C++ – Declarations and Definitions as Statements in C and C++

c++if-statementlanguage-lawyer

I was confused when this wouldn't compile in C:

int main()
{
    for (int i = 0; i < 4; ++i)
        int a = 5; // A dependent statement may not be declaration

    return 0;
}

I'm used to C++ where this will compile. I just stared dumbfounded for a while until I remembered an answer here on SO about how in C and C++ different things are considered "statements". This was in regard to a switch statement. A "statement" after the for loop brackets must be present both in C and C++. This can be done in both either adding a semicolon or creating a { } squiggly bracket block.

In C++ "int a = 7;" is considered a declaration, definition, and initialisation. In C I believe it is also considered all of these, however in C it is not considered a "statement".

Could someone exactly clarify why in C this isn't a statement whereas in C++ it is? This is confusing my concept of what a statement is, because one language says it is, and another says it isn't, so I'm kind of confused.

Best Answer

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;
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