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This book describes the JsonCpp library (also called jsoncpp and json-cpp), JsonCpp is probably the most popular library for working with JSON databases in C++. It can parse and save databases, and provides an extensive library for accessing and manipulating their members. JsonCpp works with both files and strings.

For a library which exists for years, JsonCpp is surprisingly poorly documented. You can find either a very simple example of usage, or a computer-generated list of all classes and methods.

I am not an author, nor a contributor, of the library.

JSON (JavaScript Object Notation) is an ASCII format for representing various data structures. It is pretty language-independent, machine-independent, simple, and easy readable by both humans and computers.

JSON is fully described atjson.org.

In brief, a JSON value can be one of the following (seejson.org for full details):

• null
• true orfalse
• Anumber.
  • Numbers cannot start with 0 (or -0) followed by another digit (e. g.0 and0.14 are valid numbers, while03,012.34, and-015 are not).
  • If a number contains a decimal point, it should also contain at least one digit both before and after the decimal point (e. g. numbers15. and.15 are invalid).
• Astring. Strings are enclosed in double quotes and may contain the following escape sequences:

   \" - quote   \\ - backslash   \/ - slash   \n - newline   \t - tabulation   \r - carriage return   \b - backspace   \f - form feed   \uxxxx , wherex is a hexadecimal digit - any 2-byte symbol

• Anarray. It is denoted by a comma-separated list of any values in square brackets. Example:

   [1, 2, 3, "Hello world\n", true]

• Anobject, also called associative array, keyed list etc. It is denoted by comma-separated list ofpairs in braces. A pair has a form

   key : value

where key is a string, value is any JSON value. Example:

   {"foo":1, "bar":2, "baz":3.14, "hello world":[1,2,3]}

Whitespaces can be inserted between any tokens.

Array elements are accessed by their number, while object elements are accessed by key. Arrays and objects can be empty. Arrays and objects can recursively contain another arrays or objects.

While strict JSON syntax does not allow any comments, and requires the root value to be array or object, JsonCpp allows both C-style and C++-style comments, and allows the root value to be of any type.

As for February 2016, there are totally hundreds of libraries for parsing and generating JSON on 62 languages, including 22 different libraries for C++.[1]

JsonCpp is probably the most popular C++ library. Another popular library israpidjson, which is very fast.

The easiest way to use it from Ubuntu or another flavor of Debian Linux, is to install is as:

   sudo apt-get install libjsoncpp-dev

(You may useapt instead ofapt-get.)

One drawback is that it will not install the last version. For Ubuntu 18.04.1, the version is 1.7.4, while the last version as per August 2018 is 1.8.4. The problem was much worse for 14.04, which installed 0.6.0, while the last version as per February 2016 was 1.7.4.

To use JsonCpp, include:

   #include <jsoncpp/json/json.h>

To compile a file, add flag

   -ljsoncpp

The header files will be installed to /usr/include/jsoncpp/json. In case you are curious, the libraries will be most probably installed to /usr/lib/x86_64-linux-gnu (but you hardly need their location). If you want to discover, try:

   ls /usr/lib/*/*jsoncpp*   ls /usr/lib/*jsoncpp*

To use JsonCpp with amalgamated source, you don't need to download or make any binary files. You will have a single cpp and two .h files which you should include into your projects. These files will be system-independent.

• Download and unzip the source fromthe official repository. Go to the directory.

• Run

   python amalgamate.py

It will create three files

   dist/jsoncpp.cpp, the source file to be added to your project   dist/json/json.h, the correspondent header file   dist/json/json-forwards.h, which contains forward declarations of JSON types.

You don't need anything except these three files.

• Download and unzip the source fromthe official repository. Go to the directory.
• Install cmake. Under Ubuntu or another flavour of Debian Linux:

   sudo apt-get install cmake

• Create the build directory and enter it:

   mkdir -p build   cd build

• Run the cmake command

   cmake -DCMAKE_BUILD_TYPE=release -DBUILD_STATIC_LIBS=ON -DBUILD_SHARED_LIBS=OFF -DARCHIVE_INSTALL_DIR=. -G "Unix Makefiles" ..

