Dev C++ Directives Four Types

08.06.2020by
Preprocessor directives are lines included in the code of programs preceded by a hash sign (#). These lines are not program statements but directives for the preprocessor. The preprocessor examines the code before actual compilation of code begins and resolves all these directives before any code is actually generated by regular statements.
These preprocessor directives extend only across a single line of code. As soon as a newline character is found, the preprocessor directive is ends. No semicolon (;) is expected at the end of a preprocessor directive. The only way a preprocessor directive can extend through more than one line is by preceding the newline character at the end of the line by a backslash ().
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  • May 24, 2012  In c all the preprocessor directives start with # characterexcept define operator. C Programming Language; Types of Preprocessor Directives in C Language FILE INCLUSIVE DIRECTIVE #include MACRO SUBSTITUTION DIRECTIVE #define UNDEFINE SYMBOL DIRECTIVE #undef CONDITIONAL DIRECTIVE # if #elif #else #endif #ifdef #ifndef MISCELLANEOUS DIRECTIVE #.
  • May 17, 2017  This (‘#’) symbol at the beginning of a statement in a C/C program indicates that it is a pre-processor directive. We can place these preprocessor directives anywhere in our program. Examples of some preprocessor directives are: #include, #define, #ifndef etc. There are 4 main types of preprocessor directives: Macros; File Inclusion.

macro definitions (#define, #undef)

To define preprocessor macros we can use #define. Its syntax is:
#define identifier replacement

When the preprocessor encounters this directive, it replaces any occurrence of identifier in the rest of the code by replacement. This replacement can be an expression, a statement, a block or simply anything. The preprocessor does not understand C++ proper, it simply replaces any occurrence of identifier by replacement.
After the preprocessor has replaced TABLE_SIZE, the code becomes equivalent to:

#define can work also with parameters to define function macros:
This would replace any occurrence of getmax followed by two arguments by the replacement expression, but also replacing each argument by its identifier, exactly as you would expect if it was a function:

Defined macros are not affected by block structure. A macro lasts until it is undefined with the #undef preprocessor directive:
This would generate the same code as:

Function macro definitions accept two special operators (# and ##) in the replacement sequence:
The operator #, followed by a parameter name, is replaced by a string literal that contains the argument passed (as if enclosed between double quotes):
This would be translated into:

The operator ## concatenates two arguments leaving no blank spaces between them:
This would also be translated into:

Because preprocessor replacements happen before any C++ syntax check, macro definitions can be a tricky feature. But, be careful: code that relies heavily on complicated macros become less readable, since the syntax expected is on many occasions different from the normal expressions programmers expect in C++.

Conditional inclusions (#ifdef, #ifndef, #if, #endif, #else and #elif)


These directives allow to include or discard part of the code of a program if a certain condition is met.
#ifdef allows a section of a program to be compiled only if the macro that is specified as the parameter has been defined, no matter which its value is. For example:
In this case, the line of code int table[TABLE_SIZE]; is only compiled if TABLE_SIZE

A parent file is the file that contains the #include directive. For example, if you include a file named file2 in a file named file1, file1 is the parent file. Include files can be 'nested': An #include directive can appear in a file that's named by another #include directive. For example, file2 could include file3.

was previously defined with #define, independently of its value. If it was not defined, that line will not be included in the program compilation.

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#ifndef serves for the exact opposite: the code between #ifndef and
#endif directives is only compiled if the specified identifier has not been previously defined. For example:

In this case, if when arriving at this piece of code, the TABLE_SIZE macro has not been defined yet, it would be defined to a value of 100. If it already existed it would keep its previous value since the #define directive would not be executed.
The #if, #else and #elif

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(i.e., 'else if') directives serve to specify some condition to be met in order for the portion of code they surround to be compiled. The condition that follows #if or #elif can only evaluate constant expressions, including macro expressions. For example:
Notice how the entire structure of #if, #elif and #else chained directives ends with #endif.
The behavior of #ifdef and #ifndef can also be achieved by using the special operators defined and !defined respectively in any #if or #elif directive:


Line control (#line)

When we compile a program and some error happens during the compiling process, the compiler shows an error message with references to the name of the file where the error happened and a line number, so it is easier to find the code generating the error.
The #line directive allows us to control both things, the line numbers within the code files as well as the file name that we want that appears when an error takes place. Its format is:
#line number 'filename'

Where number is the new line number that will be assigned to the next code line. The line numbers of successive lines will be increased one by one from this point on.
'filename' is an optional parameter that allows to redefine the file name that will be shown. For example:
This code will generate an error that will be shown as error in file 'assigning variable', line 20.

Error directive (#error)

This directive aborts the compilation process when it is found, generating a compilation error that can be specified as its parameter:

This example aborts the compilation process if the macro name __cplusplus is not defined (this macro name is defined by default in all C++ compilers).

