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89 lines
2.4 KiB
Markdown
89 lines
2.4 KiB
Markdown
# Trailing Commas
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Like [comments](comments.md), this library does not support trailing commas in arrays and objects *by default*.
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You can set parameter `ignore_trailing_commas` to `#!cpp true` in the [`parse`](../api/basic_json/parse.md) function to allow trailing commas in arrays and objects. Note that a single comma as the only content of the array or object (`[,]` or `{,}`) is not allowed, and multiple trailing commas (`[1,,]`) are not allowed either.
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This library does not add trailing commas when serializing JSON data.
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For more information, see [JSON With Commas and Comments (JWCC)](https://nigeltao.github.io/blog/2021/json-with-commas-comments.html).
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!!! example
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Consider the following JSON with trailing commas.
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```json
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{
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"planets": [
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"Mercury",
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"Venus",
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"Earth",
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"Mars",
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"Jupiter",
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"Uranus",
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"Neptune",
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]
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}
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```
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When calling `parse` without additional argument, a parse error exception is thrown. If `ignore_trailing_commas` is set to `#!cpp true`, the trailing commas are ignored during parsing:
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```cpp
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#include <iostream>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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std::string s = R"(
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{
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"planets": [
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"Mercury",
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"Venus",
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"Earth",
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"Mars",
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"Jupiter",
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"Uranus",
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"Neptune",
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]
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}
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)";
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try
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{
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json j = json::parse(s);
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}
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catch (json::exception& e)
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{
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std::cout << e.what() << std::endl;
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}
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json j = json::parse(s,
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/* callback */ nullptr,
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/* allow exceptions */ true,
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/* ignore_comments */ false,
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/* ignore_trailing_commas */ true);
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std::cout << j.dump(2) << '\n';
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}
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```
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Output:
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```
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[json.exception.parse_error.101] parse error at line 11, column 9: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal
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{
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"planets": [
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"Mercury",
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"Venus",
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"Earth",
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"Mars",
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"Jupiter",
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"Uranus",
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"Neptune"
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]
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}
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```
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