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Using type definitions (typedef) can often improve code readability.  However, type definitions to pointer types can make it more difficult to write const-correct code because the const qualifier will be applied to the pointer type, not to the underlying declared type.

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The following type definition improves readability at the expense of introducing a const-correctness issue.   In this example, the const qualifier applies to the typedef itself instead of applying to the underlying object type.   So Consequently, func does not take a pointer to a const struct obj, but is instead taking takes a const pointer to a struct obj.

struct obj {
  int i;
  float f;
};
typedef struct obj *ObjectPtr;
 
void func(const ObjectPtr o) {
  /* Can actually modify o's contents, against expectations. */
}

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This compliant solution makes use of type definitions , but does not declare a pointer type and so cannot be used in a const-incorrect manner.:

struct obj {
  int i;
  float f;
};
typedef struct obj Object;
 
void func(const Object *o) {
  /* Cannot modify o's contents. */
}

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The Win32 SDK headers make use of type definitions for most of the types involved in Win32 APIs, but the following noncompliant solution demonstrates a const-correctness bug.:

#include <Windows.h>
/* typedef char *LPSTR; */
 
void func(const LPSTR str) {
  /* Can mutate str's contents, against expectations. */
}

Compliant Solution (Windows)

The This compliant solution demonstrates a common naming convention found in the Win32 APIs, using the proper const type.:

#include <Windows.h>
/* typedef const char *LPCSTR; */
 
void func(LPCSTR str) {
  /* Cannot modify str's contents. */
}

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Note that many structures in the Win32 API are declared with pointer type definitions , but not pointer-to const  type -const  type definitions (LPPOINT, LPSIZE, et al.).   In these cases, it is suggested to that you create your own type definition from the base structure type.

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The following declaration of the signal() function is difficult to read and comprehend.:

void (*signal(int, void (*)(int)))(int);

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This compliant solution makes use of type definitions to specify the same type as in the noncompliant code example.:

typedef void SighandlerType(int signum);
extern SighandlerType *signal(
  int signum,
  SighandlerType *handler
);

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Related Vulnerabilities

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