Efficient Exception Handling in Resource-Constrained Embedded Systems

1. Introduction

This article provides a comprehensive analysis of exception handling mechanisms specifically designed for embedded systems, where resource constraints and performance optimization are paramount. In such environments, traditional C++ exception handling is often deemed impractical due to the substantial overhead it incurs. Consequently, developers frequently resort to disabling exceptions using compiler flags such as -fno-exceptions. Despite this, the necessity for robust error management remains critical, especially in scenarios where system stability must be preserved in the face of unexpected errors.

To address these challenges, this article explores a range of alternative error-handling strategies that are more suitable for embedded systems. The discussion begins with low-level constructs such as goto statements and function pointers, which provide direct and efficient methods for error management. It then advances to more sophisticated techniques involving setjmp and longjmp, which enable the simulation of exception handling with minimal resource consumption. Through detailed and practical examples, we illustrate how these methods can be effectively implemented in resource-constrained environments, ensuring that errors are managed gracefully without compromising system performance.

In addition to these low-level approaches, the article also examines modern C++ techniques that offer lightweight alternatives to traditional exceptions. Constructs like std::optional and std::variant provide structured and efficient methods for error handling, allowing for more granular and type-safe error management with minimal overhead. These modern techniques are particularly advantageous in embedded systems where performance and memory efficiency are critical.

Furthermore, the article addresses the complexities introduced by these alternative methods, particularly concerning resource management and cleanup. Practical solutions are presented to prevent issues such as memory leaks, which are essential for ensuring the long-term reliability of embedded systems. By the conclusion of this article, readers will have gained a thorough understanding of how to implement robust and efficient error-handling mechanisms, leveraging both traditional and modern C++ approaches. This knowledge is vital for enhancing the reliability and sustainability of mission-critical applications in embedded environments.

2. Understanding setjmp and longjmp

2.1. Low-Level Error Handling: A Pragmatic Approach

Before exploring advanced error-handling constructs like setjmp and longjmp, it is instructive to examine a fundamental, low-level approach to error management using function pointers and the goto statement. This method operates closer to the hardware, offering a highly efficient solution in embedded contexts where minimizing computational overhead is essential.

Consider the following code snippet, which demonstrates this approach:

#include <iostream>

// environment
void* try_block = nullptr;
volatile int errorCode = 0;

void riskyFunction() {

    // Simulate the effect of longjmp
    std::cout << "Starting a risky function.\n";
    
    // Mimic throwing an exception (error code: 1)
    if (try_block) {
        errorCode = 1;        // Simulate setting an error code
        goto *try_block;      // Jump back to the try block
    }

    std::cout << "No errors detected.\n";
}

int main() {

    // Save the address of the try block
    try_block = &&try_block_label;

try_block_label:
    if (errorCode == 0) {
        riskyFunction();
    } else {
        std::cout << "An error occurred!\n";
    }

    return 0;
}

The output:

>>>
Starting a risky function.
An error occurred!

• void* try_block:

  This pointer stores the address of a specific code location, effectively serving as a jump target.

• volatile int errorCode:

  This global variable indicates whether an error has occurred. The volatile qualifier ensures that the variable’s value is always read directly from memory, which is crucial in environments where memory consistency is a concern.

• riskyFunction:

  This function simulates a risky operation. If an error is detected, the function jumps back to the location pointed to by try_block, updating the errorCode to signal the error.

• goto:

  The goto statement allows the program to jump back to the try_block_label if an error is detected, thereby simulating a rudimentary exception handling mechanism.

While this approach is relatively straightforward, it can be effective in highly constrained environments where even the overhead of setjmp and longjmp might be undesirable. However, it is less flexible and more prone to errors compared to higher-level constructs.

2.2. Advanced Error Handling: Leveraging setjmp and longjmp

For scenarios that demand a higher level of abstraction and control, setjmp and longjmp offer a powerful mechanism to save and restore the program’s execution context. The setjmp function captures the current environment—including the stack context and registers—into a buffer (typically of type jmp_buf), while longjmp restores this environment, effectively reverting the program’s state to the point where setjmp was initially invoked.

