Declarations/definitions as statements in C and C++
C++ allowed that the "substatement" of an iteration statement was implicitly a compound statement ([stmt.iter])
If the substatement in an iteration-statement is a single statement and not a compound-statement, it is as if it was rewritten to be a compound-statement containing the original statement. Example:
while (--x >= 0)
int i;
can be equivalently rewritten as
while (--x >= 0) {
int i;
}
the C standard does not have this language.
Additionally, the definition of a statement changed in C++ to include a declaration statement, so even if the above change wasn't made, it would still be legal.
The reason that adding braces makes it work is because your declaration now becomes a compound-statement which can include declarations.
You are allowed to have an identifier in a loop body without braces, so you can do this instead:
int a = 5;
for (int i = 0; i < 4; ++i)
a;
In C++, a statement is (C++17 standard draft)
excerpt from [gram.stmt]
statement:
labeled-statement
attribute-specifier-seqopt expression-statement
attribute-specifier-seqopt compound-statement
attribute-specifier-seqopt selection-statement
attribute-specifier-seqopt iteration-statement
attribute-specifier-seqopt jump-statement
declaration-statement
attribute-specifier-seqopt try-block
init-statement:
expression-statement
simple-declaration
declaration-statement:
block-declaration
...
Note that there are declaration statements in C++, which are declarations, and are statements. Similarly, simple declarations are init statements. Not all declarations are statements though. The grammar of declarations contains things that are not in the list of statements:
excerpt from [gram.dcl]
declaration:
block-declaration
nodeclspec-function-declaration
function-definition
template-declaration
deduction-guide
explicit-instantiation
explicit-specialization
linkage-specification
namespace-definition
empty-declaration
attribute-declaration
block-declaration:
simple-declaration
asm-definition
namespace-alias-definition
using-declaration
using-directive
static_assert-declaration
alias-declaration
opaque-enum-declaration
simple-declaration:
decl-specifier-seq init-declarator-listopt ;
attribute-specifier-seq decl-specifier-seq init-declarator-list ;
attribute-specifier-seqopt decl-specifier-seq ref-qualifieropt [ identifier-list ] initializer ;
...
The list of declaration grammars continues on for a few pages.
In C, a statement is (C11 standard draft)
excerpt from Statements and blocks
statement:
labeled-statement
compound-statement
expression-statement
selection-statement
iteration-statement
jump-statement
Note that there are no declarations that are statements in C.
So, the meaning of statement is clearly different in the languages. Statement in C++ appears to have a broader meaning than statement in C.