C++ arduino 库返回具有不正确值的整数数组

Question

I am currently writing a library for Arduino using C++ to take an 8 character hexadecimal RFID string from a reader and convert that string into bytes, treating every two characters as part of a single byte such that "1234ABCD" becomes 0X12, 0X34, 0XAB, 0XCD. These bytes are then converted into the integer values that represent them such that the previous values become 18, 52, 171 and 205 respectively. These integers will then be placed in an array and returned to the caller. As the RFID string is hexadecimal I am multiplying the integer value of the first character in each pair by 16.

The C++ library:

int AccessPortal::hexToInt(char character) {
  if(int(character) -  '0' < 10) {
    return int(character)  - '0';
  }
  else {
    if(isupper(character)) {
      return int(character) - '7';
    }
    else {
      return int(character) - 'W';
    }
  }
}

int* AccessPortal::getByteArray(std::string uid) {
  int byteArray[4];
  int intArray[8];
  int counter = 0;
  for(int i = 0; i < 8; i++) {
    if(i  % 2 == 0) {
      intArray[i] = hexToInt(uid[i]) * 16;
    }
    else {
      intArray[i] = hexToInt(uid[i]);
    }
  }
  for(int i = 1; i < 8; i = i + 2) {
    byteArray[counter] = intArray[i - 1]  + intArray[i];
    counter ++;
  }
  return byteArray;
}

The Arduino code:

 void setup() {
      // put your setup code here, to run once:
      AccessPortal access(); //constructor 
      int *h;
      h = access.getByteArray("1234ABCD");
      for(int i = 0; i < 4; i++) {
        Serial.println(h[i]);
      } 
    }

    void loop() {
      // put your main code here, to run repeatedly:

    }

However, when I run the program the following is printed to serial:

1073741740
1073741680
1073670364
1075849820

This is strange because when I run the functions in the C++ library separately and print the results to the terminal the values are correct. I am guessing this means the error is occurring in the way the array is being returned and/or accessed in the Arduino code, however I am unsure.

Answer 1

As others have said, a pointer to a local variable becomes invalid as soon as the local variable goes out of scope, so it shouldn't be returned. You can instead create a ByteArray object that you can return-by-value from your functions. Example using ArduinoSTL:

#include <ArduinoSTL.h>
#include <string>
#include <cstdlib>

struct ByteArray {
    uint32_t m_value;
    bool m_valid;

    // convert hex string to uint32_t value
    ByteArray(const std::string& uid) :
        m_value(std::strtoul(uid.c_str(), nullptr, 16)),
        m_valid(true)
    {
        if(uid.size()!=8) m_valid = false;
    }

    // converting assignment operator
    ByteArray& operator=(const std::string& uid) {
        m_value = std::strtoul(uid.c_str(), nullptr, 16);
        if(uid.size()==8) m_valid = true;
        else m_valid = false;
        return *this;
    }

    // subscript operator to access the individual bytes (as ints)
    int operator[](size_t x) const {
        if(x>3) return -1;
        return (m_value>>((3-x)*8))&0xFF;
    }
};

struct AccessPortal {
    // change the getByteArray signature to this
    ByteArray getByteArray(const std::string&);
};

ByteArray AccessPortal::getByteArray(const std::string& uid) {
    // return by value - copy elision
    return ByteArray(uid);
}

void setup() {
    AccessPortal access; //default constructing
    ByteArray h = access.getByteArray("1234ABCD");

    Serial.println(h.m_value); // dec: 305441741  hex: 1234ABCD

    for(int i = 0; i < 4; i++) {
        Serial.println(h[i]); // dec: 18, 52, 171, 205  hex: 12, 34, AB, CD
    }
}

void loop() {
    //
}