Consider the following implementation:

#include <iostream>
#include <csetjmp>

jmp_buf environment;

void riskyFunction() {
    if (/* some error condition */) {
        // Simulates throw Error(code=1)
        longjmp(environment, 1);
    }
    std::cout << "No errors detected.\n";
}

int main() {
    int errorCode = setjmp(environment);
    if (errorCode == 0) {
        // Simulates Try block
        riskyFunction();
    } else {
        // Simulates Catch block
        std::cout << "An error occurred!\n";
    }
    return 0;
}

This example illustrates the practical utility of setjmp and longjmp in managing control flow amidst errors—a particularly valuable capability in embedded systems where traditional exception handling is not feasible.

2.3. Handling Typed Exceptions with setjmp and longjmp

The conceptual framework provided by setjmp and longjmp can be extended to handle multiple error types, each requiring a distinct response. This approach allows developers to implement typed exceptions, even in environments where conventional C++ exceptions are unavailable.

#include <iostream>
#include <csetjmp>

jmp_buf environment;

int constexpr ValueException  = 1;
int constexpr SyntaxException = 2;

void riskyFunction() {

    if (/* some error condition for value correctness*/) {
        // Simulates throw ValueException;
        longjmp(environment, ValueException);
    }
    
    if (/* some error condition for syntax correctness*/) {
        // Simulates throw SyntaxException;
        longjmp(environment, SyntaxException);
    }

    std::cout << "No errors detected.\n";
}

int main() {
    int errorCode = setjmp(environment);
    if (errorCode == 0) {
    
        // Simulates Try block
        riskyFunction();
        
    } else if (errorCode == ValueException) {
    
        // Handle Value Error
        std::cout << "ValueException ocurred\n";
        
    } else if (errorCode == SyntaxException) {
    
        // Handle Syntax Error
        std::cout << "SyntaxException ocurred\n";
        
    }
    return 0;
}

This code demonstrates how longjmp can be employed to simulate typed exception handling, a practice commonly seen in state-based error handling within C. This approach not only enhances the robustness of error management but also introduces a level of granularity that aligns with the nuanced requirements of embedded systems.

2.4. Exception Propagation: Implementing Exception Cascades

In more complex embedded applications, it is often necessary to propagate exceptions through multiple layers of function calls. The following example illustrates how longjmp can be utilized to create exception cascades, allowing errors to be propagated and handled at different levels of the program.

#include <iostream>
#include <csetjmp>

jmp_buf environment;

void riskyFunction() {
    longjmp(environment, 1);
}

void secondFunction() {
    longjmp(environment, 2);
}

int main() {
    int errorCode = setjmp(environment);
    if (errorCode == 0) {
    
        // Calling a risky function
        riskyFunction();
        
    } else if (errorCode == 1) {
    
        // Handling an error (which can also be handling)
        std::cout << "riskyFunction called longjmp\n";
        secondFunction();
        
    } else if (errorCode == 2) {
    
        // Exception was thrown while handling another exception
        std::cout << "secondFunction called longjmp\n";
        
    }
    return 0;
}

This implementation showcases the flexibility of longjmp in orchestrating complex control flows within an embedded system. By allowing control to be transferred between disparate parts of the program, longjmp enables a more nuanced and responsive error-handling strategy, essential in high-stakes embedded applications.

3. Simulating Exception Handling in Embedded Systems Using setjmp and longjmp

3.1. Designing a Basic Exception Handling Framework

The utility of setjmp and longjmp extends beyond simple error signaling; these functions can be taken advantage of to construct a rudimentary exception-handling framework that mimics the try-catch paradigm of higher-level programming languages. Such a framework is particularly advantageous in embedded systems where traditional C++ exceptions are either impractical or outright disabled.

3.2. Practical Example: Integrating Resource Management with Exception Handling

In embedded systems, resource management is a critical concern, especially given the limited availability of memory and processing power. The following example demonstrates how to implement a simple yet effective exception-handling mechanism that integrates resource management, ensuring that resources are properly released even in the presence of errors.