• Run the make command:

   make

(You may use-j flag to parallelize the building, e.g.make -j 4.)

Under Unix, it will create the file src/lib_json/libjsoncpp.a in your build directory. The include files will be in the ../include/json . Install the files (make install might help), and use

   #include <jsoncpp/json/json.h>

and

   -ljsoncpp

• Download and unzip the source fromthe official repository.
• In this unzipped source tree, under makefiles/msvc2010 (for MS Visual Studio 2010) or vs71 (MS Visual Studio 2003) you will find several Visual Studio project files which you can up convert and build.

Install JsonCpp.

Create file alice.json with the following contents:

{"book":"Alice in Wonderland","year":1865,"characters":[{"name":"Jabberwock","chapter":1},{"name":"Cheshire Cat","chapter":6},{"name":"Mad Hatter","chapter":7}]}

Create file alice.cpp with following contents:

#include<iostream>#include<fstream>#include<jsoncpp/json/json.h> // or jsoncpp/json.h , or json/json.h etc.usingnamespacestd;intmain(){ifstreamifs("alice.json");Json::Readerreader;Json::Valueobj;reader.parse(ifs,obj);// reader can also read stringscout<<"Book: "<<obj["book"].asString()<<endl;cout<<"Year: "<<obj["year"].asUInt()<<endl;constJson::Value&characters=obj["characters"];// array of charactersfor(inti=0;i<characters.size();i++){cout<<"    name: "<<characters[i]["name"].asString();cout<<" chapter: "<<characters[i]["chapter"].asUInt();cout<<endl;}}

Compile it:

   g++ -o alice alice.cpp -ljsoncpp

Then run it:

   ./alice

You will hopefully receive the following:

Book: Alice in WonderlandYear: 1865    name: Jabberwock chapter: 1    name: Cheshire Cat chapter: 6    name: Mad Hatter chapter: 7

Everything is in theJson namespace.

Names of classes and other types use upper CamelCase notation (capitalize first letter of each word). Examples:Int,ArrayIndex,ValueType. Names of member functions, fields, enum values use lower camelCase notation (capitalize first letter of each word except the first word). Examples:stringValue,isInt,size.

JsonCpp performs extensive validity checking. If an operation is invalid, it throws thestd::runtime_error exception with relevant message.

JsonCpp stores each number as either 64-bit integer (long long int or__int64), or 64-bit unsigned integer (unsigned long long int orunsigned __int64), ordouble. Below, we are going to call these int, uint, and real respectively.

An array or an object may contain at most${\displaystyle 2^{32}-1}$ elements. A string may contain at most${\displaystyle 2^{32}-1}$ characters. An object key may contain at most${\displaystyle 2^{30}-1}$ characters.

JsonCpp provides several auxiliary types.

The following types are defined inconfig.h :

• Int - defined asint
• UInt - defined asunsigned int
• Int64 - a 64-bit signed integer, defined as__int64 for Microsoft Visual Studio, otherwiselong long int
• UInt64 - a 64-bit unsigned integer, defined asunsigned __int64 for Microsoft Visual Studio, otherwiseunsigned long long int
• LargestInt - a largest possible signed integer, defined asInt64
• LargestUInt - a largest possible unsigned integer, defined asUInt64

ArrayIndex is a type for array indices. It is defined asunsigned int, which means that an array or object may contain at most${\displaystyle 2^{32}-1}$ items.

ValueType is an enum describing a type of JSON value. It is defined as:

enumValueType{nullValue=0,///< 'null' valueintValue,///< signed integer valueuintValue,///< unsigned integer valuerealValue,///< double valuestringValue,///< UTF-8 string valuebooleanValue,///< bool valuearrayValue,///< array value (ordered list)objectValue///< object value (collection of name/value pairs).};

The simplest way to do input/output is viaoperator<< andoperator>> . The program below reads a JSON value from standard input and writes it to standard output. In case of syntax error,operator<< throwsruntime_error exception.

