Ada编程语言详解：面向C++或Java开发者全面指南

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"Ada for the C or Java Developer" 是一本介绍Ada编程语言的教程，针对那些熟悉C++或Java的开发者设计。Ada作为一种源于美国军方、旨在统一军事系统编程语言的高级通用编程语言，其历史背景与Ada Lovelace的名字有关，体现了现代软件开发的许多核心原则，如清晰性、可靠性、有效性及可维护性。本书的核心内容包括： 1. ** Preface**：简介Ada语言的起源和目的，以及其在软件工程中的重要地位，强调它突破了John von Neumann传统思维模式的局限，并提供了独特的Ada文化和支持环境（APSE）。 2. ** Basics**：这部分介绍了Ada的基本概念，包括声明和语句，帮助读者建立起对语言基础的理解。 3. ** CompilationUnitStructure**：讨论了Ada的编译单元结构，确保代码组织有序且易于管理。 4. ** Statements and Declarations**：详细讲解了Ada中的语句和声明，如何定义变量、常量和类型。 5. ** Type System**：强调了Ada的强类型系统，包括语言内置类型、应用自定义类型、类型范围，以及更复杂的类型合同（如子类型谓词）和属性。还涵盖了数组、字符串和异构数据结构的处理，以及指针的使用。 6. ** Functions and Procedures**：阐述了函数和过程的一般形式，包括重载功能和子程序合同的概念，这些都是Ada面向过程编程的核心。 7. ** Packages**：探讨了包的使用，包括保护声明、分层包和从包中引用实体，体现了Ada的模块化和封装特性。 8. ** Classes and Object-Oriented Programming (OOP)**：介绍了面向对象编程在Ada中的应用，涉及原始子程序、派生和动态调度、构造函数和析构函数，以及封装等面向对象的核心概念。通过阅读这本书，C++或Java开发者可以学习到如何转换思维方式，掌握Ada这种强大而全面的编程语言，从而提高软件开发的效率和质量。 Ada的独特之处在于它在设计之初就考虑到了软件工程的实践需求，是值得深入学习的现代编程语言之一。

CHAPTER

FOUR

STATEMENTS, DECLARATIONS, AND

CONTROL STRUCTURES

4.1 Statements and Declarations

The following code samples are all equivalent, and illustrate the use of comments and working with integer variables:

[Ada]

-- Ada program to declare and modify Integers

procedure Main is

-- Variable declarations

A, B : Integer := 0;

C : Integer := 100;

: Integer;

begin

-- Ada uses a regular assignment statement for incrementation.

A := A + 1;

-- Regular addition

D := A + B + C;

end Main;

[C++]

C++ program to declare and modify ints

int main(int argc, const char

argv[]) {

// Variable declarations

int a = 0, b = 0, c = 100, d;

// C++ shorthand for incrementation

a++;

// Regular addition

d = a + b + c;

}

[Java]

Ada for the C++ or Java Developer, Release 1.0

Java program to declare and modify ints

public class Main {

public static void main(String [] argv) {

// Variable declarations

int a = 0, b = 0, c = 100, d;

// Java shorthand for incrementation

a++;

// Regular addition

d = a + b + c;

}

Statements are terminated by semicolons in all three languages. In Ada, blocks of code are surrounded by the reserved

words begin and end rather than by curly braces. We can use both multi-line and single-line comment styles in the

C++ and Java code, and only single-line comments in the Ada code.

Ada requires variable declarations to be made in a speciﬁc area called the declarative part, seen here before the begin

keyword. Variable declarations start with the identiﬁer in Ada, as opposed to starting with the type as in C++ and Java

(also note Ada’s use of the : separator). Specifying initializers is different as well: in Ada an initialization expression

can apply to multiple variables (but will be evaluated separately for each), whereas in C++ and Java each variable is

initialized individually. In all three languages, if you use a function as an initializer and that function returns different

values on every invocation, each variable will get initialized to a different value.

Let’s move on to the imperative statements. Ada does not provide ++ or -- shorthand expressions for incre-

ment/decrement operations; it is necessary to use a full assignment statement. The := symbol is used in Ada to

perform value assignment. Unlike C++’s and Java’s = symbol, := can not be used as part of an expression. So, a

statement like A := B := C; doesn’t make sense to an Ada compiler, and neither does a clause like “if A := B then ....”

Both are compile-time errors.

You can nest a block of code within an outer block if you want to create an inner scope:

with Ada.Text_IO; use Ada.Text_IO;

procedure Main is

begin

Put_Line ("Before the inner block");

declare

Alpha : Integer := 0;

begin

Alpha := Alpha + 1;

Put_Line ("Now inside the inner block");

end;

Put_Line ("After the inner block");

end Main;

It is OK to have an empty declarative part or to omit the declarative part entirely—just start the inner block with begin

if you have no declarations to make. However it is not OK to have an empty sequence of statements. You must at least

provide a null; statement, which does nothing and indicates that the omission of statements is intentional.

8 Chapter 4. Statements, Declarations, and Control Structures

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