Let’s define an ExceptionHandler class that replicates the behavior of try-catch blocks in a controlled embedded environment:

class ExceptionHandler {
public:

    // Register Try block
    ExceptionHandler& Try(std::function<void()> func);

    // Register Catch block
    ExceptionHandler& Catch(std::function<void(int)> func);

    // Execute try/catch
    void Execute();

    // Reset the handler
    void reset();

    // throw an exception
    void throwException(int error = 1);

private:
    bool                     isConfigured_ = false;
    jmp_buf                  environment_;
    std::function<void()>    tryBlock_;
    std::function<void(int)> catchBlock_;
};

In this implementation, the Try and Catch methods serve as configuration functions that register their respective blocks of code:

ExceptionHandler& ExceptionHandler::Try(std::function<void()> func) {
    tryBlock_     = func;
    isConfigured_ = true;
    return *this;
}

ExceptionHandler& ExceptionHandler::Catch(std::function<void(int)> func) {
    catchBlock_ = func;
    return *this;
}

The throwException method triggers the exception handling by calling longjmp with the appropriate error code:

void ExceptionHandler::throwException(int error) {
    longjmp(env_, error);
}

The Execute method coordinates the execution of the Try and Catch blocks, managing control flow based on whether an exception has been thrown:

• Setting up the jump environment.
• Executing the Try block.
• If an error occurs, executing the Catch block and passing the error code.
• Resetting the handler for the next pair of try-catch blocks.

void ExceptionHandler::Execute() {

    // Ensure the Try and Catch blocks are properly configured
    if (!isConfigured_)  return;
    
    // Set up the environment for potential exceptions
    int result = setjmp(env_);
    if (result == 0) {
        tryBlock_();
    } else {
        catchBlock_(result);
    }

    // Reset the handler state for future use
    reset();
}


void ExceptionHandler::reset() {
    tryBlock_     = nullptr;
    catchBlock_   = nullptr;
    isConfigured_ = false;
}

Here is an example of how this class might be used:

int main() {
    ExceptionHandler handler;

    handler.Try([&] {
        // Try block

        std::cout << "Try block: an error is about to occur.\n";

        // Trigger an exception
        handler.throwException(2);

        std::cout << "This will not be printed.\n";

    }).Catch([](int error) {
    
        // Handle the exception in the Catch block
        std::cout << "Catch block: caught error code " << error << "!\n";

    }).Execute();

    std::cout << "Program continues after try/catch.\n";

    return 0;
}

The output of this program would be:

>>>
Try block: an error is about to occur.
Catch block: An exception was caught with error code (2)!
Program continues after try/catch.

4. C++ Resources

One of the strengths of C++ is its ability to automatically invoke the destructor of a class when it goes out of scope, whether the scope ends normally (by }) or through an exception. This ensures that resources are freed properly, even when they are allocated within nested functions. This feature, known as stack unwinding, is absent when using the goto keyword. A critical question arises: do we retain this feature when using longjmp?

4.1. Cleaning Up with Longjmp

The answer is compiler-dependent. For instance, with the MSVC compiler, stack unwinding is supported with longjmp, as evidenced by the following statement: (see here)

In Microsoft C++ code on Windows, longjmp uses the same stack-unwinding semantics as exception-handling code. It’s safe to use in the same places that C++ exceptions can be raised.

However, this would be assuming exception semantics are allowed, which is not the case in embedded environments.

In GCC and ARM compilers, exception semantics are excluded from longjmp. To demonstrate this, consider a Resource class that tracks resource allocation and deallocation:

class Resource {
public:
    Resource(const std::string& name) : name_(name) {
        std::cout << name_ << " constructed!\n";
    }

    ~Resource() {
        std::cout << name_ << " destructed!\n";
    }

private:
    std::string name_;
};

Now, let’s use this class as follows:

// Global Exception Handler
ExceptionHandler handler;

int main() {

    handler.Try([&] {
        // Try block

        // This will not be destructed with throwException/longjmp
        Resource res("MyResource"); 

        std::cout << "Try block: an error is about to occur.\n";

        // Throw Exception
        handler.throwException(2);

        std::cout << "This will not be printed.\n";

    }).Catch([](int error) {

        // Catch an error
        std::cout << "Catch block: caught error code (" << error << ")!\n";

    }).Execute();

    std::cout << "Program continues after try/catch.\n";

    return 0;
}

The output would be:

>>>
MyResource constructed!
Try block: an error is about to occur.
Catch block: An exception was caught with error code (2)!
Program continues after try/catch.

As observed, the destructor for MyResource is not invoked, potentially leading to memory leaks—a serious concern in embedded systems.

