack of printf is not entirely true  In C  it is true  But in C  there are no real classes  In C    it is possible to overload cast operator  so  overloading a char  operator and using printf like this      Foo bar       printf   s  bar     can be possible  if Foo overload the good operator  Or if you made a good method  In short  printf is as extensible as cout for me     Technical argument I can see for C   streams  in general    not only cout   are    Typesafety   And  by the way  if I want to print a single   n  I use putchar   n      I will not use a nuke-bomb to kill an insect      Simpler to learn   no  complicated  parameters to learn  just to use  lt  lt  and  gt  gt  operators    Work natively with std  string  for printf there is std  string  c str    but for scanf     For printf I see      Easier  or at least shorter  in term of characters written  complex formatting  Far more readable  for me  matter of taste I guess     Better control of what the function made  Return how many characters where written and there is the  n formatter   Nothing printed  The argument must be a pointer to a signed int  where the number of characters written so far is stored    from printf - C   Reference  Better debugging possibilities  For same reason as last argument      My personal preferences go to printf  and scanf  functions  mainly because I love short lines  and because I don t think type problems on printing text are really hard to avoid  The only thing I deplore with C-style functions is that std  string is not supported  We have to go through a char  before giving it to printf  with the std  string  c str   if we want to read  but how to write

[c++] 'printf' vs. 'cout' in C++

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