c / expert
Snippet
Encapsulation via Opaque Pointers
Opaque pointers (or 'PIMPL' in C) hide the implementation details of a struct from the user. By only providing a forward declaration in the header, the compiler prevents users from accessing internal fields or assuming the struct's size, ensuring a stable binary interface (ABI).
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// In public header: session.htypedef struct Session Session;Session* session_create(int id);void session_destroy(Session* s);// In private implementation: session.c#include <stdlib.h>struct Session {int id;char internal_key[32];};Session* session_create(int id) {Session* s = malloc(sizeof(struct Session));if (s) s->id = id;return s;}void session_destroy(Session* s) { free(s); }
Breakdown
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typedef struct Session Session;
Creates an alias for an incomplete struct type, usable only via pointers in external code.
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struct Session { ... };
The full definition is hidden in the .c file, preventing direct access to 'internal_key' by library users.
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Session* s = malloc(sizeof(struct Session));
The library manages memory allocation because only the library knows the actual size of the struct.