4.2. Manual Stack Unwinding

To address the previous issue, we can manually manage resource cleanup by extending the ExceptionHandler class:

template <typename T>
void registerCleanup(T& resource)
{
    cleanupStack_.emplace_back([&resource](){
      resource.~T();
    });
}

The cleanupStack_ is a vector of destructor functions that are invoked when an exception is thrown:

std::vector<std::function<void()>> cleanupStack_;

When an exception is thrown, all registered variables are destructed:

void ExceptionHandler::throwException(int error)
{
    // Execute destructors first
    for (auto it = cleanupStack_.rbegin(); it != cleanupStack_.rend(); ++it) (*it)();
    cleanupStack_.clear();

    // Then perform long jump
    longjmp(env_, error);
}

This solution requires developers to manually register each local resource, as demonstrated below:

// Global Exception Handler
ExceptionHandler handler;

void riskyFunction()
{
    // Resource
    Resource localRes("ResourceInFunction");

    // Register the source before any exception might be thrown
    handler.registerCleanup(localRes);

    // Throw an exception
    handler.throwException(4);

    std::cout << "functionWithLocalResource: This line won't be executed if an exception is thrown.\n";
}

int main() {

    handler.Try([&] {
        // Try block

        // This will not be destructed with throwException/longjmp
        Resource res("MyResource"); 

        // Register the source before any exception might be thrown
        handler.registerCleanup(res);

        std::cout << "Try block: an error is about to occur.\n";

        // Call a risky function
        riskyFunction(handler);

        std::cout << "This will not be printed.\n";

    }).Catch([](int error) {

        // Catch an error
        std::cout << "Catch block: caught error code (" << error << ")!\n";

    }).Execute();

    std::cout << "Program continues after try/catch.\n";

    return 0;
}

The output now reflects proper resource cleanup:

>>>
MyResource constructed!
ResourceInFunction constructed!
ResourceInFunction destructed!
MyResource destructed!
Catch block: An exception was caught with error code (2)!
Program continues after try/catch.

Now, the destructors are invoked as expected. However, it is easy to overlook registering a local variable, which could lead to subtle bugs.

4.3. Considerations for Embedded Systems: Balancing Complexity and Efficiency

While setjmp and longjmp offer a powerful means of simulating exception handling in embedded systems, they also introduce significant complexity, particularly in terms of managing the cleanup stack. This complexity is exacerbated in resource-constrained environments, where developers must meticulously balance performance, memory usage, and error handling. The manual cleanup required by GCC and ARM compilers adds another layer of difficulty, emphasizing the importance of thorough testing and validation to ensure system reliability.

5. Alternative Approaches: Simplifying with goto

In some embedded systems, simplicity and efficiency may outweigh the benefits of structured exception handling. In such cases, using the goto statement can provide a straightforward and pragmatic error-handling mechanism, albeit at the cost of abstraction and safety.

#include <iostream>


int riskyFunction() {
    Resource localRes("ResourceInFunction");

    // Assume an error happened
    int errorCode = doSomething(); // returns errorCode 4

    // Check for an error
    if (errorCode != 0) {
        goto cleanup;
    }

    std::cout << "riskyFunction: This line won't be executed if an error occurs.\n";

cleanup:
    // Clean up resources if necessary
    localRes.release();

    // Return the error code
    return errorCode;
}


int main() {
    int errorCode = 0;
    Resource res("MyResource");

    std::cout << "Try block: an error is about to occur.\n";

    // Call the risky function and handle errors
    errorCode = riskyFunction();
    if (errorCode != 0) {
        goto cleanup;
    }

    std::cout << "This will not be printed if an error occurs.\n";

cleanup:
    // Clean up resources
    res.release();

    if (errorCode != 0) {
        std::cout << "Catch block: caught error code (" << errorCode << ")!\n";
    }

    std::cout << "Program continues after try/catch.\n";

    return errorCode;
}

6. Alternative Approaches: Modern C++

Beyond the low-level techniques previously discussed, modern C++ introduces several lightweight alternatives to traditional exceptions that are particularly advantageous in resource-constrained embedded systems. These alternatives, including std::optional, std::variant, and error codes encapsulated within classes, offer a structured and efficient approach to error handling. Their minimal overhead makes them well-suited for environments where performance and memory efficiency are paramount.