#include<iostream>#include<jsoncpp/json/json.h> // or somethingusingnamespacestd;intmain(){Json::Valueval;cin>>val;cout<<val;}

For example:

$ g++ -o copy-json copy-json.cpp -ljsoncpp$ echo '{"a":[1,2],"b":[3,4]}' | ./copy-json{        "a" : [ 1, 2 ],        "b" : [ 3, 4 ]}

For reading this way from a string, or writing to a string, you may usestd::istringstream andstd::ostringstream respectively, but there are alternative ways to do it.

The methodtoStyledString converts any value to a formatted string. Its declaration is:

std::stringtoStyledString()const;

Another, and more robust, way to read JSON values is via theReader class. Its most useful public methods are (here and below, comments are mine, and the order of methods is also mine):

Reader();// the default constructor~Reader();// Read a value from a JSON document and store it to root.// If collectComments is true, comments are stored, otherwise they are ignored.// In case of syntax error, it returns false, and the value of root may be arbitrary.boolparse(conststd::string&document,Value&root,boolcollectComments=true);// from std::stringboolparse(constchar*beginDoc,constchar*endDoc,Value&root,boolcollectComments=true);// from C-style stringboolparse(std::istream&is,Value&root,boolcollectComments=true);// from input stream// Returns a user friendly string that list errors in the parsed document.std::stringgetFormattedErrorMessages()const;

Example:

#include<iostream>#include<jsoncpp/json/json.h> // or somethingusingnamespacestd;intmain(){Json::Valueval;Json::Readerreader;boolb=reader.parse(cin,val);if(!b)cout<<"Error: "<<reader.getFormattedErrorMessages();elsecout<<val;}

UnlikeReader, the classWriter is abstract. There are two classes which implement it:

• FastWriter produces unformatted, non-human-readable documents. Everything will be written in a single line.
• StyledWriter produces formatted, human-readable documents, similar tooperator<<, but with less indentation and without empty lines.

FastWriter has the following public methods (default constructor and destructor are not shown):

// omit the word "null" when printing null values// this contradicts the JSON standard, but accepted by JavaScript// this function is not available in old versionsvoiddropNullPlaceholders();// don't add newline as last character// this function is not available in old versionsvoidomitEndingLineFeed();// print space after ":" in objectsvoidenableYAMLCompatibility();// write JSON object to a stringvirtualstd::stringwrite(constValue&root);

StyledWriter has the following public method (default constructor and destructor are not shown):

virtualstd::stringwrite(constValue&root);// write JSON object to a string

Finally, there is alsoStyledStreamWriter class, for writing to streams. It is directly called byoperator<< . This class is not a descendant ofWriter or any other class. Its public methods are:

StyledStreamWriter(std::stringindentation="\t");~StyledStreamWriter();voidwrite(std::ostream&out,constValue&root);

StyledStreamWriter is not very useful, asoperator<< is more convenient. You may want to use it if you want non-standard indentation.

Example:

#include<iostream>#include<jsoncpp/json/json.h> // or somethingusingnamespacestd;intmain(){Json::Valueval;cin>>val;Json::FastWriterfast;Json::StyledWriterstyled;stringsFast=fast.write(val);stringsStyled=styled.write(val);cout<<"Fast:\n"<<sFast<<"Styled:\n"<<sStyled;cout<<"Styled stream:\n";Json::StyledStreamWriterstyledStream;styledStream.write(cout,val);}

The following method of Value returns some info about it:

// get the type (intValue etc.)ValueTypetype()const;// get the number of elements in array or object (returns 0 for anything else, including string)ArrayIndexsize()const;// returns true for empty array, empty object, or nullboolempty()const;// returns true for null onlybooloperator!()const;// returns true for specific type onlyboolisNull()const;boolisBool()const;boolisString()const;boolisArray()const;boolisObject()const;// see explanations in textboolisInt()const;boolisInt64()const;boolisUInt()const;boolisUInt64()const;boolisIntegral()const;boolisDouble()const;boolisNumeric()const;

isInt(),isInt64(),isUInt(),isUInt64() returntrue only if all the following conditions satisfied:

• the type is numeric (int, uint, or real)
• if the type is real, the value should have zero fractional part
• the value should be within the range of the given type (Int,Int64,UInt,UInt64 respectively)

isDouble() andisNumeric() currently returntrue if the type is int, uint, or real.

isIntegral() always returnstrue for an int or uint. For a real value, it returnstrue if the value has zero fractional part, and within the range ofInt64 orUInt64. For all other types, it returnsfalse.

is... functions are not backwards-compatible. In versions 0.*,isInt() andisUInt() just checked the type,isInt64() andisUInt64() did not exist,isArray() andisObject() also returned true for null value,isIntegral also returned true for booleans etc.

