Reading json files in C

Question

I m trying to read in a JSON file  So far I have focused on using the jsoncpp library  However  the documentation is quite hard to understand for me  Could anyone explain in lay terms what it does    Say I have a people json which looks like this     Anna              age   18         profession    student      Ben             age     nineteen          profession    mechanic        What happens when I read this in  Can I create some sort of data structure people which I can index by Anna and Ben as well as age and profession  What would be the data type of people  I thought it would be something similar to a  nested  map  but map values always have to have the same type  don t they   I have worked with python before and my  goal   which may be ill-set for C    is to obtain the equivalent of a nested python dictionary

User · Answer

storing peoples like this     Anna          age   18     profession    student     Ben         age     nineteen      profession    mechanic        will cause problems  particularly if differents peoples have same name    rather use array storing objects like this       peoples                          name   Anna                 age   18              profession    student                                 name   Ben               age     nineteen               profession    mechanic                    like this  you can enumerates objects  or acces objects by numerical index  remember that json is storage structure  not dynamically sorter or indexer  use data stored in json to build indexes as you need and acces data

User · Answer

Here is another easier possibility to read in a json file    include  json json h   std  ifstream file input  input json    Json  Reader reader  Json  Value root  reader parse file input  root   cout  lt  lt  root    You can then get the values like this   cout  lt  lt  root  key

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Yes you can create a nested data structure people which can be indexed by Anna and Ben  However  you can t index it directly by age and profession  I will get to this part in the code   The data type of people is of type Json  Value  which is defined in jsoncpp   You are right  it is similar to the nested map  but Value is a data structure which is defined such that multiple types can be stored and accessed  It is similar to a map with a string as the key and Json  Value as the value  It could also be a map between an unsigned int as key and Json  Value as the value  In case of json arrays      Here s the code    include  lt json value h gt   include  lt fstream gt   std  ifstream people file  people json   std  ifstream  binary   people file  gt  gt  people   cout lt  lt people    This will print the entire json object     The following lines will let you access the indexed objects  cout lt  lt people  Anna      Prints the value for  Anna  cout lt  lt people  ben      Prints the value for  Ben  cout lt  lt people  Anna    profession      Prints the value corresponding to  profession  in the json for  Anna   cout lt  lt people  profession      NULL  There is no element with key  profession   Hence a new empty element will be created    As you can see  you can index the json object only based on the hierarchy of the input data

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Essentially javascript and C   work on two different principles  Javascript creates an  associative array  or hash table  which matches a string key  which is the field name  to a value  C   lays out structures in memory  so the first 4 bytes are an integer  which is an age  then maybe we have a fixed-wth 32 byte string which represents the  profession    So javascript will handle things like  age  being 18 in one record and  nineteen  in another  C   can t   However C   is much faster    So if we want to handle JSON in C    we have to build the associative array from the ground up  Then we have to tag the values with their types  Is it an integer  a real value  probably return as  double    boolean  a string  It follows that a JSON C   class is quite a large chunk of code  Effectively what we are doing is implementing a bit of the javascript engine in C    We then pass our JSON parser the JSON as a string  and it tokenises it  and gives us functions to query the JSON from C

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You can use c    boost  property tree  ptree for parsing json data  here is the example for your json  data  this would be more easy if you shift name inside each child nodes   include  lt iostream gt                                                                include  lt string gt                                                                  include  lt tuple gt                                                                    include  lt boost property tree ptree hpp gt                                           include  lt boost property tree json parser hpp gt    int main           namespace pt   boost  property tree                                              pt  ptree loadPtreeRoot                                                           pt  read json  example json   loadPtreeRoot                                      std  vector lt std  tuple lt std  string  std  string  std  string gt  gt  people             pt  ptree temp                                                                   pt  ptree tage                                                                   pt  ptree tprofession                                                             std  string age                                                                  std  string profession                                                             Get first child                                                                temp   loadPtreeRoot get child  Anna                                             tage   temp get child  age                                                       tprofession   temp get child  profession                                          age    tage get value lt std  string gt                                                profession    tprofession get value lt std  string gt                                  std  cout  lt  lt   age     lt  lt  age  lt  lt    n   lt  lt   profession     lt  lt  profession  lt  lt    n          push tuple to vector                                                           people push back std  make tuple  Anna   age  profession                            Get Second child                                                               temp   loadPtreeRoot get child  Ben                                              tage   temp get child  age                                                       tprofession   temp get child  profession                                          age    tage get value lt std  string gt                                                profession     tprofession get value lt std  string gt                                 std  cout  lt  lt   age     lt  lt  age  lt  lt    n   lt  lt   profession     lt  lt  profession  lt  lt    n          push tuple to vector                                                           people push back std  make tuple  Ben   age  profession                           for  const auto amp  tmppeople  people                                                   std  cout  lt  lt   Child    lt  lt  std  get lt 0 gt  tmppeople   lt  lt          lt  lt     age               lt  lt  std  get lt 1 gt  tmppeople   lt  lt    n    profession      lt  lt  std  get lt 2 gt  tmppeople   lt  lt    n

