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, many mistakes in C++ are not caught at compile time and only surface at runtime — making them difficult to debug and sometimes catastrophic.

Segmentation Faults

A segmentation fault occurs when your program tries to access memory it is not allowed to access — one of the most frequent runtime errors in C++.

int* p = nullptr;
*p = 5;  // Segfault: dereferencing null pointer

Common causes:

• Dereferencing null or uninitialized pointers
• Accessing arrays or vectors out of bounds
• Using memory after it has been freed (use-after-free)

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];  // UB: out-of-bounds access

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: integer overflow, invalid pointer arithmetic, double deletion, uninitialized variables.

Memory Leaks

A memory leak occurs when dynamically allocated memory is never released. Memory usage grows over time, degrading performance or crashing long-running applications.

int* p = new int(5);
// forgot delete p; — memory is leaked

Especially dangerous in servers, games, and embedded systems that run continuously.

Unhandled Exceptions

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

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

How to Avoid Runtime Hell

Modern C++ provides tools and patterns that significantly reduce runtime errors:

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

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.