Common Runtime Errors in C++ (and How to Avoid Them)

C++ is a powerful systems programming language that gives developers fine-grained control over memory, performance, and hardware. However, this power comes with responsibility. Unlike higher-level languages, many mistakes in C++ are not caught at compile time and only surface when the program is running.

These runtime errors can be difficult to debug, unpredictable, and sometimes catastrophic. In this article, we explore the most common runtime errors in C++, why they occur, and how modern C++ practices can help you avoid them.

Segmentation Faults

A segmentation fault occurs when your program tries to access memory that it is not allowed to access. This is one of the most frequent runtime errors encountered by C++ developers, especially beginners.

int* p = nullptr;
*p = 5;

In the example above, the pointer p does not point to a valid memory location. Dereferencing it leads to undefined behavior and usually results in a segmentation fault.

Dereferencing null or uninitialized pointers
Accessing arrays or vectors out of bounds
Using memory after it has been freed

Undefined Behavior

Undefined behavior (UB) is a category of errors where the C++ standard imposes no requirements on what should happen. The program may crash, appear to work, or behave inconsistently across systems.

int arr[10];
int x = arr[100];

Accessing memory beyond the bounds of an array is undefined behavior. Compilers are allowed to assume that undefined behavior never occurs, which can lead to aggressive optimizations that make bugs extremely hard to trace.

Common sources of undefined behavior include integer overflow, invalid pointer arithmetic, double deletion, and accessing uninitialized variables.

Memory Leaks

A memory leak occurs when dynamically allocated memory is never released. While the program may continue running, memory usage grows over time, which can degrade performance or crash long-running applications.

int* p = new int(5);
// no delete

In this case, the allocated memory is never freed. Repeating this pattern in loops or large applications leads to serious memory issues.

Memory leaks are especially dangerous in servers, games, and embedded systems where applications are expected to run continuously.

Exceptions

C++ uses exceptions to signal error conditions at runtime. If exceptions are not properly handled, they can cause abrupt program termination.

std::out_of_range — accessing containers with invalid indices
std::bad_alloc — failure to allocate memory

Failing to catch exceptions or relying on unchecked operations can result in unexpected crashes. Proper exception handling improves robustness and reliability.

How to Avoid Runtime Hell

Modern C++ provides tools and patterns that significantly reduce runtime errors. Adopting these practices can make your code safer, cleaner, and easier to maintain.

RAII (Resource Acquisition Is Initialization) for automatic resource management
Smart pointers like std::unique_ptr and std::shared_ptr
Sanitizers such as AddressSanitizer and UndefinedBehaviorSanitizer
Modern C++ containers instead of raw arrays

Most runtime bugs disappear when you stop using raw pointers and let the language manage lifetimes for you.

Conclusion

C++ does not aim to protect you from every mistake — it empowers you to write efficient, low-level code when you need it. Understanding runtime errors and their root causes is a key milestone in becoming a proficient C++ developer.

By following modern C++ practices and developing a strong mental model of memory and object lifetimes, you can avoid most runtime failures and write code that is both powerful and reliable.