1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
|
# Partial Read
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](./JMESPath.md).
# Partial reading with glz::opts
`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](./wrappers.md) by the same name also exists.
Example: read only the first two elements into a `std::tuple`
```c++
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`
```c++
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`
```c++
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
```c++
struct partial_struct
{
std::string string{};
int32_t integer{};
};
```
```c++
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);
```
## Unit Test Examples
```c++
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);
};
};
```
|
