C++ Primer, Fifth Edition

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, ...