C++ Primer 第5版：C++ 文の構造

C++ において、抽象的な値から実行へと至る道のりは文から始まります。式文は、式の末尾にセミコロンを付けるだけで作成され、コンパイラにその式を評価させ、 flow of control sequentially.

1. The Null Statement

A null statement (;) is a placeholder used when the language requires a statement but your logic does not. While useful in some loops, beware the extraneous null statement—an accidental semicolon after a while or if header can lead to devastating logic errors where the intended body is ignored.

⚠️ Warning (p. 235): An accidental semicolon after a loop header creates a null statement as the body, often resulting in infinite loops.

2. Compound Statements (Blocks)

A compound statement, or block, is a sequence of statements enclosed in curly braces { }. It is treated as a single unit of execution. Blocks define their own scope; names defined inside are invisible outside.

Note (p. 235): Unlike simple statements, a block is not terminated by a semicolon.

TERMINALbash — 80x24

> Ready. Click "Run" to execute.

QUESTION 1

Which of the following creates an 'expression statement'?

An expression followed by a semicolon.

Any line of code within curly braces.

A variable declaration without an assignment.

A comment followed by a semicolon.

QUESTION 2

Exercise 5.2: What is a block and when might you use one?

A block is a group of functions; used to organize large projects.

A block is a single semicolon; used to end a program.

A block is a sequence of statements in braces; used where the syntax requires one statement but logic requires many.

A block is a reserved memory segment; used for global variables.

QUESTION 3

What is the primary danger of an 'extraneous null statement' after a while loop header?

The compiler will throw a syntax error.

The loop body will be treated as the null statement, causing the intended logic to execute only once or never.

It automatically converts the loop into a do-while loop.

It speeds up the execution by skipping checks.

QUESTION 4

Exercise 5.3: Does using the comma operator to compress a while loop into a single statement without a block improve readability?

Yes, less lines always means clearer code.

No, it typically diminishes readability by hiding the loop's intent and side effects.

Yes, it is the standard way to write C++ loops.

No, the comma operator cannot be used in while loops.

QUESTION 5

Which statement about block termination is correct?

Every block must end with a semicolon after the closing brace.

A block is NOT terminated by a semicolon.

Only blocks inside functions require a semicolon.

A block must end with a null statement.