Source file inclusion (#include)

This directive has been used assiduously in other sections of this tutorial. When the preprocessor finds an #include directive it replaces it by the entire content of the specified header or file. There are two ways to use #include:
In the first case, a header is specified between angle-brackets <>. This is used to include headers provided by the implementation, such as the headers that compose the standard library (iostream, string,..). Whether the headers are actually files or exist in some other form is implementation-defined, but in any case they shall be properly included with this directive.
The syntax used in the second #include uses quotes, and includes a file. The file is searched for in an implementation-defined manner, which generally includes the current path. In the case that the file is not found, the compiler interprets the directive as a header inclusion, just as if the quotes (') were replaced by angle-brackets (<>).

Pragma directive (#pragma)

This directive is used to specify diverse options to the compiler. These options are specific for the platform and the compiler you use. Consult the manual or the reference of your compiler for more information on the possible parameters that you can define with #pragma.
If the compiler does not support a specific argument for #pragma, it is ignored - no syntax error is generated.

Predefined macro names

The following macro names are always defined (they all begin and end with two underscore characters, _):
macrovalue
__LINE__Integer value representing the current line in the source code file being compiled.
__FILE__A string literal containing the presumed name of the source file being compiled.
__DATE__A string literal in the form 'Mmm dd yyyy' containing the date in which the compilation process began.
__TIME__A string literal in the form 'hh:mm:ss' containing the time at which the compilation process began.
__cplusplusAn integer value. All C++ compilers have this constant defined to some value. Its value depends on the version of the standard supported by the compiler:
  • 199711L: ISO C++ 1998/2003
  • 201103L: ISO C++ 2011
Non conforming compilers define this constant as some value at most five digits long. Note that many compilers are not fully conforming and thus will have this constant defined as neither of the values above.
__STDC_HOSTED__1 if the implementation is a hosted implementation (with all standard headers available)
0 otherwise.

The following macros are optionally defined, generally depending on whether a feature is available:
macrovalue
__STDC__In C: if defined to 1, the implementation conforms to the C standard.
In C++: Implementation defined.
__STDC_VERSION__In C:
  • 199401L: ISO C 1990, Ammendment 1
  • 199901L: ISO C 1999
  • 201112L: ISO C 2011
In C++: Implementation defined.
__STDC_MB_MIGHT_NEQ_WC__1 if multibyte encoding might give a character a different value in character literals
__STDC_ISO_10646__A value in the form yyyymmL, specifying the date of the Unicode standard followed by the encoding of wchar_t characters
__STDCPP_STRICT_POINTER_SAFETY__1 if the implementation has strict pointer safety (see get_pointer_safety)
__STDCPP_THREADS__1 if the program can have more than one thread

Particular implementations may define additional constants.

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For example:

Previous:
Exceptions

Index
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Input/output with files
  • C++ Basics
  • C++ Object Oriented
  • C++ Advanced
  • C++ Useful Resources
  • Selected Reading

An operator is a symbol that tells the compiler to perform specific mathematical or logical manipulations. C++ is rich in built-in operators and provide the following types of operators −

  • Arithmetic Operators
  • Relational Operators
  • Logical Operators
  • Bitwise Operators
  • Assignment Operators
  • Misc Operators

This chapter will examine the arithmetic, relational, logical, bitwise, assignment and other operators one by one.

Arithmetic Operators

There are following arithmetic operators supported by C++ language −

Assume variable A holds 10 and variable B holds 20, then −

OperatorDescriptionExample
+Adds two operandsA + B will give 30
-Subtracts second operand from the firstA - B will give -10
*Multiplies both operandsA * B will give 200
/Divides numerator by de-numeratorB / A will give 2
%Modulus Operator and remainder of after an integer divisionB % A will give 0
++Increment operator, increases integer value by oneA++ will give 11
--Decrement operator, decreases integer value by oneA-- will give 9

Relational Operators

There are following relational operators supported by C++ language

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Assume variable A holds 10 and variable B holds 20, then −

OperatorDescriptionExample
Checks if the values of two operands are equal or not, if yes then condition becomes true.(A B) is not true.
!=Checks if the values of two operands are equal or not, if values are not equal then condition becomes true.(A != B) is true.
>Checks if the value of left operand is greater than the value of right operand, if yes then condition becomes true.(A > B) is not true.
<Checks if the value of left operand is less than the value of right operand, if yes then condition becomes true.(A < B) is true.
>=Checks if the value of left operand is greater than or equal to the value of right operand, if yes then condition becomes true.(A >= B) is not true.
<=Checks if the value of left operand is less than or equal to the value of right operand, if yes then condition becomes true.(A <= B) is true.

Logical Operators

There are following logical operators supported by C++ language.

