String to const char* Arduino

In Arduino programming, working with strings is a common task, whether you’re sending data over serial communication, displaying messages on an LCD, or interacting with other hardware components. The String class in Arduino provides a convenient way to handle text, but there are situations where you need to convert this String into a C-style string (a const char*) to work with certain functions or libraries. This conversion is crucial when you need to pass strings to functions that expect traditional C strings, which are simply arrays of characters terminated by a null character. Understanding how to perform this conversion and handle the resulting pointer correctly is key to ensuring your Arduino projects run smoothly. In this example, we’ll demonstrate a use case where converting a String to const char* is necessary, and we’ll walk through the code step by step.

But if you are only looking for string formatting, we have covered this in one of our previous articles where we explained the Arduino sprintf() function. We had also explained How to receive string input from serial in Arduino?

Using the c_str() Method

The String class in Arduino includes the c_str() method, which converts the String object into a const char*. This method returns a pointer to the internal buffer that holds the character data of the string, allowing you to use it seamlessly with C-style functions. For instance, if you have a String object defined as String myString = "Hello";, you can obtain the C-style string with const char* myCharPointer = myString.c_str();. This simple line of code enables the use of the string in a wide range of functions that do not support the String class directly. However, it is important to understand that this conversion is a one-way street; while you can easily convert String to const char*, modifying the string afterward could invalidate the pointer, leading to potential bugs.

Example Code: Sending a String via Serial Communication

Scenario

Imagine you are building a system where an Arduino sends data to a computer via serial communication. You need to send a message stored in a String object, but the Serial.write() function, which is more efficient for sending data than Serial.print(), requires a const char* as an argument. This is a perfect scenario to use the c_str() method to convert your String to a const char*

Example Code:

Now here is the example code for the above-mentioned scenario.

void setup() {
  // Initialize serial communication at 9600 baud
  Serial.begin(9600);
  
  // Define a String object with the message to be sent
  String myString = "Hello, Arduino!";
  
  // Convert the String to a const char*
  const char* myCharPointer = myString.c_str();
  
  // Send the C-style string via Serial.write()
  Serial.write(myCharPointer);
}

void loop() {
  // No repeated actions required
}

Code language: Arduino (arduino)

Caution When Using the Converted Pointer

While the c_str() method provides a straightforward way to convert String to const char*, there are important considerations to keep in mind. The pointer returned by c_str() is only valid as long as the String object remains unchanged. If the String is modified after the conversion, the internal buffer might be reallocated, rendering the previously obtained const char* invalid. This could lead to unexpected behavior or even crashes if the pointer is used after such a modification. Therefore, it’s crucial to either avoid modifying the String after the conversion or to immediately use the const char* before any changes are made to the String. This careful handling ensures that your code remains robust and free from hard-to-trace bugs.

Short Summary

In Arduino programming, there may be situations where you need to convert a String object to a const char*, which is a pointer to a character array, for use with functions that require C-style strings. To do this, you can use the c_str() method provided by the String class. This method returns a const char* that points to the internal buffer of the String object. For example, if you have a String myString = "Hello";, you can convert it using const char* myCharPointer = myString.c_str();. This conversion is useful when you need to pass a string to functions like Serial.print(), which expect a const char*. Keep in mind that the pointer returned by c_str() is only valid as long as the String object remains unchanged, so avoid modifying the String before you’re done using the const char*.