What are macros and what are its advantages and disadvantages?  

Syntax

macro-directive ::= #define identifier [replacement-list] #define identifier ( [identifier-list] ) [replacement-list] #undef identifier

replacement-list ::= token replacement-list token

Description

A #define preprocessing directive of the form:

1. define identifier [replacement-list]

defines the identifier as a macro name that represents the replacement list. The macro name is then replaced by the list of tokens wherever it appears in the source file (except inside of a string or character constant, or comment). A macro definition remains in force until it is undefined through the use of the #undef directive or until the end of the translation unit.

Macros can be redefined without an intervening #undef directive. Any parameters used must agree in number and spelling, and the replacement lists must be identical. All white space is treated equally.

The replacement-list may be empty. If the token list is not provided, the macro name is replaced with no characters.

If the define takes the form

1. define identifier ([identifier-list]) replacement-list

a macro with formal parameters is defined. The macro name is the identifier and the formal parameters are provided by the identifier-list which is enclosed in parentheses. The first parenthesis must immediately follow the identifier with no intervening white space. If there is a space between the identifier and the (, the macro is defined as if it were the first form and that the replacement list begins with the ( character.

The formal parameters to the macro are separated with commas. They may or may not appear in the replacement list. When the macro is invoked, the actual arguments are placed in a parentheses-enclosed list following the macro name. Comma tokens enclosed in additional matching pairs of parentheses do not separate arguments but are themselves components of arguments.

The actual arguments replace the formal parameters in the token string when the macro is invoked.

If a formal parameter in the macro definition directive's token string follows a # operator, it is replaced by the corresponding argument from the macro invocation, preceded and followed by a double-quote character (") to create a string literal. This feature may be used to turn macro arguments into strings. This feature is often used with the fact that the compiler concatenates adjacent strings.
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Answered by Nishu%27s at 11:10 PM on November 04, 2008

well, macro-directive ::= #define identifier [replacement-list] #define identifier ( [identifier-list] ) [replacement-list] #undef identifier

replacement-list ::= token replacement-list token

Description

A #define preprocessing directive of the form:

1. define identifier [replacement-list]

defines the identifier as a macro name that represents the replacement list. The macro name is then replaced by the list of tokens wherever it appears in the source file (except inside of a string or character constant, or comment). A macro definition remains in force until it is undefined through the use of the #undef directive or until the end of the translation unit.

Macros can be redefined without an intervening #undef directive. Any parameters used must agree in number and spelling, and the replacement lists must be identical. All white space is treated equally.

The replacement-list may be empty. If the token list is not provided, the macro name is replaced with no characters.

If the define takes the form

1. define identifier ([identifier-list]) replacement-list

a macro with formal parameters is defined. The macro name is the identifier and the formal parameters are provided by the identifier-list which is enclosed in parentheses. The first parenthesis must immediately follow the identifier with no intervening white space. If there is a space between the identifier and the (, the macro is defined as if it were the first form and that the replacement list begins with the ( character.

The formal parameters to the macro are separated with commas. They may or may not appear in the replacement list. When the macro is invoked, the actual arguments are placed in a parentheses-enclosed list following the macro name. Comma tokens enclosed in additional matching pairs of parentheses do not separate arguments but are themselves components of arguments.

The actual arguments replace the formal parameters in the token string when the macro is invoked.

If a formal parameter in the macro definition directive's token string follows a # operator, it is replaced by the corresponding argument from the macro invocation, preceded and followed by a double-quote character (") to create a string literal. This feature may be used to turn macro arguments into strings. This feature is often used with the fact that the compiler concatenates adjacent strings.

After all replacements have taken place during macro invocation, each instance of the special ## token is deleted and the tokens preceding and following the ## are concatenated into a single token. This is useful in forming unique variable names within macros.

Answered by Jai Sri Ram at 4:24 PM on November 03, 2008