Void Pointers in C and C : Why are Implicit Conversions Different?

Understanding Void Pointers: Differences Between C and C

Void pointers are a key aspect of both C and C , but there are notable differences in their handling. This article explores these differences and explains why seemingly contradictory behaviors can occur.

Implicit Conversions in C

In C, conversions between a pointer to a specific type (e.g., int) and a void pointer (void) are always implicit. This means that the following statement is valid:

<code class="c">int* p = malloc(sizeof(int));</code>

The malloc function returns a void*, which is then assigned to the int* pointer. However, in C , such implicit conversions are not permitted.

Explicit Casting in C

In C , converting from a pointer to a void pointer is still implicit, but converting from a void pointer to a specific type requires an explicit cast. This explains why the following code compiles in both C and C :

<code class="c">void foo(void* vptr)
{
}

int main()
{
    int* p = (int*) malloc(sizeof(int));
    foo(p);
    return 0;
}</code>

The malloc function returns a void*, which is passed to the foo function. Even though the int* pointer is assigned to the void* parameter, C allows this implicit conversion. However, the subsequent call to p requires an explicit cast to the int* type.

The Standard's Perspective

According to the K&R2 specification, any pointer to an object can be safely converted to void* without information loss. Furthermore, converting back to the original pointer type restores the original value. In C , the standard specifies that implicit conversions from any pointer type to void* are allowed, but explicit casting is necessary for conversions from void* to any other type.

Conclusion

The key difference between C and C void pointer handling lies in the requirement for explicit casting when converting from void* to a specific type in C . This stricter approach helps prevent undefined behavior and maintains type safety in C code.

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