Is "else if" a single keyword?

I am new to C++. I often see conditional statement like below:

if 
  statement_0;
else if
  statement_1;

Question:

Syntactically, shall I treat else if as a single keyword? Or is it actually an nested if statement within the outer else like below?

if 
  statement_0;
else 
  if
    statement_1;

They are not a single keyword if we go to the draft C++ standard section 2.12 Keywords table 4 lists both if and else separately and there is no else if keyword. We can find a more accessible list of C++ keywords by going to cppreferences section on keywords.

The grammar in section 6.4 also makes this clear:

selection-statement:
 if ( condition ) statement
 if ( condition ) statement else statement

The if in else if is a statement following the else term. The section also says:

> [...]The substatement in a selection-statement (each substatement, in the > else form of the if statement) implicitly defines a block scope (3.3). > If the substatement in a selection-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 substatement.

and provides the following example:

if (x)
 int i;
  
can be equivalently rewritten as

if (x) {  
  int i;
}

So how is your slightly extended example parsed?

if 
  statement_0;
else 
  if
    statement_1;
  else
    if
      statement_2 ;

will be parsed like this:

if 
{
  statement_0;
}
else
{ 
    if
    {
      statement_1;
    }
    else
    {
        if
        {
         statement_2 ;
        }
    }
}

Note

We can also determine that else if can not be one keyword by realizing that keywords are identifiers and we can see from the grammar for an identifier in my answer to Can you start a class name with a numeric digit? that spaces are not allowed in identifiers and so therefore else if can not be a single keyword but must be two separate keywords.

Syntactically, it's not a single keyword; keywords cannot contain white space. Logically, when writing lists of else if, it's probably better if you see it as a single keyword, and write:

if ( c1 ) {
    //  ...
} else if ( c2 ) {
    //  ...
} else if ( c3 ) {
    //  ...
} else if ( c4 ) {
    //  ...
} // ...

The compiler literally sees this as:

if ( c1 ) {
    //  ...
} else {
    if ( c2 ) {
        //  ...
    } else {
        if ( c3 ) {
            //  ...
        } else {
            if ( c4 ) {
                //  ...
            } // ...
        }
    }
}

but both forms come out to the same thing, and the first is far more readable.

No, it is not.
They are two keywords and, moreover, the second "if" is a substatement "inside" the scope determined by the first "else" statement.

You can see the scope by using curly braces:

if(X) {
  statement_0;
}
else {
  if(Y) {
    statement_1;
  }  
}

And normally implemented with two distinct keywords, one is if and one is else.

As already answered, it isn't. They are two keywords. It's start of two statements one following each one other. To try make it a bit more clear, here's the BNF gramar which deal with if and else statements in C++ language.

 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

   selection-statement: 
         if ( condition ) statement
  	 if ( condition ) statement else statement

Note that statement itself include selection-statement. So, combinations like:

if (cond1)
   stat
else if(cond2)
   stat
else
   stat

are possible and valid according to C++ standard/semantics.

Note: C++ grammar take from this page.

else and if are two different C++ keywords. An if statement can be followed by an optional else if...else statement. An if statement can have zero or more else if's and they must come before the else.

You can find syntax and example in this if...else statement tutorial

An if statement can be followed by an optional else if...else statement, which is very useful to test various conditions using single if...else if statement.

When using if , else if , else statements there are few points to keep in mind.

>An if can have zero or one else's and it must come after any else if's. > >An if can have zero to many else if's and they must come before the else. > >Once an else if succeeds, none of he remaining else if's or else's will be tested.

have a look if...else statement tutorial.

I would just like to add my point of view to all these explanations. As I see it, if you can use these keywords separately, they must be TWO keywords. Maybe you can have a look at c++ grammar, from this link in stackoverflow: https://stackoverflow.com/questions/2849795/is-there-a-standard-c-grammar

Regards