Hello! Today's expedition is about understanding C++'s error messages. We'll examine how these error messages work, their structure, and the most common types that we're likely to encounter. Let's get started!

C++ handles errors using exceptions and error codes. Exceptions are a way to signal that an error has occurred, allowing the program to handle the error gracefully or terminate the program if necessary. Error codes, on the other hand, are returned by functions to indicate that something went wrong. We'll focus primarily on the error messages generated by the compiler and runtime exceptions in C++.

C++ error messages typically comprise the following components generated by compilers like g++:

1. Description: This string describes what went wrong.
2. Location: Indicates where the error occurred in the code, specifying the file name and line number.

To illustrate, let's consider this code error:

The code:

C++
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1#include <iostream>
2int main() {
3    std::cout << "Hello, World!"
4}

The error:

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1example.cpp: In function ‘int main()’:
2example.cpp:3:32: error: expected ‘;’ before ‘}’ token
3    3 |     std::cout << "Hello, World!"
4      |                                ^
5      |                                ;
6    4 | }

Although the error message might initially seem intimidating, it clearly indicates that a semicolon is missing before the closing brace.

For this error:

• Description is expected ‘;’ before ‘}’ token
• Location is example.cpp:3:32

Every error message provides these details to help you understand and locate the issue.

Syntax errors occur when the code violates C++'s language rules, preventing the compiler from parsing and, thus, compiling the code. These errors are usually caught at compile time. Examples include:

• Missing Brackets: Forgetting to close a bracket or parenthesis can lead to a syntax error.

  C++
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  1#include <iostream>
  2int main() {
  3    std::cout << "Hello, C++!";

  This raises:

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  1example.cpp: In function ‘int main()’:
  2example.cpp:4:5: error: expected ‘}’ at end of input

• Misplaced Keywords: Using keywords in the wrong context can also cause syntax errors.

  C++
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  1int main() {
  2    int class = 5; // Incorrect use of "class" as a variable name
  3}

  This would typically raise:

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  1example.cpp: In function ‘int main()’:
  2example.cpp:2:9: error: expected primary-expression before ‘int’

Logical errors happen when the code does not perform as intended or produces unexpected results. These errors are not caught at compile time because the code is syntactically correct. Let's see examples:

C++
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1#include <iostream>
2int main() {
3    for (int i = 0; i < 10; i--) {
4        std::cout << i << std::endl;
5    }
6    return 0;
7}

Although it compiles, the loop runs infinitely because i decreases instead of increasing.

Using a variable that is different from the intended one can cause logical errors.

C++
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1#include <iostream>
2int main() {
3    int total = 10;
4    int count = 2;
5    int average = total / count;
6    std::cout << "Average: " << total << std::endl; // Wrong variable used here
7    return 0;
8}

This prints the total instead of the average.

Trying to access an element outside the array's bounds can lead to unexpected behavior. Unlike many other languages, C++ won't throw an exception in this situation. Take a look at this code:

C++
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1#include <iostream>
2int main() {
3    int arr[5] = {1, 2, 3, 4, 5};
4    std::cout << arr[5] << std::endl; // Trying to access an element out of bounds
5    return 0;
6}

This may compile successfully but will result in undefined behavior. Arrays in C++ are zero-indexed, so valid indices for an array of size 5 are 0 to 4. Accessing arr[5] reads a memory location beyond the array, leading to unpredictable results or even a segmentation fault if the memory is inaccessible. Most probably, in the codesignal environment, it will simply print out 0, but there is no way to predict for sure.

Runtime errors occur during the execution of a program, after it has successfully compiled. These errors are typically harder to track since they occur under specific conditions.

In C++, a std::runtime_error is a common exception thrown due to an unrecoverable issue, such as division by zero or a null pointer dereference. When a runtime error occurs, the program typically terminates with an error message.

• Division by Zero: Attempting to divide a number by zero.

C++
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1#include <iostream>
2int main() {
3    int numerator = 10;
4    int denominator = 0;
5    int result = numerator / denominator; // Division by zero
6    std::cout << "Result: " << result << std::endl;
7    return 0;
8}

This causes a runtime error that typically results in the program terminating with a divide by zero exception.

• Null Pointer Dereference: Attempting to access a member of a null pointer.

C++
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1#include <iostream>
2int main() {
3    int* ptr = nullptr; // Declaring a null pointer
4    std::cout << *ptr << std::endl; // This will cause a runtime error
5    return 0;
6}

This would likely cause a segmentation fault due to null pointer dereference.

Congratulations! In this lesson, we've broadened our understanding of C++ error messages by categorizing them into syntax errors, logical errors, and runtime errors. You've seen examples of each and should now have a better understanding of the types of errors you may encounter when coding in C++. Identifying the type of error quickly is crucial in debugging efficiently. As you practice and encounter these errors firsthand, you'll develop a sharper instinct for resolving them. Next, we'll move on to hands-on exercises where you'll get to tackle these error types in real C++ code scenarios. Happy debugging!