Keywords and Reserved words tells about the predefined identifiers which can be redefined and which can't be redefined.

1. Keywords and Reserved Words :
Keywords are predefined identifiers, that are used in the syntax
and can be used as identifiers again.
Reserved words are the words in a programming language
which has a fixed meaning and cannot be redefined by the programmer.
They cannot be used as identifers(variables, functions etc).
• All reserved words and keywords are in lowercase letters.
• But be aware that this terminology is not standard. For example, In
some books it was mentioned that Reserved words are also called as
Reserved Keywords and some authors will use keyword in the same
sense that we have used Reserved word.
• Infact all keywords are subset of Reserve words.
• Keywords can be redefined while Reserved words cannot be
redefined and used.
For example :
#include<iostream>
using namespace std;
int main() // here main is the keyword
{
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2. int main; // it can be redefined
main = 1;
cout<< main;
return 0;
}
Here main is the keyword. So it can be redefined and used as an
identifier.
Output :
1
#include <iostream>
using namespace std;
int main()
{
int auto;
auto = 1;
cout<< auto;
return 0;
}
Here auto is an Reserved word. It cannot be redefined. So it
gives an error.
In general reserved words and keywords need not coincide, but
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3. in most modern programming languages, keywords are a subset of
reserved words, as this makes parsing easier, since keywords cannot be
confused with identifiers. In some languages, like C or Python, reserved
words and keywords coincide, while in other languages, like Java, all
keywords are reserved words, but some reserved words are not keywords –
these are "reserved for future use".
Here is a list of Reserved words and keywords.
C++ Rerserved words :
The reserved words of C++ may be conveniently placed into 3
groups. In the first group we put those that were also present in the C
programming language and have been carried over into C++. There are
32 of these, and here they are:
auto const double float int short struct unsigned
break continue else for long signed switch void
case default enum goto register sizeof typedef volatile
char do extern if return static union while
There are another 30 reserved words that were not in C, are
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4. therefore new to C++, and here they are:
asm dynamic_cast namespace reinterpret_cast try
bool explicit
export new static_cast typeid
catch false operator template using
class friend private this typename
const_cast inline public throw virtual
delete mutable protected true wchar_t
The following 11 C++ reserved words have been added to
provide more readable alternatives for some of the C++ operators, but
they are not essential when the standard ASCII character set is being used.
and bitand compl not_eq or_eq xor_eq
and_eq bitor not or xor
Predefined Identifiers :
Here is a very short list of some of the predefined identifiers :
cin endl INT_MIN iomanip main npos std
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5. cout include INT_MAX iostream MAX_RAND NULL string
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