Example:

#include<iostream>#include<jsoncpp/json/json.h> // or somethingusingnamespacestd;intmain(){Json::Valueval;cin>>val;switch(val.type()){caseJson::nullValue:cout<<"nullValue\n";break;caseJson::intValue:cout<<"intValue\n";break;caseJson::uintValue:cout<<"uintValue\n";break;caseJson::realValue:cout<<"realValue\n";break;caseJson::stringValue:cout<<"stringValue\n";break;caseJson::booleanValue:cout<<"booleanValue\n";break;caseJson::arrayValue:cout<<"arrayValue\n";break;caseJson::objectValue:cout<<"objectValue\n";break;default:cout<<"wrong type\n";break;}}

To get the numerical, boolean, or string value itself, the class Value provides following methods:

constchar*asCString()const;std::stringasString()const;IntasInt()const;UIntasUInt()const;Int64asInt64()const;UInt64asUInt64()const;LargestIntasLargestInt()const;LargestUIntasLargestUInt()const;floatasFloat()const;doubleasDouble()const;boolasBool()const;

Some of these methods may throwstd::runtime_exception. A simple rule: ifisFoo() returns true, then it is safe to callasFoo(), but the opposite is not necessarily true.

Another rule, it is always safe to call

• asString() for string
• asLargestInt() for int
• asLargestUInt() for uint
• asFloat() orasDouble() for any number (int, uint, or real)
• asBool() for boolean

Below are the details.

The methodsasInt(),asUInt,asInt64(),asUInt64(),asLargestInt(),asLargestUInt() do the following:

• If the original value is numeric, check if it is within the range of the destination type. If not, throwstd::runtime_error. Then cast the value to the destination type. The casting is plain, so the real value of 3.9, when sent toasInt(), becomes 3.
• If the original value is boolean or null, return 1 for true, 0 for false, and 0 for null.
• For strings, arrays, and objects, throwstd::runtime_error.

The methodsasFloat() andasDouble() do the following:

• If the original value is numeric, cast it tofloat ordouble.
• If the original value is boolean or null, return 1.0 for true, 0.0 for false, and 0.0 for null.
• For strings, arrays, and objects, throwstd::runtime_error.

The methodasBool() accepts anything.

• false, null, 0, 0.0, or empty string/array/object is converted tofalse
• true, non-zero number, or non-empty string/array/object is converted totrue

The methodasString() is robust, slow, and high level. It returnsstd::string. It correctly treats strings with zero characters. It accepts everything except arrays and objects. For a null, the method returns""; for a boolean, it returns"true" or"false"; for a number, it returns it's string representation. For arrays and objects, it throwsstd::runtime exception.

The methodasCString() is the fast, low-level method. It accepts only strings (otherwise it throwsstd::runtime exception), and directly returns the C-style string which is stored internally. The method doesn't allocate anything. Don't callfree() ordelete[] on the returned pointer! You should keep in mind two things:

• Since C-style strings should not contain zero characters, the method is only suitable for strings without zero characters.
• For empty strings, it sometimes returns 0.

If you need a C-style string, but don't want to be confused by zero characters, newer versions of JsonCpp add the following method:

boolgetString(charconst**begin,charconst**end)const;

This method stores the pointer to the first character to*begin, stores the pointer to the final zero character to*end, and returnstrue. For non-strings and sometimes for empty strings, it returnsfalse.

