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Concept
Type punning
Résumé
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 union — C++ adds reference type conversion and reinterpret_cast 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.
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:
int bind(int sockfd, struct sockaddr *my_addr, socklen_t addrlen);
The bind function is usually called as follows:
struct sockaddr_in sa = {0};
int sockfd = ...;
sa.sin_family = AF_INET;
sa.sin_port = htons(port);
bind(sockfd, (struct sockaddr )&sa, sizeof sa);
The Berkeley sockets library fundamentally relies on the fact that in C, a pointer to struct sockaddr_in is freely convertible to a pointer to struct sockaddr; and, in addition, that the two structure types share the same memory layout. Therefore, a reference to the structure field my_addr->sin_family (where my_addr is of type struct sockaddr) will actually refer to the field sa.sin_family (where sa is of type struct sockaddr_in). 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.
Not all examples of type punning involve structures, as the previous example did.
Source officielle
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