At times it is not necessary to read the entire JSON document, but rather just a header or some of the initial fields.
This document describes a limited approach using the partial_read option, to quickly exit after parsing fields of interest.
For more advanced (and still performant) partial reading, Glaze provides compile-time and run-time JMESPath support.
partial_read is a compile time flag in glz::opts that indicates only existing array and object elements should be read into, and once the memory has been read, parsing returns without iterating through the rest of the document.
Note
Once any sub-object is read, the parsing will finish. This ensures high performance with short circuiting.
A wrapper by the same name also exists.
Example: read only the first two elements into a std::tuple
std::string s = R"(["hello",88,"a string we don't care about"])";
std::tuple<std::string, int> obj{};
expect(!glz::read<glz::opts{.partial_read = true}>(obj, s));
expect(std::get<0>(obj) == "hello");
expect(std::get<1>(obj) == 88);Example: read only the first two elements into a std::vector
std::string s = R"([1,2,3,4,5])";
std::vector<int> v(2);
expect(!glz::read<glz::opts{.partial_read = true}>(v, s));
expect(v.size() == 2);
expect(v[0] = 1);
expect(v[1] = 2);Example: read only the allocated element in a std::map
std::string s = R"({"1":1,"2":2,"3":3})";
std::map<std::string, int> obj{{"2", 0}};
expect(!glz::read<glz::opts{.partial_read = true}>(obj, s));
expect(obj.size() == 1);
expect(obj.at("2") = 2);Example: read only the fields present in a struct and then short circuit the parse
struct partial_struct
{
std::string string{};
int32_t integer{};
};std::string s = R"({"integer":400,"string":"ha!",ignore})";
partial_struct obj{};
expect(!glz::read<glz::opts{.partial_read = true}>(obj, s));
expect(obj.string == "ha!");
expect(obj.integer == 400);struct Header
{
std::string id{};
std::string type{};
};
struct HeaderFlipped
{
std::string type{};
std::string id{};
};
struct NestedPartialRead
{
std::string method{};
Header header{};
int number{};
};
suite partial_read_tests = [] {
using namespace ut;
static constexpr glz::opts partial_read{.partial_read = true};
"partial read"_test = [] {
Header h{};
std::string buf = R"({"id":"51e2affb","type":"message_type","unknown key":"value"})";
expect(!glz::read<partial_read>(h, buf));
expect(h.id == "51e2affb");
expect(h.type == "message_type");
};
"partial read 2"_test = [] {
Header h{};
// closing curly bracket is missing
std::string buf = R"({"id":"51e2affb","type":"message_type","unknown key":"value")";
expect(!glz::read<partial_read>(h, buf));
expect(h.id == "51e2affb");
expect(h.type == "message_type");
};
"partial read unknown key"_test = [] {
Header h{};
std::string buf = R"({"id":"51e2affb","unknown key":"value","type":"message_type"})";
expect(glz::read<partial_read>(h, buf) == glz::error_code::unknown_key);
expect(h.id == "51e2affb");
expect(h.type.empty());
};
"partial read unknown key 2"_test = [] {
Header h{};
std::string buf = R"({"id":"51e2affb","unknown key":"value","type":"message_type"})";
expect(!glz::read<glz::opts{.error_on_unknown_keys = false, .partial_read = true}>(h, buf));
expect(h.id == "51e2affb");
expect(h.type == "message_type");
};
"partial read don't read garbage"_test = [] {
Header h{};
std::string buf = R"({"id":"51e2affb","unknown key":"value","type":"message_type"garbage})";
expect(!glz::read<glz::opts{.error_on_unknown_keys = false, .partial_read = true}>(h, buf));
expect(h.id == "51e2affb");
expect(h.type == "message_type");
};
"partial read missing key"_test = [] {
Header h{};
std::string buf = R"({"id":"51e2affb","unknown key":"value"})";
expect(glz::read<glz::opts{.error_on_unknown_keys = false, .partial_read = true}>(h, buf) != glz::error_code::missing_key);
expect(h.id == "51e2affb");
expect(h.type.empty());
};
"partial read missing key 2"_test = [] {
Header h{};
std::string buf = R"({"id":"51e2affb","unknown key":"value"})";
expect(!glz::read<glz::opts{.error_on_unknown_keys = false, .partial_read = true}>(h, buf));
expect(h.id == "51e2affb");
expect(h.type.empty());
};
"partial read HeaderFlipped"_test = [] {
HeaderFlipped h{};
std::string buf = R"({"id":"51e2affb","type":"message_type","unknown key":"value"})";
expect(not glz::read<partial_read>(h, buf));
expect(h.id == "51e2affb");
expect(h.type == "message_type");
};
"partial read HeaderFlipped unknown key"_test = [] {
HeaderFlipped h{};
std::string buf = R"({"id":"51e2affb","unknown key":"value","type":"message_type"})";
expect(glz::read<partial_read>(h, buf) == glz::error_code::unknown_key);
expect(h.id == "51e2affb");
expect(h.type.empty());
};
"partial read unknown key 2 HeaderFlipped"_test = [] {
HeaderFlipped h{};
std::string buf = R"({"id":"51e2affb","unknown key":"value","type":"message_type","another_field":409845})";
expect(glz::read<glz::opts{.error_on_unknown_keys = false, .partial_read = true}>(h, buf) == glz::error_code::none);
expect(h.id == "51e2affb");
expect(h.type == "message_type");
};
};
suite nested_partial_read_tests = [] {
using namespace ut;
static constexpr glz::opts partial_read{.partial_read = true};
"nested object partial read"_test = [] {
NestedPartialRead n{};
std::string buf =
R"({"method":"m1","header":{"id":"51e2affb","type":"message_type","unknown key":"value"},"number":51})";
expect(not glz::read<partial_read>(n, buf));
expect(n.method == "m1");
expect(n.header.id == "51e2affb");
expect(n.header.type == "message_type");
expect(n.number == 0);
};
"nested object partial read, don't read garbage"_test = [] {
NestedPartialRead n{};
std::string buf =
R"({"method":"m1","header":{"id":"51e2affb","type":"message_type","unknown key":"value",garbage},"number":51})";
expect(not glz::read<partial_read>(n, buf));
expect(n.method == "m1");
expect(n.header.id == "51e2affb");
expect(n.header.type == "message_type");
expect(n.number == 0);
};
};