C++ writing JSON with JSONCPP

Question

I was trying to use the JSONCPP library to work with JSON documents in C++. Reading another question here, I saw this example and tried to implement:

#include "json/json.h"

int main(){
    Json::Value event;
    Json::Value vec(Json::arrayValue);
    vec.append(Json::Value(1));
    vec.append(Json::Value(2));
    vec.append(Json::Value(3));

    event["competitors"]["home"]["name"] = "Liverpool";
    event["competitors"]["away"]["code"] = 89223;
    event["competitors"]["away"]["name"] = "Aston Villa";
    event["competitors"]["away"]["code"] = vec;

    std::cout << event << std::endl;
}

But I keep getting this error:

json2.cpp:(.text+0x1c): referência não definida para "Json::Value::Value(Json::ValueType)"
/usr/bin/ld: json2.cpp:(.text+0x30): referência não definida para "Json::Value::Value(Json::ValueType)"

json2.cpp is the file's name, as it looks like. Being a /usr/bin/ld a link error, I tried to add as parameter the directory path to the compile command but the error persists

Does anyone know how to fix it?

Answer 1

It’s all about how you build/install the library and specify the headers in the source. Since you didn’t mention what method you took to build the library, I’ll demonstrate three solutions I got to work on Ubuntu 20.04 LTS.

Install the Package

Most straightforward way is to install via apt:

$ sudo apt-get install libjsoncpp-dev

The header files will be installed to /usr /include/jsoncpp/json and can be included in json2.cpp as:

#include <jsoncpp/json/json.h>

And compile by adding the linker flag as:

$ g++ json2.cpp -o json2 -ljsoncpp

Amalgamated Source

Inside of the top-level directory of the jsoncpp repository, you can amalgamate the source by:

$ python amalgamate.py

By default, this will generate all necessary source and header files in the dist/ directory, and there is no need to link the library. Include in json2.cpp as:

#include <json/json.h>

As indicated in the build instructions, jsoncpp.cpp will have to be incorporated into your project:

$ g++ json2.cpp dist/jsoncpp.cpp -Idist -o json2

Cmake

From the top-level directory of the jsoncpp repository:

$ mkdir build
$ cd build
$ cmake -DCMAKE_BUILD_TYPE=release -DBUILD_STATIC_LIBS=ON -DBUILD_SHARED_LIBS=OFF -DARCHIVE_INSTALL_DIR=. -G "Unix Makefiles" ..
$ make; make install

This will install the header files to /usr/local/include/json. Include in json2.cpp as:

#include <json/json.h>

And compile by adding the linker flag as:

$ g++ json2.cpp -o json2 -ljsoncpp