6.1. Utilizing std::optional for Error Handling

The std::optional type, introduced in C++17, encapsulates an object that may or may not hold a value. This type is especially beneficial for functions that might fail to produce a meaningful result, allowing the function to return an “empty” state rather than throwing an exception or returning an error code.

6.1.1. Example

#include <iostream>  
#include <optional>  
  
std::optional<int> riskyFunction(int value) { 
  
    // Return an empty optional to indicate an error  
    if (value < 0) return std::nullopt;  
  
    // Return the result as an optional  
    return value * 2;  
}  
  
int main() {  
    auto result = riskyFunction(-5);  
    
    // Handle the error
    if (!result) {  
       std::cout << "An error occurred: invalid input.\n";  
       exit(0);  // Early termination
    }  
  
    // Valid input, retrieve and display the value
    std::cout << "Result: " << result.value() << "\n";  
    
    return 0;  
}

In this example, the function riskyFunction returns an std::optional<int>, which either contains a valid result or std::nullopt if an error occurs. This approach avoids the overhead of exceptions while providing a clear mechanism for error checking.

6.2. Leveraging std::variant for Typed Errors

std::variant, also introduced in C++17, is a type-safe union that can store one of several types, making it a versatile tool for returning either a result or an error type from a function. This is analogous to using tagged unions in C but with the added safety and flexibility offered by modern C++.

6.2.1. Example for variant

#include <iostream>
#include <variant>
#include <string>

// Value Error Type
struct ValueError {  
    std::string message;  
};  
  
// Syntax Error Type
struct SyntaxError {  
    std::string message;  
};  

// Result type can be an integer, a value error, or a syntax error
using Result = std::variant<int, ValueError, SyntaxError>;  
  
Result riskyFunction(int value) {  

    // Return a ValueError if the input is negative
    if (value < 0)  return ValueError{"Value cannot be negative"}; 
     
    // Return a SyntaxError if the input is zero
    if (value == 0) return SyntaxError{"Value cannot be zero"};  

    // Return the value
    return value * 2;  
}  
  
int main() {  
    auto result = riskyFunction(0); 
     
    // Check for a ValueError
    if (std::holds_alternative<ValueError>(result)) {
       std::cout << "Value error: " << std::get<ValueError>(result).message << "\n";  
       exit(0);
    }
    
    // Check for a SyntaxError
    if (std::holds_alternative<SyntaxError>(result)) {
       std::cout << "Syntax error: " << std::get<SyntaxError>(result).message << "\n";  
       exit(0);
    }

    // Otherwise, an integer result is assumed
    std::cout << "Result: " << std::get<int>(result) << "\n";  
  
    return 0;  
}

In this instance, riskyFunction returns a std::variant<int, ValueError, SyntaxError>, allowing it to return either a valid result or detailed error information. This approach increases the granularity of error handling while avoiding the performance drawbacks associated with exceptions.

6.3. Encapsulating Error Codes within Classes

Another modern C++ approach involves encapsulating error codes within a class structure. This method enhances code readability and maintainability by associating specific error codes with meaningful class types, as opposed to relying on plain integers.

We will first define a basic ErrorCode class to encapsulate error types and messages, then construct a Result class template that encapsulates either a value or an error. Finally, we will demonstrate how to utilize these classes in a function that performs a potentially risky operation and returns either a result or an error.

6.3.1. Defining the ErrorCode Class

The initial step in constructing our error-handling framework involves defining an ErrorCode class. This class will represent various types of errors that may occur and will provide a mechanism to retrieve an error message associated with each error type.

Here is the basic structure of the ErrorCode class:

// ErrorCode class to represent possible errors
class ErrorCode {
 public:
    // Enum to define different types of error codes
    enum Code { None, ValueError, SyntaxError };

    // Constructor takes an optional error code and an error message
    explicit ErrorCode(Code code = None, std::string message = "")
        : code_(code), message_(std::move(message)) {}

    // Getter for the error message
    std::string message() const { return message_; }

    // Getter for the error code
    Code code() const { return code_; }

 private:
    Code code_;                ///< error code / type
    std::string message_;      ///< error message
};

In this structure:

• Enum Code: Enumerates different error types (None, ValueError, SyntaxError).
• Constructor: Initializes the error code and its associated message.
• Getters: Provide access to the error code and message.