Example :

#include<iostream>#include<jsoncpp/json/json.h> // or somethingusingnamespacestd;intmain(){Json::Valueval;cin>>val;switch(val.type()){caseJson::nullValue:cout<<"null\n";break;caseJson::intValue:cout<<"int "<<val.asLargestInt()<<"\n";break;caseJson::uintValue:cout<<"uint "<<val.asLargestUInt()<<"\n";break;caseJson::realValue:cout<<"real "<<val.asDouble()<<"\n";break;caseJson::stringValue:cout<<"string "<<val.asString()<<"\n";break;/*        -or-        case Json::stringValue: {            const char *begin;            const char *end;            val.getString(&begin, &end);            cout << "string of length " << end - begin << "\n";        }        break;        */caseJson::booleanValue:cout<<"boolean "<<val.asBool()<<"\n";break;caseJson::arrayValue:cout<<"array of length "<<val.size()<<"\n";break;caseJson::objectValue:cout<<"object of length "<<val.size()<<"\n";break;default:cout<<"wrong type\n";break;}}

The class Json::Value provides following constructors:

Value(ValueTypetype=nullValue);Value(Intvalue);Value(UIntvalue);Value(Int64value);Value(UInt64value);Value(doublevalue);Value(constchar*value);Value(constchar*beginValue,constchar*endValue);Value(conststd::string&value);Value(boolvalue);Value(constValue&other);

The first constructor creates null, false, 0, 0.0, or empty string/array/object. The other constructors are self-explanatory.

Assignment, swap, and all comparison operators are also provided (as methods).

Value&operator=(constValue&other);voidswap(Value&other);booloperator<(constValue&other)const;booloperator<=(constValue&other)const;booloperator>=(constValue&other)const;booloperator>(constValue&other)const;booloperator==(constValue&other)const;booloperator!=(constValue&other)const;intcompare(constValue&other)const;

Arrays have their own methods. These methods also work for null.

Some of them are similar to C++ STL's vectors:

ArrayIndexsize()const;boolempty()const;voidclear();voidresize(ArrayIndexsize);Value&operator[](ArrayIndexindex);Value&operator[](intindex);constValue&operator[](constArrayIndexindex)const;constValue&operator[](constintindex)const;

Note that ArrayIndex is defined as unsigned int.

resize() changes the array size by either removing last values or appending null values.

Ifoperator[] receives a negative index, it throws thestd::runtime_error exception. If it receivesindex which is equal to or greater than the current size,

• non-constantoperator[] appendsindex-size()+1 null values, then returns the last value
• constantoperator[] returns the null value

To append a value, useappend:

Value&append(constValue&value);

This is similar to C++ methodvector::push_back(). In other words,foo.append(bar) is equivalent tofoo[foo.size()]=bar.

The methodget returnsindex-th element, ordefaultValue ifindex is greater or equal to size:

Valueget(ArrayIndexindex,constValue&defaultValue)const;

Note that it returns value NOT by reference, so calling this method may be very expensive.

To check validity of an index, you may want to useisValidIndex:

boolisValidIndex(ArrayIndexindex)const;

This is not very useful, asvalue.isValidIndex(index) is equivalent toindex < value.size().

You can also remove one value withremoveIndex:

boolremoveIndex(ArrayIndexi,Value*removed);

No miracles, this method takes linear time. Ifi is greater or equal to size, it returnsfalse.

Iffoo is null, the methods above treat it as an empty array:

• foo.empty() returnstrue
• foo.size() returns 0
• foo.clear() does nothing
• foo.resize(0) does nothing
• foo.resize(size) for positive size transformsfoo into an array ofsize nulls.
• foo[i] (non-constant) transformsfoo into an array ofi+1 nulls, then returns the last value
• foo[i] (constant) returns the null value
• foo.isValidIndex(i) always returnsfalse
• foo.get(index, defaultValue) always returnsdefaultValue
• foo.append(bar) makesfoo an array of one element, which equals tobar
• foo.removeIndex always returns false

The methodsclear(),empty() andsize() also work for objects.

Other than this, calling any of the methods above for something which is neither array nor null is pretty useless. They either return something trivial or throw thestd::runtime_error exception:

• foo.empty() returnsfalse, unlessfoo is an empty object
• foo.size() returns 0, unlessfoo is an object
• foo.clear() throws thestd::runtime_error exception, unlessfoo is an object
• resize,append,get,operator[] throw thestd::runtime_error exception
• isValidIndex always returnsfalse
• removeIndex always returnsfalse

Objects have their own methods. These methods also work for null.

