Original

Multiple and indirect unit references

The order in which units appear in the uses clause determines the order of their initialization (see The initialization section)and affects the way identifiers are located by the compiler. If two units declare a variable, constant, type, procedure, or function with the same name, the compiler uses the one from the unit listed last in the uses clause. (To access the identifier from the other unit, you would have to add a qualifier: UnitName.Identifier.)

A uses clause need include only units used directly by the program or unit in which the clause appears. That is, if unit A references constants, types, variables, procedures, or functions that are declared in unit B, then A must use B explicitly. If B in turn references identifiers from unit C, then A is indirectly dependent on C; in this case, C needn’t be included in a uses clause in A, but the compiler must still be able to find both B and C in order to process A.

The example below illustrates indirect dependency.

program Prog;

uses Unit2;

const a = b;

...

unit Unit2;

interface

uses Unit1;

const b = c;

...

unit Unit1;

interface

const c = 1;

...

In this example, Prog depends directly on Unit2, which depends directly on Unit1. Hence Prog is indirectly dependent on Unit1. Because Unit1 does not appear in Prog’s uses clause, identifiers declared in Unit1 are not available to Prog.

To compile a client module, the compiler needs to locate all units that the client depends on, directly or indirectly. Unless the source code for these units has changed, however, the compiler needs only their .dcu (Windows) or .dcu/.dpu (Linux) files, not their source (.pas) files.

When changes are made in the interface section of a unit, other units that depend on it must be recompiled. But when changes are made only in the implementation or other sections of a unit, dependent units don’t have to be recompiled. The compiler tracks these dependencies automatically and recompiles units only when necessary.

Topic groups

Programs and units: Overview

Program structure and syntax: Overview

The program heading

The program uses clause

The block

Unit structure and syntax: Overview

The unit heading

The interface section

The implementation section

The initialization section

The finalization section

Unit references and the uses clause

The syntax of a uses clause

Multiple and indirect unit references

Circular unit references

译文

多重和间接单元引用

单元在uses子句中出现的位置决定了其初始化节执行的顺序（初始化节）并且对编译器定位该单元中标识符的方式有影响。如果两个单元声明了同名的变量、常量、类型、过程或函数，那么编译器将根据uses子句中的单元列表从后向前查找，也就是说，列于uses子句中靠后位置的单元，其标识符优先被编译器认可。（要访问其他单元的标识符，可以在标识符前加上限定词，例如：UnitName.Identifier。这样就可以消除编译器自动匹配可能带来的影响。）

uses子句中只需要包括其直接使用的单元。对于程序（program），只需要包括其语句块中直接使用的标识符所在的单元。对于单元，在其接口节使用的标识符所在的单元需要列在其接口节的uses子句中，此时其实现节也可以使用被引用的单元中的公共实体；在其实现节使用的标识符所在的单元需要列在其实现节的uses子句中，此时其接口节中不能使用被引用单元的公共实体。例如，如果单元A引用了在单元B中声明的常量、类型、变量、过程或函数，那么单元A必需通过uses子句显式地使用单元B。如果此时单元B又引用了单元C，那么称单元A间接引用了单元C。在此情况下，单元C不必包括在单元A的uses子句中，而为了成功编译单元A，编译器仍必需能找到单元B和单元C。