forked from bblanchon/ArduinoJson
Minor corrections to the doc
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Avoiding common pitfalls in Arduino JSON
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Avoiding common pitfalls in Arduino JSON
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========================================
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========================================
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As `StaticJsonBuffer` is the corner stone of this library, you'll see that every pitfall listed here is related to a wrong understanding of the memory model.
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As `StaticJsonBuffer` is the corner stone of this library, you'll see that every pitfall listed here are related to a wrong understanding of the memory model.
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Make sure you read [Arduino JSON memory model](Memory model.md) before going further.
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Make sure you read [Arduino JSON memory model](Memory model.md) before going further.
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@ -14,7 +14,7 @@ There are basically two reasons why they may fail:
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1. the JSON string is invalid,
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1. the JSON string is invalid,
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2. the JSON string contains more values that the buffer can store.
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2. the JSON string contains more values that the buffer can store.
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So, if you are sure the JSON string is correct and you still can't parse it, you should slightly increase the number of token of the parser.
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So, if you are sure the JSON string is correct and you still can't parse it, you should try to increase the size of the `StaticJsonBuffer`.
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## 2. Make sure everything fits in memory
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## 2. Make sure everything fits in memory
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@ -25,8 +25,8 @@ To diagnose this, look at every big objects in you code and sum their size to ch
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For example, don't do this:
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For example, don't do this:
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char json[1024]; // 1 KB
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char json[1024]; // 1 KB
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JsonParser<512> parser; // 514 B
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StaticJsonBuffer<512> buffer; // 514 B
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because it may be too big for a processor with only 2 KB: you need free memory to store other variables and the call stack.
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because it may be too big for a processor with only 2 KB: you need free memory to store other variables and the call stack.
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@ -34,9 +34,9 @@ That is why an 8-bit processor is not able to parse long and complex JSON string
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## 3. Keep the `StaticJsonBuffer` in memory long enough
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## 3. Keep the `StaticJsonBuffer` in memory long enough
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Remember that the function of `StaticJsonBuffer` return references.
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Remember that `StaticJsonBuffer`'s function return references.
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References don't store data, they are just pointer to the actual.
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References don't contain data, they are just pointer to the actual.
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This will only work if the data actual is still in memory.
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So they can only work if the actual data is in memory.
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For example, don't do this:
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For example, don't do this:
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@ -48,12 +48,13 @@ For example, don't do this:
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because the local variable `buffer` will be *removed* from memory when the function `parseArray()` returns, and the `JsonArray&` will point to an invalid location.
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because the local variable `buffer` will be *removed* from memory when the function `parseArray()` returns, and the `JsonArray&` will point to an invalid location.
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## 4. Don't make `StaticJsonBuffer` global
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## 4. Don't reuse the same `StaticJsonBuffer`
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If you read the previous point, you may come to the idea of using a global variable for your `StaticJsonBuffer`.
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During is lifetime a `StaticJsonBuffer` growth until it's discarded. If you try to reuse the same instance several time, it will rapidly get full.
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This is probably a bad idea because `StaticJsonBuffer` can be quite big (depending on your requirement) and would be eating a lot of memory, even when you don't use it.
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There are some cases were a `StaticJsonBuffer` can be a global variable, but must of the time you should declare it in a local scope, in a function which unique role is to handle the JSON serialization.
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For this reason, you should not use a global variable for your `StaticJsonBuffer`. I don't think there is any scenario in which a global `StaticJsonBuffer` would be a valid option.
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The best practice is to declare it in a local scope, so that it's discarded as soon as possible. My advice it to declare it in a function which unique role is to handle the JSON serialization.
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## 5. Keep the JSON string in memory long enough
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## 5. Keep the JSON string in memory long enough
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@ -69,21 +70,21 @@ For instance, let's imagine that you parse `["hello","world"]`, like this:
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const char* first = array[0];
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const char* first = array[0];
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const char* second = array[1];
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const char* second = array[1];
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In that case, both `first` and `second` are pointer to the content of the original string `json`.
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In that case, both `first` and `second` are pointers to the content of the original string `json`.
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So this will only work if `json` is still in memory.
