fast-json

Build & run example

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;
}