user35228: Which library I must use for JSON? Where is RPC call example ?
How to use RPC of bitcoind or bitcoin-cli? or better Electrum API to get all new (maybe pending) transactions which appears in Bitcoin net?

user26697: 
How to use RPC of bitcoind or bitcoin-cli?


bitcoind - bitcoin deamon = core value of the software (bitcoind -printtoconsole -debug=1)
bitcoin-cli - bitcoin command line interface = allow you to "query' the blockchain that you had download previously with bitcoind. (bitcoin-cli getinfo)

So bitcoind provide the RPC "interface" in which you can query with bitcoin-cli (or a library in c++ you will find...).
You must run bitcoind before using bitcoin-cli.
Basically bitcoin-cli communicate with your node bitcoind so in other word your current blockchain state.
bitcoin-qt - Graphical bitcoin software = bitcoind + graphical interface 
(You can find a graphical console embedded in which you will use bitcoin-cli command)

user4334: Even though it is a C library, you could use Jansson in C++ apps, too. See: https://en.bitcoin.it/wiki/API_reference_(JSON-RPC)#C

user30017: I am not sure why you chose C++
If you are going to build something where you need the speed of C++ then understandably stick with it.
Otherwise to simply perform some RPC calls use something simpler like python or PHP.   
Here is a tutorial on how to do RPC calls via python:
Bitcoin & Blockchain Programming #007 – RPCs via Python 
And via PHP:
Bitcoin & Blockchain Programming #005 – RPCs via PHP

user75032: Your direct question is about a fine JSON-RPC library in C++, and the direct answer to it is:

libjson-cpp &
libjson-rpc-cpp

But if I wanna target your intention, there is a well-developed wrapper for Bitcoind-json/rpc interface here. It will save you massive time!

35228: <p>Which library I must use for JSON? Where is RPC call example ?
How to use RPC of bitcoind or bitcoin-cli? or better <strong>Electrum</strong> API to get all new (maybe pending) transactions which appears in Bitcoin net?</p>
 26697: <blockquote>
  <p>How to use RPC of bitcoind or bitcoin-cli?</p>
</blockquote>

<ol>
<li>bitcoind - bitcoin deamon = core value of the software (bitcoind -printtoconsole -debug=1)</li>
<li>bitcoin-cli - bitcoin command line interface = allow you to "query' the blockchain that you had download previously with bitcoind. (bitcoin-cli getinfo)<br></li>
</ol>

<p>So bitcoind provide the RPC "interface" in which you can query with bitcoin-cli <br>(or a library in c++ you will find...).<br>
You must run bitcoind before using bitcoin-cli.<br></p>

<p>Basically bitcoin-cli communicate with your node bitcoind so in other word your current blockchain state.<br></p>

<p>bitcoin-qt - Graphical bitcoin software = bitcoind + graphical interface 
<br>(You can find a graphical console embedded in which you will use bitcoin-cli command)</p>
 4334: <p>Even though it is a C library, you could use <a href="https://jansson.readthedocs.org/en/latest/" rel="nofollow">Jansson</a> in C++ apps, too. See: <a href="https://en.bitcoin.it/wiki/API_reference_(JSON-RPC)#C" rel="nofollow">https://en.bitcoin.it/wiki/API_reference_(JSON-RPC)#C</a></p>
 30017: <p>I am not sure why you chose C++<br>
If you are going to build something where you need the speed of C++ then understandably stick with it.<br>
Otherwise to simply perform some RPC calls use something simpler like python or PHP.   </p>

<p>Here is a tutorial on how to do RPC calls via python:<br>
<a href="http://blog.cryptoiq.ca/?p=347" rel="nofollow">Bitcoin &amp; Blockchain Programming #007 – RPCs via Python</a>   </p>

<p>And via PHP:<br>
<a href="http://blog.cryptoiq.ca/?p=261" rel="nofollow">Bitcoin &amp; Blockchain Programming #005 – RPCs via PHP</a></p>
 75032: <p>Your direct question is about a fine JSON-RPC library in C++, and the direct answer to it is:</p>

<ul>
<li><a href="https://github.com/open-source-parsers/jsoncpp" rel="nofollow noreferrer">libjson-cpp</a> &amp;</li>
<li><a href="https://github.com/cinemast/libjson-rpc-cpp" rel="nofollow noreferrer">libjson-rpc-cpp</a></li>
</ul>

<p>But if I wanna target your intention, there is a well-developed wrapper for Bitcoind-json/rpc interface <a href="https://github.com/minium/bitcoin-api-cpp" rel="nofollow noreferrer">here</a>. It will save you massive time!</p>
