When a pointer to a particular type (say int, char, float, ..) is incremented, its value is increased by the size of that data type. If a void pointer which points to data of size x is incremented, how does it get to point x bytes ahead? How does the compiler know to add x to value of the pointer?
9 Answers
Final conclusion: arithmetic on a void* is illegal in both C and C++.
GCC allows it as an extension, see Arithmetic on void- and Function-Pointers (note that this section is part of the "C Extensions" chapter of the manual). Clang and ICC likely allow void* arithmetic for the purposes of compatibility with GCC. Other compilers (such as MSVC) disallow arithmetic on void*, and GCC disallows it if the -pedantic-errors flag is specified, or if the -Werror-pointer-arith flag is specified (this flag is useful if your code base must also compile with MSVC).
The C Standard Speaks
Quotes are taken from the n1256 draft.
The standard's description of the addition operation states:
6.5.6-2: For addition, either both operands shall have arithmetic type, or one operand shall be a pointer to an object type and the other shall have integer type.
So, the question here is whether void* is a pointer to an "object type", or equivalently, whether void is an "object type". The definition for "object type" is:
6.2.5.1: Types are partitioned into object types (types that fully describe objects) , function types (types that describe functions), and incomplete types (types that describe objects but lack information needed to determine their sizes).
And the standard defines void as:
6.2.5-19: The
voidtype comprises an empty set of values; it is an incomplete type that cannot be completed.
Since void is an incomplete type, it is not an object type. Therefore it is not a valid operand to an addition operation.
Therefore you cannot perform pointer arithmetic on a void pointer.
