Declaration Statements (C++)
Declaration statements introduce new names into the current scope. These names can be:
Type names (class, struct, union, enum, typedef, and pointer-to-member).
Declarations may be any of the following:
block-declaration function-definition template-declaration explicit-instantiation explicit-specialization linkage-specification namespace-definition
If a declaration within a block introduces a name that is already declared outside the block, the previous declaration is hidden for the duration of the block. After termination of the block, the previous declaration is again visible.
Multiple declarations of the same name in the same block are illegal.