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basic_json_document gets a second template parameter, Editable (false by default), plus the aliases json_editable_document, json_editable_view, ordered_json_editable_document and ordered_json_editable_view. Editable documents can change values and structure without rewriting the source text: set()/push_back() on values, keys, array indices and JSON pointers; insert() before an array element; erase() of an object key, array index or JSON pointer. New values and element sequences go into edit storage that the document owns and never moves, so views keep referring to their value across edits and a parsed node never moves. Read-only documents walk the plain node array and are unaffected. Strings are checked for UTF-8 on entry, so dump() of an editable document never throws type_error.316. Binary values cannot be stored (type_error.319). A seeded differential test applies random edits to an editable document and to the equivalent ordered_json and compares both after every step. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
39 lines
1.5 KiB
C++
39 lines
1.5 KiB
C++
#include <iostream>
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#include <nlohmann/json_view.hpp>
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using json = nlohmann::json;
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using json_editable_document = nlohmann::json_editable_document;
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using json_editable_view = nlohmann::json_editable_view;
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int main()
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{
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// a deprecated field is dropped from a configuration file, and a
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// decommissioned replica is removed from the list -- "price" keeps its
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// trailing zero, and the fields around the removed ones keep their order
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const std::string text = R"({
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"name": "cache",
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"legacy_host": "db0",
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"host": "db1",
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"price": 19.90,
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"replicas": ["db2", "db3", "db4"]
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})";
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json_editable_document doc = json_editable_document::parse(text);
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doc.erase(doc.root(), "legacy_host"); // (1) an object member
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doc.erase(doc.root()["replicas"], 1); // (2) an array element ("db3")
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const std::size_t removed = doc.erase(json::json_pointer("/replicas/0")); // (3) via a JSON pointer
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std::cout << removed << '\n';
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std::cout << doc.root().dump(2, ' ', false, json_editable_view::number_format::source) << "\n\n";
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// the same edits on a plain json value: object_t is a std::map, so
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// parsing already sorted the keys, and dump() rewrites every number to
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// its shortest form, even "price", which was never touched
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json plain = json::parse(text);
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plain.erase("legacy_host");
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plain["replicas"].erase(1);
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plain["replicas"].erase(0);
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std::cout << plain.dump(2) << '\n';
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}
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