C++ vs C

C++ began as “C with Classes” and stays mostly source-compatible with C, but adds type safety, RAII, generics, and exceptions — and is not a strict superset.

Why it matters

You constantly cross this boundary: calling C libraries (POSIX, OpenSSL, SQLite) from C++, or exposing a C ABI from a C++ library so other languages can bind to it. Knowing exactly where the languages diverge prevents linker errors, silent ABI mismatches, and double-frees. The stable, flat C ABI is also why C — not C++ — is the lingua franca for FFI.

How it works

C++ adds whole subsystems C lacks: classes and inheritance, templates, exceptions, namespaces, references, operator-overloading, and the STL. It also tightens existing C rules.

• Not a superset. Valid C that breaks in C++: int new; (keyword), implicit void*→T* conversion (malloc needs a cast in C++), and tentative definitions.
• Linkage. C++ name-mangles symbols to encode types for overloading; to call/expose C symbols you wrap them in extern "C" { ... } so the names stay flat.
• Idiom shift. C uses malloc/free and manual cleanup with goto fail; C++ uses raii + smart-pointers-unique-ptr-shared-ptr-weak-ptr so destructors free resources automatically — no leaks on early return or exception.

Concern	C	C++
Allocation	malloc / free	new / delete, RAII
Polymorphism	function pointers	virtual functions
Generics	macros / void*	templates
Error path	return codes / errno	exceptions or codes
Symbol names	flat	mangled

Example

// Make a C++ function callable from C (no mangling):
extern "C" int add(int a, int b) { return a + b; }
 
// Consume a C header from C++:
extern "C" {
  #include <sqlite3.h>   // its symbols keep C linkage
}

Without extern "C", the linker emits _Z3addii and the C side’s reference to add goes unresolved.

Pitfalls

• Assuming superset semantics — sizeof('a') is sizeof(int) in C but sizeof(char) (1) in C++; char literals differ.
• Mixing malloc with delete (or new with free) is UB; never cross allocators, and never free a non-trivially-destructible object.
• Throwing a C++ exception across a C stack frame is UB — the C frames don’t know how to unwind.
• One extern "C" function can’t be overloaded — C linkage has no mangling to disambiguate.

See also

• raii
• smart-pointers-unique-ptr-shared-ptr-weak-ptr