Some of them are similar to C++ STL's maps:

ArrayIndexsize()const;boolempty()const;voidclear();Value&operator[](constchar*key);constValue&operator[](constchar*key)const;Value&operator[](conststd::string&key);constValue&operator[](conststd::string&key)const;

These are self-explanatory. Foroperator[], if the key does not exist, non-constantoperator[] inserts the (key, null) pair and returns the reference to this null, while constantoperator[] just returns the reference to some null.

The following methods take a key as either C++ string, C string, or a pair of pointers specifying beginning and end of the string. The last form does not exist in old versions of JsonCpp. It is useful, for example, if a string contains zero characters.

The methodisMember checks whether there exists a member with given key:

boolisMember(constchar*key)const;boolisMember(conststd::string&key)const;boolisMember(constchar*begin,constchar*end)const;// only in newer versions

The methodremoveMember removes an element. The first two forms return the removed value NOT by reference, which may be pretty expensive.

ValueremoveMember(constchar*key);// deprecated in versions 1.*ValueremoveMember(conststd::string&key);// deprecated in versions 1.*boolremoveMember(constchar*key,Value*removed);// only since versions 1.*boolremoveMember(std::stringconst&key,Value*removed);// only since versions 1.*boolremoveMember(constchar*begin,constchar*end,Value*removed);// only since versions 1.*

To iterate through the members of an object, you need the full list of their keys. This is performed bygetMemberNames:

Value::MembersValue::getMemberNames()const;

Value::Members is defined as:

typedefstd::vector<std::string>Members;

The methodget returns value for the given key, or, in its absence,defaultValue. Just like with arrays, it returns value NOT by reference, so calling this method may be very expensive.

Valueget(constchar*key,constValue&defaultValue)const;Valueget(constchar*begin,constchar*end,constValue&defaultValue)const;Valueget(conststd::string&key,constValue&defaultValue)const;// only in newer versions

The methodfind exists only in newer versions. It receives the key in (begin, end) form and returns pointer to the found value. If not found, it returnsNULL pointer.

constValue*find(constchar*begin,constchar*end)const;// only in newer versions

The methods above treat null value as an empty object:

• clear() does nothing
• size() returns 0
• empty() returns true
• non-constantoperator[] transforms the value into one-element object, with given key and null value
• constantoperator[] returns null
• removeMember() returns null (deprecated variants) orfalse (new variants).
• getMemberNames() returns empty vector of strings
• get() returnsdefaultValue
• find() returnsNULL

The methodsclear(),size(),empty() work also for arrays. Other than this, for a value which is neither null nor an object, the methods above return trivial value or throw thestd::runtime_error exception.

Creating a complex structure:

#include<iostream>#include<jsoncpp/json/json.h> // or somethingusingnamespacestd;intmain(){// create the characters arrayJson::Valuech;ch[0]["name"]="Jabberwock";ch[0]["chapter"]=1;ch[1]["name"]="Cheshire Cat";ch[1]["chapter"]=6;ch[2]["name"]="Mad Hatter";ch[2]["chapter"]=7;// create the main objectJson::Valueval;val["book"]="Alice in Wonderland";val["year"]=1865;val["characters"]=ch;cout<<val<<'\n';}

Recursive function for printing any value (of course, it already exists, but we implement it from scratch):

#include<cstdlib>#include<iostream>#include<fstream>#include<jsoncpp/json/json.h> // or somethingusingnamespacestd;voidIndent(ostream&ofs,intindent){for(inti=0;i<indent;i++)ofs<<' ';}voidMyPrint(ostream&ofs,constJson::Value&val,intindent=0){switch(val.type()){caseJson::nullValue:ofs<<"null";break;caseJson::booleanValue:ofs<<(val.asBool()?"true":"false");break;caseJson::intValue:ofs<<val.asLargestInt();break;caseJson::uintValue:ofs<<val.asLargestUInt();break;caseJson::realValue:ofs<<val.asDouble();break;caseJson::stringValue:ofs<<'"'<<val.asString()<<'"';break;caseJson::arrayValue:{Json::ArrayIndexsize=val.size();if(size==0)ofs<<"[]";else{ofs<<"[\n";intnewIndent=indent+4;for(Json::ArrayIndexi=0;i<size;i++){Indent(ofs,newIndent);MyPrint(ofs,val[i],newIndent);ofs<<(i+1==size?"\n":",\n");}Indent(ofs,indent);ofs<<']';}break;}caseJson::objectValue:{if(val.empty())ofs<<"{}";else{ofs<<"{\n";intnewIndent=indent+4;vector<string>keys=val.getMemberNames();for(size_ti=0;i<keys.size();i++){Indent(ofs,newIndent);conststring&key=keys[i];ofs<<'"'<<key<<'"'<<" : ";MyPrint(ofs,val[key],newIndent);ofs<<(i+1==keys.size()?"\n":",\n");}Indent(ofs,indent);ofs<<'}';}break;}default:cerr<<"Wrong type!"<<endl;exit(0);}}intmain(){ifstreamifs("alice.json");Json::Valueval;ifs>>val;MyPrint(cout,val);cout<<'\n';}

