'dustDensity' is not captured - c++

Question

I'm trying to merge some code from another Arduino project into my current project. What i'm try to do is post the readings that I get from my current project to a web server, so that I can access it from my laptop or mobile device.
When I try to merge server code, I get the error 'dustDensity' is not captured. Not really sure as to why this is happening, and would appreciate some help to get around this issue.
Trying to compile this code in Arduino IDE but it won't.
#include <WiFi.h>
#include <ESPAsyncWebServer.h>
#include <SPIFFS.h>
#include <WiFi.h>
#include <ESPAsyncWebServer.h>
#include <SPIFFS.h>
#define USE_AVG
#define LED_BUILTIN 13
// Replace with your network credentials
const char* ssid = "ssid";
const char* password = "password";
// Create AsyncWebServer object on port 80
AsyncWebServer server(80);
// Arduino pin numbers.
const int sharpLEDPin = 18; // Arduino digital pin 7 connect to sensor LED.
const int sharpVoPin = 25; // Arduino analog pin 5 connect to sensor Vo.
// For averaging last N raw voltage readings.
#ifdef USE_AVG
#define N 100
static unsigned long VoRawTotal = 0;
static int VoRawCount = 0;
#endif // USE_AVG
// Set the typical output voltage in Volts when there is zero dust.
static float Voc = 0.6;
// Use the typical sensitivity in units of V per 100ug/m3.
const float K = 0.5;
/////////////////////////////////////////////////////////////////////////////
// Helper functions to print a data value to the serial monitor.
void printValue(String text, unsigned int value, bool isLast = false) {
Serial.print(text);
Serial.print("=");
Serial.print(value);
if (!isLast) {
Serial.print(", ");
}
}
void printFValue(String text, float value, String units, bool isLast = false) {
Serial.print(text);
Serial.print("=");
Serial.print(value);
Serial.print(units);
if (!isLast) {
Serial.print(", ");
}
}
/////////////////////////////////////////////////////////////////////////////
// Arduino setup function.
void setup() {
// Set LED pin for output.
pinMode(sharpLEDPin, OUTPUT);
pinMode(LED_BUILTIN, OUTPUT);
// Start the hardware serial port for the serial monitor.
Serial.begin(9600);
// Wait two seconds for startup.
delay(2000);
Serial.println("");
Serial.println("GP2Y1014AU0F Demo");
Serial.println("=================");
}
// Arduino main loop.
void loop() {
// Turn on the dust sensor LED by setting digital pin LOW.
digitalWrite(sharpLEDPin, LOW);
// Wait 0.28ms before taking a reading of the output voltage as per spec.
delayMicroseconds(280);
// Record the output voltage. This operation takes around 100 microseconds.
int VoRaw = analogRead(sharpVoPin);
// Turn the dust sensor LED off by setting digital pin HIGH.
digitalWrite(sharpLEDPin, HIGH);
// Wait for remainder of the 10ms cycle = 10000 - 280 - 100 microseconds.
delayMicroseconds(9620);
// Print raw voltage value (number from 0 to 1023).
#ifdef PRINT_RAW_DATA
printValue("VoRaw", VoRaw, true);
Serial.println("");
#endif // PRINT_RAW_DATA
// Use averaging if needed.
float Vo = VoRaw;
#ifdef USE_AVG
VoRawTotal += VoRaw;
VoRawCount++;
if ( VoRawCount >= N ) {
Vo = 1.0 * VoRawTotal / N;
VoRawCount = 0;
VoRawTotal = 0;
} else {
return;
}
#endif // USE_AVG
// Compute the output voltage in Volts.
Vo = Vo / 1024.0 * 5.0;
printFValue("Vo", Vo*1000.0, "mV");
// Convert to Dust Density in units of ug/m3.
float dV = Vo - Voc;
if ( dV < 0 ) {
dV = 0;
Voc = Vo;
}
float dustDensity = dV / K * 100.0;
printFValue("DustDensity", dustDensity, "ug/m3", true);
Serial.println("");
if (dustDensity > 85.0) {
digitalWrite(LED_BUILTIN,HIGH);
}
else {
digitalWrite(LED_BUILTIN,LOW);
}
if(!SPIFFS.begin()){
Serial.println("An Error has occurred while mounting SPIFFS");
return;
}
// Connect to Wi-Fi
WiFi.begin(ssid, password);
while (WiFi.status() != WL_CONNECTED) {
delay(1000);
Serial.println("Connecting to WiFi..");
}
// Print ESP32 Local IP Address
Serial.println(WiFi.localIP());
// Route for root / web page
server.on("/", HTTP_GET, [](AsyncWebServerRequest *request){
request->send(SPIFFS, "/index.html");
});
server.on("/temperature", HTTP_GET, [](AsyncWebServerRequest *request){
request->send_P(200, "text/plain", dustDensity().c_str());
});
server.on("/humidity", HTTP_GET, [](AsyncWebServerRequest *request){
request->send_P(200, "text/plain", VoRaw().c_str());
});
// Start server
server.begin();
} // END PROGRAM
So not sure, why it won't compile. I'm trying to send dustDensity to a website from my ESP32.

Answer 1

Your code is currently
server.on("/temperature", HTTP_GET, [](AsyncWebServerRequest *request){
request->send_P(200, "text/plain", dustDensity().c_str());
});
This code:
[](AsyncWebServerRequest *request){
request->send_P(200, "text/plain", dustDensity().c_str());
}
is a "lambda expression". It's a function that won't be evaluated until request->send_P() is called. That means that dustDensity won't be defined at that point. For it to be available to the function it has to be "captured", which means that you must tell the lambda expression explicitly to make it available.
You'll also need to turn it into a C-string or String since the method wants a string argument.
So you need to rewrite this code to look more like this:
server.on("/temperature", HTTP_GET, [dustDensity](AsyncWebServerRequest *request){
request->send(200, "text/plain", String(dustDensity()));
});
The changes here are:
- add dustDensity to the brackets in the lambda so that it's captured and available inside the function
- change send_P to send because send_P is meant for sending strings stored in PROGMEM
- make a String from dustDensity so that send() has some text to send
ESPAsyncWebServer is extensively documented with lots of examples. I highly recommend reading the documentation if you're using it.