fast-json

C++ header-only library for processing of JSON documents, with focus on small memory footprint and performance.

Build & run example

$ ./build_and_run

Example Usage

#include "fast-json.hpp"
#include <iostream>
#include <string>
using namespace std;


int main(int argc, char** argv){
    Arena<1> arena_allocator;        // fast-json comes with an arena allocator which supports reallocation,
                                    // the arena API is pretty simple which allows for a straight forward adoption
                                    // of user defined allocators.
    fast_json<Arena<1>> jsn;        
    string json = "[1,2,3,4,5,6]";
    auto r {jsn.read(json,&arena_allocator,0)};
    if(r){
        double sum{};
        // fast-json provides a traverse - function which takes a functor f.
        // The traversal occurs in document order, i.e. pre-order traversal of the binary representation
        // (root node, then children from left to right). If an inner node, i.e. array or object,
        // is visited the first time 'enter' is set to true, and false in the case the node is exited, i.e.
        // after all children have been visited.
        traverse_binary_representation(r->first, r->second.second, [&sum](msg_node*m, bool enter){
            if (m->what== msg_node::F64) sum += ((msg_node_f64*)m)->value; // all nodes derive from msg_node,
                    //dispatch according to the 'what' field, cast if necessary (see fast-json.hpp for details)
        });
        cout << " sum = " << sum << '\n';
    } else {
        cerr << "Failed to read JSON.\n";
        return 1;
    }
    {
     // Access via the index operator []
     std::string json = R"({
      "pi":3.141592655359,
      "jsonrpc":"2.0",
      
      "id":0,"method":"initialize",
      "params":
       {
        "protocolVersion":"2025-11-25",
        "capabilities":{
          "sampling":{},
          "elicitation":{},
          "roots":{"listChanged":true},
          "tasks":{
            "list":{},
            "cancel":{},
            "requests":{
            "sampling":{"createMessage":{}},
            "elicitation":{"create":{}}
           }
          }
        },
       "clientInfo":{
         "name":"inspector-client",
         "version":"0.20.0"
        }
       }
      })";
     fast_json<Arena<1>> jsn{json, arena_allocator, 0}; 
     if (!jsn.valid()){
        std::cerr << "Failed to read >>"+json+"<<.\n";
        return 2;
     }
     std::string jsonrpc = jsn["jsonrpc"];
     std::string method = jsn["method"];
     double pi = jsn["pi"];
     std::string params_protocolVersion = jsn["params"]["protocolVersion"];
     std::cerr << "jsonrpc=" << jsonrpc <<" "<<"method="<< method << " pi=" << pi << " params.protocolVersion=" << params_protocolVersion << "\n"; 
   }
    return 0;
}