Free pascal datetime8/19/2023 Starting with Delphi 3 you can write the following: When you define a string constant, instead of writing: In such a case, simply declare a variable, instead. Assigning a new value to a constant disables all the compiler optimizations on constants. Although this is the default, you are strongly advised not to use this trick as a general programming technique. The 32-bit version still permits this behavior for backward compatibility when you enable the $J compiler directive, or use the corresponding Assignable typed constants check box of the Compiler page of the Project Options dialog box. Note: The 16-bit version of Delphi allows you to change the value of a typed constant at run-time, as if it was a variable. This second approach makes sense particularly for simple constants. When you declare a constant, the compiler can choose whether to assign a memory location to the constant, and save its value there, or to duplicate the actual value each time the constant is used. If you want to tell Delphi to use a specific type you can simply add the type name in the declaration, as in: In the example above, the Thousand constant is assumed to be of type SmallInt, the smallest integral type which can hold it. Here are some sample declarations:ĭelphi determines the constant's data type based on its value. The compiler will look at the value and automatically use its proper data type. To declare a constant you don't need to specify a data type, but only assign an initial value. Pascal also allows the declaration of constants to name values that do not change during program execution. This initialization technique works only for global variables, not for variables declared inside the scope of a procedure or method. In Delphi you can assign an initial value to a global variable while you declare it. In some cases, this conversion is automatic, but usually you need to call a specific system function that changes the internal representation of the data. Of course, it is often possible to convert the value of a variable from one type into a different type. It only warns you that there is something wrong in the code. You should not blame Delphi for these errors. Usually, errors like this are programming errors, because it does not make sense to assign a True or False value to a variable of the Integer data type. If you try to compile this code, Delphi issues a compiler error with this description: Incompatible types: 'Integer' and 'Boolean'. Using simple assignments, we can write the following code:īut the next statement is not correct, because the two variables have different data types: You cannot mix Booleans and integers (as you can with the C language). For example, you can use the Boolean value in a test and the integer value in a numerical expression. Once you have defined a variable of a given type, you can perform on it only the operations supported by its data type. You can write more than one variable name on a single line, as in the last statement above. After the var keyword comes a list of variable names, followed by a colon and the name of the data type. The var keyword can be used in several places in the code, such as at the beginning of the code of a function or procedure, to declare variables local to the routine, or inside a unit to declare global variables. Here are some sample variable declarations: Every time you declare a variable, you must specify a data type. Pascal requires all variables to be declared before they are used. The concept of type is stronger in Pascal than in C, where the arithmetic data types are almost interchangeable, and much stronger than in the original versions of BASIC, which had no similar concept. The type determines the values a variable can have, and the operations that can be performed on it. The original Pascal language was based on some simple notions, which have now become quite common in programming languages. The cover of the 4th edition of Essential Pascal,
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