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>usingnamespacestd;intmain(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";
return1;
}
{
// 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";
return2;
}
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"; }
return0;
}