Type punning

In computer science, a type punning is any programming technique that subverts or circumvents the type system of a programming language in order to achieve an effect that would be difficult or impossible to achieve within the bounds of the formal language.

In C and C++, constructs such as pointer type conversion and  — C++ adds reference type conversion and   to this list — are provided in order to permit many kinds of type punning, although some kinds are not actually supported by the standard language.

In the Pascal programming language, the use of records with variants may be used to treat a particular data type in more than one manner, or in a manner not normally permitted.

Sockets example
One classic example of type punning is found in the Berkeley sockets interface. The function to bind an opened but uninitialized socket to an IP address is declared as follows:

The  function is usually called as follows:

The Berkeley sockets library fundamentally relies on the fact that in C, a pointer to  is freely convertible to a pointer to  ; and, in addition, that the two structure types share the same memory layout. Therefore, a reference to the structure field  (where   is of type  ) will actually refer to the field   (where   is of type  ). In other words, the sockets library uses type punning to implement a rudimentary form of polymorphism or inheritance.

Often seen in the programming world is the use of "padded" data structures to allow for the storage of different kinds of values in what is effectively the same storage space. This is often seen when two structures are used in mutual exclusivity for optimization.

Floating-point example
Not all examples of type punning involve structures, as the previous example did. Suppose we want to determine whether a floating-point number is negative. We could write:

However, supposing that floating-point comparisons are expensive, and also supposing that  is represented according to the IEEE floating-point standard, and integers are 32 bits wide, we could engage in type punning to extract the sign bit of the floating-point number using only integer operations:

Note that the behaviour will not be exactly the same: in the special case of  being negative zero, the first implementation yields   while the second yields. Also, the first implementation will return  for any NaN value, but the latter might return   for NaN values with the sign bit set.

This kind of type punning is more dangerous than most. Whereas the former example relied only on guarantees made by the C programming language about structure layout and pointer convertibility, the latter example relies on assumptions about a particular system's hardware. Some situations, such as time-critical code that the compiler otherwise fails to optimize, may require dangerous code. In these cases, documenting all such assumptions in comments, and introducing static assertions to verify portability expectations, helps to keep the code maintainable.

Practical examples of floating-point punning include fast inverse square root popularized by Quake III, fast FP comparison as integers, and finding neighboring values by incrementing as an integer (implementing nextafter).

C and C++
In addition to the assumption about bit-representation of floating-point numbers, the above floating-point type-punning example also violates the C language's constraints on how objects are accessed: the declared type of  is   but it is read through an expression of type. On many common platforms, this use of pointer punning can create problems if different pointers are aligned in machine-specific ways. Furthermore, pointers of different sizes can alias accesses to the same memory, causing problems that are unchecked by the compiler. Even when data size and pointer representation match, however, compilers can rely on the non-aliasing constraints to perform optimizations that would be unsafe in the presence of disallowed aliasing.

Use of pointers
A naive attempt at type-punning can be achieved by using pointers: (The following running example assumes IEEE-754 bit-representation for type .)

The C standard's aliasing rules state that an object shall have its stored value accessed only by an lvalue expression of a compatible type. The types  and   are not compatible, therefore this code's behavior is undefined. Although on GCC and LLVM this particular program compiles and runs as expected, more complicated examples may interact with assumptions made by strict aliasing and lead to unwanted behavior. The option  will ensure correct behavior of code using this form of type-punning, although using other forms of type punning is recommended.

Use of
In C, but not in C++, it is sometimes possible to perform type punning via a.

Accessing  after most recently writing to the other member, , is an allowed form of type-punning in C, provided that the member read is not larger than the one whose value was set (otherwise the read has unspecified behavior ). The same is syntactically valid but has undefined behavior in C++, however, where only the last-written member of a  is considered to have any value at all.

For another example of type punning, see Stride of an array.

Use of
In C++20, the  function allows type punning with no undefined behavior. It also allows the function be labeled.

Pascal
A variant record permits treating a data type as multiple kinds of data depending on which variant is being referenced. In the following example, integer is presumed to be 16 bit, while longint and real are presumed to be 32, while character is presumed to be 8 bit: In Pascal, copying a real to an integer converts it to the truncated value. This method would translate the binary value of the floating-point number into whatever it is as a long integer (32 bit), which will not be the same and may be incompatible with the long integer value on some systems.

These examples could be used to create strange conversions, although, in some cases, there may be legitimate uses for these types of constructs, such as for determining locations of particular pieces of data. In the following example a pointer and a longint are both presumed to be 32 bit: Where "new" is the standard routine in Pascal for allocating memory for a pointer, and "hex" is presumably a routine to print the hexadecimal string describing the value of an integer. This would allow the display of the address of a pointer, something which is not normally permitted. (Pointers cannot be read or written, only assigned.) Assigning a value to an integer variant of a pointer would allow examining or writing to any location in system memory: This construct may cause a program check or protection violation if address 0 is protected against reading on the machine the program is running upon or the operating system it is running under.

The reinterpret cast technique from C/C++ also works in Pascal. This can be useful, when eg. reading dwords from a byte stream, and we want to treat them as float. Here is a working example, where we reinterpret-cast a dword to a float:

C#
In C# (and other .NET languages), type punning is a little harder to achieve because of the type system, but can be done nonetheless, using pointers or struct unions.

Pointers
C# only allows pointers to so-called native types, i.e. any primitive type (except ), enum, array or struct that is composed only of other native types. Note that pointers are only allowed in code blocks marked 'unsafe'.

Struct unions
Struct unions are allowed without any notion of 'unsafe' code, but they do require the definition of a new type.

Raw CIL code
Raw CIL can be used instead of C#, because it doesn't have most of the type limitations. This allows one to, for example, combine two enum values of a generic type:

This can be circumvented by the following CIL code:

The  CIL opcode allows for some other tricks, such as converting a struct to a byte array: