Compound Statements (Blocks)

A compound statement, usually referred to as a block, is a (possibly empty) sequence of statements and declarations surrounded by a pair of curly braces. A block is a scope (§ 2.2.4, p. 48). Names introduced inside a block are accessible only in that block and in blocks nested inside that block. Names are visible from where they are defined until the end of the (immediately) enclosing block.

Compound statements are used when the language requires a single statement but the logic of our program needs more than one. For example, the body of a while or for loop must be a single statement, yet we often need to execute more than one statement in the body of a loop. We do so by enclosing the statements in curly braces, thus turning the sequence of statements into a block.

As one example, recall the while loop in the program in § 1.4.1 (p. 11):

while (val <= 10) {
    sum += val;  // assigns sum + val to sum
    ++val;       // add 1 to val
}

The logic of our program needed two statements but a while loop may contain only one statement. By enclosing these statements in curly braces, we made them into a single (compound) statement.

We also can define an empty block by writing a pair of curlies with no statements. An empty block is equivalent to a null statement:

while (cin >> s && s != sought)
    { } // empty block