This class forms the foundation for representing errors within our application.

6.3.2. Creating the Result Class Template

Next, we need a way to return either a valid result or an error from a function. To achieve this, we create a Result class template that can hold either a value of any type T or an ErrorCode.

The Result class is defined as a template, allowing it to accommodate any data type as the potential result of an operation. This flexibility is crucial for creating a general-purpose error-handling mechanism.

template <typename T>
class Result {
    // Class members and methods will be defined within this template.
};

By templating the class, we enable Result to handle various types of results, making it adaptable to different contexts within an application.

The Result class is designed to accommodate two primary scenarios:

1. The operation succeeds and returns a value, or
2. The operation fails and returns an error.

To facilitate these scenarios, the class provides two constructors.

public:
    // Constructor for a successful result containing a value
    explicit Result(T value) : result_(std::move(value)) {}

    // Constructor for an error result containing an ErrorCode
    Result(ErrorCode error) : result_(std::move(error)) {}

• Success Constructor: This constructor initializes the Result object with a value of type T. The use of std::move ensures efficient handling of the value, particularly when dealing with large or complex objects.

• Error Constructor: This constructor initializes the Result object with an ErrorCode, signifying that the operation encountered an issue.

These constructors are fundamental to ensuring that the Result object can accurately represent either a successful outcome or an error, providing a clear and structured approach to error handling.

To enable the Result object to hold either a value or an error, the class employs std::variant. This C++ standard library feature ensures that the object can store one of these types at a time, but not both simultaneously.

private:
    std::variant<T, ErrorCode> result_; ///< Holds either a value of type T or an ErrorCode

The std::variant type is integral to the design of the Result class. It enforces type safety by ensuring that the object contains only a value or an error at any given moment, thus preventing ambiguous states.

A common requirement when dealing with the outcome of an operation is the ability to easily determine whether it succeeded or failed. The Result class provides an overloaded bool operator to facilitate this check.

public:
    // Overloaded bool operator to check if the result contains a value (indicating success)
    explicit operator bool() const {
        return std::holds_alternative<T>(result_);
    }

• Boolean Operator: This operator checks whether the Result contains a value of type T. If it does, the operator returns true, indicating success; otherwise, it returns false.

This feature enhances the usability of the Result class by allowing developers to intuitively and efficiently check the success of an operation.

To access the value or error contained within the Result object, the class provides two methods: value() and error(). These methods return std::optional types, which offer a safe way to handle cases where the requested data might not be present.

public:
    // Retrieve the value if no error occurred, returning std::optional to handle the absence of a value
    std::optional<T> value() const {
        if (std::holds_alternative<T>(result_)) {
            return std::get<T>(result_);
        }
        return std::nullopt;
    }

    // Retrieve the error if one occurred, returning std::optional to handle the absence of an error
    std::optional<ErrorCode> error() const {
        if (std::holds_alternative<ErrorCode>(result_)) {
            return std::get<ErrorCode>(result_);
        }
        return std::nullopt;
    }

• value() Method: This method attempts to retrieve the stored value. If the Result contains a value, it is returned; if the Result contains an error, the method returns std::nullopt, indicating the absence of a value.

• error() Method: Similarly, this method retrieves the stored error if one is present. If the Result contains a value instead, it returns std::nullopt, signifying the absence of an error.

These methods provide a clear and safe mechanism for extracting the contents of the Result object, ensuring that error handling is both explicit and reliable.

The Result class template offers a comprehensive and type-safe mechanism for managing the outcomes of operations that may either succeed or fail. By encapsulating both the value and the error within a single object, it presents a clean and efficient alternative to traditional error-handling strategies, making it particularly suitable for environments where reliability and performance are critical.

6.3.3. Implementing the Result Class in a Function

With the ErrorCode and Result classes defined, we can now use them in a function that performs a potentially risky operation. The function will return a Result<int> to indicate either a successful operation (yielding a result) or an error.

