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-blockinit-statement:    expression-statement    simple-declarationdeclaration-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-declarationblock-declaration:    simple-declaration    asm-definition    namespace-alias-definition    using-declaration    using-directive    static_assert-declaration    alias-declaration    opaque-enum-declarationsimple-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 blocksstatement:    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.

According to cppreference, C++ includes following types of statements:

1. expression statements;
2. compound statements;
3. selection statements;
4. iteration statements;
5. jump statements;
6. declaration statements;
7. try blocks;
8. atomic and synchronized blocks

While C considers following types of statements:

1. compound statements
2. expression statements
3. selection statements
4. iteration statements
5. jump statements

As you can notice, declarations are not considered statements in C, while it is not this case in C++.

For C++:

int main(){                                     // start of a compound statement    int n = 1;                        // declaration statement    n = n + 1;                        // expression statement    std::cout << "n = " << n << '\n'; // expression statement    return 0;                         // return statement}                                     // end of compound statement

For C:

int main(void){                          // start of a compound statement    int n = 1;             // declaration (not a statement)    n = n+1;               // expression statement    printf("n = %d\n", n); // expression statement    return 0;              // return statement}                          // end of compound statement