User · Answer

Have a look at nlohmann s JSON Repository on GitHub  I have found that it is the most convenient way to work with JSON   It is designed to behave just like an STL container  which makes its usage very intuitive

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Example  with complete source code  to read a json configuration file   https   github com sksodhi CodeNuggets tree master json config read    gt  pwd  root CodeNuggets json config read   gt  ls Makefile  README md  ReadJsonCfg cpp  cfg json   gt  cat cfg json        Note     This is a cofiguration file       Config               server-ip         10 10 10 20           server-port       5555           buffer-length    5000             gt  cat ReadJsonCfg cpp   include  lt iostream gt   include  lt json value h gt   include  lt jsoncpp json json h gt   include  lt fstream gt   void  displayCfg const Json  Value  amp cfg root    int main         Json  Reader reader      Json  Value cfg root      std  ifstream cfgfile  cfg json        cfgfile  gt  gt  cfg root       std  cout  lt  lt          cfg root   start          lt  lt  std  endl      std  cout  lt  lt  cfg root  lt  lt  std  endl      std  cout  lt  lt          cfg root   end            lt  lt  std  endl       displayCfg cfg root             void  displayCfg const Json  Value  amp cfg root        std  string serverIP   cfg root  Config    server-ip   asString        std  string serverPort   cfg root  Config    server-port   asString        unsigned int bufferLen   cfg root  Config    buffer-length   asUInt         std  cout  lt  lt          Configuration          lt  lt  std  endl      std  cout  lt  lt   server-ip         lt  lt  serverIP  lt  lt  std  endl      std  cout  lt  lt   server-port       lt  lt  serverPort  lt  lt  std  endl      std  cout  lt  lt   buffer-length     lt  lt  bufferLen lt  lt  std  endl      gt  cat Makefile  CXX   g   PROG   readjsoncfg  CXXFLAGS    -g -O0 -std c  11  CPPFLAGS              -I            -I usr include jsoncpp  LDLIBS                      -ljsoncpp  LDFLAGS    -L usr local lib   LDLIBS   all    PROG           echo   PROG  compilation success   SRCS             ReadJsonCfg cpp OBJS   subst  cc  o    subst  cpp  o    SRCS       PROG     OBJS            CXX       LDFLAGS  -o     clean          rm -f   OBJS    PROG     depend  depend   depend   depend    SRCS          rm -f    depend           CXX    CXXFLAGS    CPPFLAGS  -MM     gt      depend   include  depend   gt  make Makefile 43   depend  No such file or directory rm -f    depend g   -g -O0 -std c  11 -I  -I usr include jsoncpp -MM ReadJsonCfg cpp  gt      depend  g   -g -O0 -std c  11 -I  -I usr include jsoncpp  -c -o ReadJsonCfg o ReadJsonCfg cpp g   ReadJsonCfg o -L usr local lib -ljsoncpp -o readjsoncfg readjsoncfg compilation success    gt    readjsoncfg         cfg root   start                   Config                                buffer-length    5000                   server-ip     10 10 10 20                    server-port     5555                      Note     This is a cofiguration file           cfg root   end                 Configuration        server-ip      10 10 10 20 server-port    5555 buffer-length  5000   gt

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