Here’s an example of such a function:

Result<int> riskyFunction(int value) {

    // If the input value is negative, return a ValueError
    if (value < 0) {
        return ErrorCode(ErrorCode::ValueError, "Value cannot be negative");
    }

    // If the input value is zero, return a SyntaxError
    if (value == 0) {
        return ErrorCode(ErrorCode::SyntaxError, "Value cannot be zero");
    }

    // Input is correct
    return Result<int>(value * 2); // Return the result on success
}

where

• Negative Value: Returns an ErrorCode for a ValueError.
• Zero Value: Returns an ErrorCode for a SyntaxError.
• Valid Value: Returns the computed result (value * 2).

This function demonstrates how to use the Result and ErrorCode classes to handle errors in a structured manner.

6.3.4. Managing the Result in the Main Function

Finally, we can handle the result of riskyFunction in the main function:

int main() {
    // Call riskyFunction with an input value of 5
    auto result = riskyFunction(5);

    // Check if the result contains an error
    if (!result) {
        // Retrieve the error from the result
        auto error = result.error();

        // Check if the error is a ValueError and print the error message
        if (error && error->code() == ErrorCode::ValueError) {
            std::cout << "Value Error: " << error->message() << "\n";
            exit(0); // or return
        }

        // Check if the error is a SyntaxError and print the error message
        if (error && error->code() == ErrorCode::SyntaxError) {
            std::cout << "Syntax Error: " << error->message() << "\n";
            exit(0); // or return
        }
    } 

    // print the successful result value
    std::cout << "Result: " << *result.value() << "\n";

    return 0;
}

where

• Check for Error: The if (!result) block checks if an error occurred.
• Handle ValueError: If the error is a ValueError, print the message and exit.
• Handle SyntaxError: If the error is a SyntaxError, print the message and exit.
• Print Result: If no error occurred, print the result.

This approach is particularly advantageous in environments where exceptions are disabled or in performance-critical applications where the overhead of exceptions is undesirable. The combination of std::variant, std::optional, and custom error codes provides a robust alternative that enhances both code readability and maintainability.

7. Conclusion

In the highly specialized field of embedded systems development, where resources are tightly constrained, the use of traditional C++ exceptions is often impractical. By adopting alternative techniques such as low-level goto and function pointers, or more sophisticated methods like setjmp and longjmp, developers can implement effective error-handling mechanisms with minimal overhead. Each approach has its advantages and trade-offs, which must be carefully evaluated based on the specific constraints and requirements of the application at hand.

For a quick recap, here is a comparison between discussed methods:

Method	Advantages	Disadvantages
goto and Function Pointers	- Extremely low overhead, close to the hardware level - Simple to implement in small, constrained environments	- Prone to errors and difficult to maintain - Lacks flexibility and scalability - No automatic cleanup
setjmp and longjmp	- Provides a mechanism for non-local jumps and structured error handling without exceptions - Suitable for more complex control flows	- Requires manual resource management (no stack unwinding) - Can be complex to implement correctly
Typed Exceptions with longjmp	- Allows handling different types of errors distinctly - Offers more granularity in error management	- Complexity in implementation - Manual resource management required
Exception Cascades with longjmp	- Enables complex control flow and exception propagation - Flexible error-handling strategy	- Even higher complexity - Difficult to debug and maintain - Requires thorough testing and validation
Custom Exception Handler	- Mimics higher-level try/catch behavior in embedded C/C++ without runtime overhead - More structured and organized error handling	- Increased complexity - Requires manual resource registration and cleanup - Potential for missed cleanup
goto for Cleanup	- Simple and straightforward for resource cleanup - Simplifies resource cleanup operations in basic scenarios	- Lacks abstraction and safety - Can lead to spaghetti code and is unsuitable for complex error handling
Raw std::optional and std::variant	- Modern C++ features with low overhead, compatible with resource-constrained environments - Type-safe and integrates well with existing codebases	- Limited expressiveness compared to full exception handling - Still requires manual checks and error handling
Result Class Template	- Encapsulates error handling in a type-safe manner - Provides clear and maintainable code structure - Avoids the runtime overhead of exceptions	- Increased code complexity - May be over-engineered for simple error-handling cases

In conclusion, there is no one-size-fits-all solution for error handling in embedded systems. The best approach often involves a careful balance between simplicity, efficiency, and flexibility, tailored to the unique challenges of the target environment. By leveraging the techniques discussed in this article, developers can ensure that their embedded systems remain resilient, even in the face of unexpected errors.