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So this will only work if `json` is still in memory.
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## 6. JSON string is altered
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## 6. JSON string is altered
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If you read carefully the previous pitfall, you may I come to the conclusion that the JSON parser modifies the JSON string.
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If you read carefully the previous section, you may have come to the conclusion that the JSON parser modifies the JSON string.
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Indeed, the parser modifies the string for two reasons:
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Indeed, the parser modifies the string for two reasons:
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1. it inserts `\0` to terminate substrings,
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1. it inserts `\0` to terminate substrings,
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2. it translate escaped characters like `\n` or `\t`.
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2. it translate escaped characters like `\n` or `\t`.
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Most of the time this wont be an issue, but it there are some corner case that can be problematic.
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Most of the time this wont be an issue, but there are some corner cases that can be problematic.
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Let take the example above:
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Let take the example bellow:
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char[] json = "[\"hello\",\"world\"]";
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char[] json = "[\"hello\",\"world\"]";
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StaticJsonBuffer<32> buffer;
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StaticJsonBuffer<32> buffer;
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@ -97,5 +98,5 @@ If you replace it by:
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Depending on your platform, you may have an exception because the parser tries to write at a location that is read-only.
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Depending on your platform, you may have an exception because the parser tries to write at a location that is read-only.
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In the first case `char json[]` declares an array of `char` initialized to the specified string.
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In the first case `char json[]` declares an array of `char` initialized to the specified string.
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In the second case `char* json` declares a pointer to a read only string, in fact it should be a `const char*` instead of a `char*`.
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In the second case `char* json` declares a pointer to a read-only string, in fact it should be a `const char*` instead of a `char*`.
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@ -7,7 +7,7 @@ If you want to contribute to the project, please:
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2. Follow the coding conventions
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2. Follow the coding conventions
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3. Write tests
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3. Write tests
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About the coding conventions: I try to follow the [Google C++ Style Guide](http://google-styleguide.googlecode.com/svn/trunk/cppguide.html) which few variations to match the Arduino conventions.
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About the coding conventions: I try to follow the [Google C++ Style Guide](http://google-styleguide.googlecode.com/svn/trunk/cppguide.html) with few variations to match the Arduino conventions.
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I use [ClangFormat](http://clang.llvm.org/docs/ClangFormat.html) to format the code for me.
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I use [ClangFormat](http://clang.llvm.org/docs/ClangFormat.html) to format the code for me.
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I use [CppLint](http://google-styleguide.googlecode.com/svn/trunk/cpplint/cpplint.py) to detect non-compliant stuff.
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I use [CppLint](http://google-styleguide.googlecode.com/svn/trunk/cpplint/cpplint.py) to detect non-compliant stuff.
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@ -1,6 +1,12 @@
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Decoding JSON with Arduino JSON
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Decoding JSON with Arduino JSON
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===============================
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===============================
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Before writing any code, don't forget to include the header:
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#include <ArduinoJson.h>
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For instructions on how to install the library, please read [Using the library with Arduino](Using the library with Arduino.md) or [Using the library without Arduino](Using the library without Arduino.md).
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## Example
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## Example
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Here an example that parse the string `{"sensor":"gps","time":1351824120,"data":[48.756080,2.302038]}`:
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Here an example that parse the string `{"sensor":"gps","time":1351824120,"data":[48.756080,2.302038]}`:
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@ -39,14 +45,14 @@ Before continuing please read the page [Arduino JSON memory model](Memory model.
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## Step 2: Parse the JSON string
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## Step 2: Parse the JSON string
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You call the JSON parser through the instance of `StaticJsonBuffer`.
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You invoke the JSON parser through the instance of `StaticJsonBuffer`.
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It exposes two function for parsing JSON:
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It exposes two functions for parsing JSON:
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1. parseArray() that returns a reference to a `JsonArray`
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1. `parseArray()` that returns a reference to a `JsonArray`
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2. parseObject() that returns a reference to a `JsonObject`
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2. `parseObject()` that returns a reference to a `JsonObject`
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Let's see an example.
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Let's see an example.
