Internally Merged Domains in .NET Incoporate Data Matrix barcode in .NET Internally Merged Domains

Internally Merged Domains use .net framework datamatrix 2d barcode generation toattach 2d data matrix barcode in .net datamatrix Some language .NET Data Matrix s maintain no semantic distinction among domains that have the same internal representation. For example, in FORTH and C, integers, truth values, and characters are represented identically.

In these languages, an integer is a truth value is a character, and a truth value or a character is an integer. A value belonging to any one of these external domains can be used in a context appropriate for any other one. An integer operation may be applied at will to a truth value or a character.

No conversion process is needed to go between these domains. This is one of the very convenient aspects of C: the translator lets the programmer decide whether it is meaningful to use a truth value as if it were an integer, and thus does not prevent the programmer from exploiting an implicit relationship between domains [Exhibit 15.10].

Of course, one could also claim that this kind of code is obscure and ought to be well documented if it is written at all. In fact, C and FORTH programmers use these tricks frequently, and the very commonness of such code reduces the di culty of understanding it. A more serious cost associated with internally merged domains is that the translator has no way of knowing which objects belong to which external domain, and so cannot help the programmer avoid unintended and meaningless operations.

This is illustrated by the very strange C code in Exhibit 15.11. Let us trace the execution of this odd expression: The ASCII codes for a and b are compared.

a is not greater than b , so the answer is FALSE, which is represented by 0 (4 bytes). The character c (one byte) is interpreted as an integer. Its ASCII code, 99, must be lengthened to 4 bytes to match the length of the integer 5.

. 450 Exhibit 1 5.11. Many misinterpretations!.

CHAPTER 15. THE SEMANTICS OF TYPES The / and t he + are nonsensical operations. They would be agged as errors in Pascal, but they are accepted in C because integers, characters, and truth values belong to one internally merged domain..

if ( ( a &g Visual Studio .NET Data Matrix barcode t; b ) / ( 5 + c ) ) x=1; else t=1;. integer 63 FALSE = 0 0 = FALSE FALSE selects the else clause. 6 8 Result is that t becomes 1. We add 5 to 99 giving 104 (4 bytes). The truth value, 0, from item (1) is interpreted as an integer and divided by 104, giving 0 (4 bytes). This integer 0 is now interpreted as the truth value FALSE, and used in the if test, selecting the else clause.

Finally, we store 1 in the variable t. This example illustrates the variety of ways that C values can be interpreted, but it is so arti cial that it does not provide a convincing example of the cost of internally merged domains. However, this kind of domain merging does frequently lead to errors, especially for programmers who commonly use more than one language.

Exhibit 15.12 illustrates a common and galling error that most former Pascal programmers make when they begin to write code in C. As a general rule, the language that has distinct internal domains representing distinct external domains is easier to learn and easier to use.

A compiler with full type checking is a powerful ally in the battle to debug a large program. The price paid for this assistance is that the programmer must explicitly indicate domain conversions..

Domain Mapping An existing d omain may be used to implement a new domain by mapping the elements of the new domain onto the elements of the old. This produces two domains with a common implementation but dissimilar semantic intent. For example, one could represent the imaginary numbers by mapping each imaginary onto the corresponding real and leaving implicit the fact that each one represents that real multiplied by i.

When a new domain, D , is implemented by mapping it onto an old domain, D, we say that every element of D is represented by an element from D. In this case, the object from D can be. 15.3. DOMAIN visual .

net gs1 datamatrix barcode IDENTITY: DIFFERENT DOMAIN/ SAME DOMAIN Exhibit 15.12. The bane of the former Pascal programmer.

. The Pascal co mparison operator is = , but C uses == for comparison and = for assignment. Intending to repeat a process many times, as long as the variable named a remains equal to zero, the absentminded programmer writes: do process to be repeated while ( a=0 ); Unfortunately, this process will be executed exactly once. The rst time the expression a=0 is evaluated, the value zero will be stored in the variable a, and also returned as the value of the assignment expression.

The while test interprets the 0 as FALSE and terminates the loop. Moreover, when the perplexed programmer looks at the value in a, she or he will see the number zero and be unable to understand why the loop did not repeat!. converted i Data Matrix 2d barcode for .NET nto an object from D, or vice versa, by changing only the domain identi er attached to the object. The physical form of the value does not need to be changed.

The same domain, D, may be used to implement other domains also, making a many-to-one relationship between implemented domains and an implementing domain [Exhibit 15.13]. Although a D-object is structurally identical to a D -object, di erent sets of functions are probably appropriate for D and D .

For example, bit strings are sometimes used to represent both integers and arrays of switches. Division is meaningful for integers but not for switches, and masking operations may be meaningful for arrays of switches but not for integers. Concatenation might be a meaningful operation for bit strings that are neither integers nor switches.

Thus the semantic intent of the object s domain, not just its physical representation, determines what operations are meaningful. The intent should be considered before applying a function to the object. Mapped Domains with Distinct Identities The primitive types in Pascal demonstrate domain mapping where all domains have separate identities.

Several Pascal primitive types are actually represented in the computer as integers [Exhibit 15.13]. Pascal also permits the programmer to de ne an enumerated type .

To do this, the. Exhibit 15.13 . Domains mapped onto integers in Pascal.

Semantic Intent integer numbers alphabetic characters machine addresses truth values (false, true) enumerated type Typical Implementations full-word bit strings full-word integers (8-bit if packed) integers 0 .. virtual memory size integers (0=false, 1=true) integers 0 .

. cardinality of the type.
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