Assume variable A holds 1 and variable B holds 0, then −

OperatorDescriptionExample
&&Called Logical AND operator. If both the operands are non-zero, then condition becomes true.(A && B) is false.
Called Logical OR Operator. If any of the two operands is non-zero, then condition becomes true.(A B) is true.
!Called Logical NOT Operator. Use to reverses the logical state of its operand. If a condition is true, then Logical NOT operator will make false.!(A && B) is true.

Bitwise Operators

Bitwise operator works on bits and perform bit-by-bit operation. The truth tables for &, , and ^ are as follows −

pqp & qp qp ^ q
00000
01011
11110
10011

Assume if A = 60; and B = 13; now in binary format they will be as follows −

A = 0011 1100

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B = 0000 1101

-----------------

A&B = 0000 1100

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A B = 0011 1101

A^B = 0011 0001

~A = 1100 0011

The Bitwise operators supported by C++ language are listed in the following table. Assume variable A holds 60 and variable B holds 13, then −

OperatorDescriptionExample
&Binary AND Operator copies a bit to the result if it exists in both operands. (A & B) will give 12 which is 0000 1100
Binary OR Operator copies a bit if it exists in either operand.(A B) will give 61 which is 0011 1101
^Binary XOR Operator copies the bit if it is set in one operand but not both.(A ^ B) will give 49 which is 0011 0001
~Binary Ones Complement Operator is unary and has the effect of 'flipping' bits. (~A ) will give -61 which is 1100 0011 in 2's complement form due to a signed binary number.
<<Binary Left Shift Operator. The left operands value is moved left by the number of bits specified by the right operand.A << 2 will give 240 which is 1111 0000
>>Binary Right Shift Operator. The left operands value is moved right by the number of bits specified by the right operand.A >> 2 will give 15 which is 0000 1111

Assignment Operators

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There are following assignment operators supported by C++ language −

OperatorDescriptionExample
=Simple assignment operator, Assigns values from right side operands to left side operand.C = A + B will assign value of A + B into C
+=Add AND assignment operator, It adds right operand to the left operand and assign the result to left operand.C += A is equivalent to C = C + A
-=Subtract AND assignment operator, It subtracts right operand from the left operand and assign the result to left operand.C -= A is equivalent to C = C - A
*=Multiply AND assignment operator, It multiplies right operand with the left operand and assign the result to left operand.C *= A is equivalent to C = C * A
/=Divide AND assignment operator, It divides left operand with the right operand and assign the result to left operand.C /= A is equivalent to C = C / A
%=Modulus AND assignment operator, It takes modulus using two operands and assign the result to left operand.C %= A is equivalent to C = C % A
<<=Left shift AND assignment operator.C <<= 2 is same as C = C << 2
>>=Right shift AND assignment operator.C >>= 2 is same as C = C >> 2
&=Bitwise AND assignment operator.C &= 2 is same as C = C & 2
^=Bitwise exclusive OR and assignment operator.C ^= 2 is same as C = C ^ 2
=Bitwise inclusive OR and assignment operator.C = 2 is same as C = C 2

Misc Operators

The following table lists some other operators that C++ supports.

Sr.NoOperator & Description
1

sizeof

sizeof operator returns the size of a variable. For example, sizeof(a), where ‘a’ is integer, and will return 4.

2

Condition ? X : Y

Conditional operator (?). If Condition is true then it returns value of X otherwise returns value of Y.

3

,

Comma operator causes a sequence of operations to be performed. The value of the entire comma expression is the value of the last expression of the comma-separated list.

4

. (dot) and -> (arrow)

Member operators are used to reference individual members of classes, structures, and unions.

5

Cast

Casting operators convert one data type to another. For example, int(2.2000) would return 2.

6

&

Pointer operator & returns the address of a variable. For example &a; will give actual address of the variable.

7

*

Pointer operator * is pointer to a variable. For example *var; will pointer to a variable var.

Operators Precedence in C++

Operator precedence determines the grouping of terms in an expression. This affects how an expression is evaluated. Certain operators have higher precedence than others; for example, the multiplication operator has higher precedence than the addition operator −

For example x = 7 + 3 * 2; here, x is assigned 13, not 20 because operator * has higher precedence than +, so it first gets multiplied with 3*2 and then adds into 7.

Here, operators with the highest precedence appear at the top of the table, those with the lowest appear at the bottom. Within an expression, higher precedence operators will be evaluated first.

Category Operator Associativity
Postfix () [] -> . ++ - - Left to right
Unary + - ! ~ ++ - - (type)* & sizeof Right to left
Multiplicative * / % Left to right
Additive + - Left to right
Shift << >> Left to right
Relational < <= > >= Left to right
Equality != Left to right
Bitwise AND & Left to right
Bitwise XOR ^ Left to right
Bitwise OR Left to right
Logical AND && Left to right
Logical OR Left to right
Conditional ?: Right to left
Assignment = += -= *= /= %=>>= <<= &= ^= = Right to left
Comma , Left to right
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