Arduino

Invalid Use Of Void Expression How To Use Function Pointer With Input

by Staff Writer
4 min read

Understanding Function Pointers in Arduino

Function pointers are a powerful feature in programming that allow you to reference and invoke functions dynamically. In the context of Arduino and C/C++, function pointers can be particularly useful when needing to pass functions as arguments to other functions. This concept can seem complex and lead to errors such as the "Invalid Use Of Void Expression". This article explores how to properly use function pointers, particularly with input parameters, to avoid common pitfalls.

The Basics of Function Pointers

A function pointer is a variable that stores the address of a function. This enables you to call a function indirectly, which can be advantageous in various scenarios, such as callbacks or event handling.

Syntax of Function Pointers

To declare a function pointer, you need to specify the return type of the function, followed by an asterisk (to denote a pointer) and the function’s parameter types in parentheses. For example:

void (*funcPtr)(int);

In this case, funcPtr is a pointer to a function that returns void and takes an integer parameter. You can assign a function to this pointer as follows:

void exampleFunction(int val) {
    // Function implementation
}

funcPtr = exampleFunction;

You can then call the function using the pointer:

funcPtr(10);

Common Pitfalls: Invalid Use of Void Expression

One frequent mistake that leads to the "Invalid Use Of Void Expression" error occurs when trying to use a function pointer that points to a void function in a context that expects a value. For instance, if you attempt to assign the result of a void function to a variable, the compiler will generate this error because void does not return a value.

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Example of the Error

Here’s an example that illustrates this error:

void dummyFunction() {
    // Intentionally left blank
}

int value = dummyFunction();  // This line creates an "Invalid Use Of Void Expression" error

In this case, the variable value is attempting to store the return of dummyFunction, which does not return any value. To avoid this error, ensure that you do not assign or expect a return value from a void function.

Using Function Pointers with Parameters

When working with function pointers that require input parameters, the process involves defining the function correctly and ensuring parameters are passed appropriately.

Step-by-Step Usage

  1. Define the Function: Create a function that accepts parameters and returns a type other than void if you want to use its result.
int compute(int number) {
    return number * number;  // Returns square of the input
}
  1. Declare the Function Pointer: Create a function pointer that matches the function’s signature.
int (*computePtr)(int);
  1. Assign the Function to the Pointer:
computePtr = compute;
  1. Using the Pointer: Call the function through the pointer, passing the necessary arguments.
int result = computePtr(5); // result now holds the value 25

Following these steps ensures that the function pointer is used correctly with parameters, avoiding common errors such as invalid void expressions.

Advanced Use Cases of Function Pointers

Function pointers can be particularly useful in callback scenarios, where the specific function to be executed is determined at runtime. This is common in event-driven programming or when implementing state machines.

Callback Example

void onSuccess(int result) {
    Serial.println("Success: " + String(result));
}

void executeCallback(int input, void (*callback)(int)) {
    int output = input + 10; // Simulating some processing
    callback(output);         // Call the passed function
}

void setup() {
    Serial.begin(9600);
    executeCallback(5, onSuccess); // Pass onSuccess as the callback
}

In this example, the executeCallback function processes some data and then calls a passed function pointer (onSuccess), showcasing the flexibility function pointers provide.

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FAQ

1. What types of functions can be pointed to using a function pointer?

Function pointers can point to any function that matches the declared signature, including functions that return different types (void, int, float, etc.) and those that take various types of parameters.

2. How do I handle multiple function pointers with different signatures?

You can use a generic function pointer or define multiple function pointers with distinct signatures, but ensure you handle them appropriately based on their specific types during function implementation.

3. Can function pointers be used with class methods in Arduino?

Yes, function pointers can reference class methods, but the syntax differs slightly, requiring the use of the class instance’s pointer and the appropriate method signature. Use typedef to simplify the management of function pointers in such cases.

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