9 Statements [stmt.stmt]

9.7 Declaration statement [stmt.dcl]

1
#
A declaration statement introduces one or more new identifiers into a block; it has the form
declaration-statement:
	block-declaration
If an identifier introduced by a declaration was previously declared in an outer block, the outer declaration is hidden for the remainder of the block, after which it resumes its force.
2
#
Variables with automatic storage duration ([basic.stc.auto]) are initialized each time their declaration-statement is executed.
Variables with automatic storage duration declared in the block are destroyed on exit from the block ([stmt.jump]).
3
#
It is possible to transfer into a block, but not in a way that bypasses declarations with initialization.
A program that jumps91 from a point where a variable with automatic storage duration is not in scope to a point where it is in scope is ill-formed unless the variable has scalar type, class type with a trivial default constructor and a trivial destructor, a cv-qualified version of one of these types, or an array of one of the preceding types and is declared without an initializer.
[ Example
: 
void f() {
  // ...
  goto lx;          // ill-formed: jump into scope of a
  // ...
ly:
  X a = 1;
  // ...
lx:
  goto ly;          // OK, jump implies destructor call for a followed by
                    // construction again immediately following label ly
}
— end example
 ]
4
#
Dynamic initialization of a block-scope variable with static storage duration ([basic.stc.static]) or thread storage duration ([basic.stc.thread]) is performed the first time control passes through its declaration; such a variable is considered initialized upon the completion of its initialization.
If the initialization exits by throwing an exception, the initialization is not complete, so it will be tried again the next time control enters the declaration.
If control enters the declaration concurrently while the variable is being initialized, the concurrent execution shall wait for completion of the initialization.92
If control re-enters the declaration recursively while the variable is being initialized, the behavior is undefined.
[ Example
: 
int foo(int i) {
  static int s = foo(2*i);      // recursive call - undefined
  return i+1;
}
— end example
 ]
5
#
The destructor for a block-scope object with static or thread storage duration will be executed if and only if it was constructed.
[ Note
:
[basic.start.term] describes the order in which block-scope objects with static and thread storage duration are destroyed.
— end note
 ]
91)
The transfer from the condition of a switch statement to a case label is considered a jump in this respect.
92)
The implementation must not introduce any deadlock around execution of the initializer.
Deadlocks might still be caused by the program logic; the implementation need only avoid deadlocks due to its own synchronization operations.