Iterators have typesJson::Value::iterator andJson::Value::const_iterator. They are bidirectional, but not random. The methods ofJson::Value are

const_iteratorbegin()const;const_iteratorend()const;iteratorbegin();iteratorend();

Only arrays and objects have non-trivial iterators. Iffoo is neither array nor object, thenfoo.begin() equals tofoo.end().

Iterators have the full set of operators: incrementing and decrementing (postfix and prefix++ and--), comparison to equality and inequality, assignment, default constructor and copy constructor. Iterators are not random, so adding an integer number to iterator is not possible. Subtracting iterator from another iterator is possible, but takes linear time.

Iffoo is an iterator to an array, then*foo is a reference to the respective array's element. Iffoo is an iterator to an object, then*foo is NOT a reference to the (key, value) pair. It is a reference to the value itself.

operator-> is available since versions 1.* .

Example:

#include<iostream>#include<fstream>#include<jsoncpp/json/json.h> // or somethingusingnamespacestd;intmain(){Json::Readerreader;Json::Valueval;reader.parse("{\"one\":1,\"two\":2,\"three\":3}",val);for(Json::Value::const_iteratorit=val.begin();it!=val.end();++it)cout<<it->asInt()<<'\n';}

will print:

132

To receive key (for iterator to object) or index (for iterator to array),Json::Value::iterator andJson::Value::const_iterator provide three methods:

// For iterator to array, returns the index// For iterator to object, returs the key.Valuekey()const;// Return the index, or -1 if it is not an array iteratorUIntindex()const;// Return the key, or "" if it is not an object iterator.constchar*memberName()const;

As explained in comments,key() returns the key for an object element or the index for an array element;index() returns the index for an array element, otherwiseUInt(-1);memberName() returns the key for an object element, otherwise empty string ("").

Example:

This works for newer version only.

#include<iostream>#include<fstream>#include<jsoncpp/json/json.h> // or somethingusingnamespacestd;intmain(){Json::Readerreader;Json::Valueval;reader.parse("{\"one\":1,\"two\":2,\"three\":3}",val);for(Json::Value::const_iteratorit=val.begin();it!=val.end();++it)cout<<it.key().asString()<<':'<<it->asInt()<<'\n';}

This prints:

one:1three:3two:2

For an older version, we cannot initialize Json::Value::const_iterator as Json::Value::iterator, and there is no operator->.

In versions 0.*, casting fromconst_iterator to iterator is not possible, while the corresponding assignment is possible. For example, ifval is not constant, we cannot write

Json::Value::const_iteratorit=val.begin();

Instead, we should useJson::Value::iterator. Besides, there is nooperator-> .

Therefore, instead of the program above, we should write:

#include<iostream>#include<fstream>#include<jsoncpp/json/json.h> // or somethingusingnamespacestd;intmain(){Json::Readerreader;Json::Valueval;reader.parse("{\"one\":1,\"two\":2,\"three\":3}",val);// We cannot declare "it" as a const_iterator, because begin() and end()// return iterator, and there is no casting from iterator to const_iteratorfor(Json::Value::iteratorit=val.begin();it!=val.end();++it)cout<<it.key().asString()<<':'<<(*it).asInt()<<'\n';// no operator-> in this version}

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