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Say we want to parse `{"sensor":"gps","time":1351824120,"data":[48.756080,2.302038]}`, it's an object so we call `parseObject` as follows:
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Say we want to parse `{"sensor":"gps","time":1351824120,"data":[48.756080,2.302038]}`, it's an object so we call `parseObject()` as follows:
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char json[] = "{\"sensor\":\"gps\",\"time\":1351824120,\"data\":[48.756080,2.302038]}";
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char json[] = "{\"sensor\":\"gps\",\"time\":1351824120,\"data\":[48.756080,2.302038]}";
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@ -5,7 +5,7 @@ Before writing any code, don't forget to include the header:
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#include <ArduinoJson.h>
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#include <ArduinoJson.h>
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If your not using the Arduino IDE, please read [Using the library without Arduino](Using the library without Arduino.md).
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For instructions on how to install the library, please read [Using the library with Arduino](Using the library with Arduino.md) or [Using the library without Arduino](Using the library without Arduino.md).
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## Example
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## Example
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@ -58,14 +58,13 @@ Then you can add strings, integer, booleans, etc:
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There are two syntaxes for floating point values:
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There are two syntaxes for floating point values:
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array.add<4>(3.1415); // 4 digits: "3.1415"
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array.add(3.1415, 4); // 4 digits: "3.1415"
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array.add(3.1415); // 2 digits: "3.14"
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array.add(3.1415); // 2 digits: "3.14"
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> ##### About floating point precision
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> ##### About floating point precision
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> The overload of `add()` with 2 parameters allows you to specify the number of decimals to save in the JSON string.
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> The overload of `add()` with 2 parameters allows you to specify the number of decimals to save in the JSON string.
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> When you use the overload with one parameter, you use the default number of decimals which is two.
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> When you use the overload with one parameter, you use the default number of decimals which is 2.
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> Note that this behavior is the exact same as Arduino's `Print::print(double,int);` which is implemented by `Serial`.
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> Note that this behavior is the exact same as Arduino's `Print::print(double,int);` which is implemented by `Serial`, so you may already be familiar with this behavior.
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> So you may already be familiar with it.
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You can add a nested array or object if you have a reference to it.
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You can add a nested array or object if you have a reference to it.
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Or simpler, you can create nested array or nested objects from the array:
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Or simpler, you can create nested array or nested objects from the array:
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@ -75,7 +74,7 @@ Or simpler, you can create nested array or nested objects from the array:
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#### Objects
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#### Objects
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You create an array like this:
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You create an object like this:
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JsonObject& object = jsonBuffer.createObject();
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JsonObject& object = jsonBuffer.createObject();
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@ -89,7 +88,7 @@ Then you can add strings, integer, booleans, etc:
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As for the arrays, there are two syntaxes for the floating point values:
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As for the arrays, there are two syntaxes for the floating point values:
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object["key4"].set<4>(3.1415); // 4 digits "3.1415"
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object["key4"].set(3.1415, 4); // 4 digits "3.1415"
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object["key5"] = 3.1415; // default: 2 digits "3.14"
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object["key5"] = 3.1415; // default: 2 digits "3.14"
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You can add a nested array or object if you have a reference to it.
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You can add a nested array or object if you have a reference to it.
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@ -137,5 +136,5 @@ And, of course if you need an indented JSON string:
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> ##### About the Print interface
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> ##### About the Print interface
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> The library is designed to send the JSON string to an implementation of the `Print` interface that is part of Arduino.
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> The library is designed to send the JSON string to an implementation of the `Print` interface that is part of Arduino.
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> In the example above we used `Serial`, but they are many other implementation that would work as well, including: `HardwareSerial`, `SoftwareSerial`, `LiquidCrystal`, `EthernetClient`, `WiFiClient`, `Wire`...
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> In the example above we used `Serial`, but they are many other implementations that would work as well, including: `HardwareSerial`, `SoftwareSerial`, `LiquidCrystal`, `EthernetClient`, `WiFiClient`, `Wire`...
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> When you use this library out of the Arduino environment, it will use it's own implementation of `Print` and everything will be the same.
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> When you use this library out of the Arduino environment, it will use it's own implementation of `Print` and everything will be the same.
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