C++ Programming Tutorials

Preprocessor directives
Preprocessor directives are lines included in the code of our programs that are not program statements but directives for the preprocessor. These lines are always preceded by a pound sign (#). The preprocessor is executed before the actual compilation of code begins, therefore the preprocessor digests all these directives before any code is generated by the statements.

These preprocessor directives extend only across a single line of code. As soon as a newline character is found, the preprocessor directive is considered to end. 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 (\).

macro definitions (#define, #undef)
To define preprocessor macros we can use #define. Its format 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++, it simply replaces any occurrence of identifier by replacement.

#define TABLE_SIZE 100
int table1[TABLE_SIZE];
int table2[TABLE_SIZE];

After the preprocessor has replaced TABLE_SIZE, the code becomes equivalent to:

int table1[100];
int table2[100];

This use of #define as constant definer is already known by us from previuos tutorials, but #define can work also with parameters to define function macros:

#define getmax(a,b) a>b?a:b

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:

// function macro
#include <iostream>
using namespace std;

#define getmax(a,b) ((a)>(b)?(a):(b))

int main()
{
int x=5, y;
y= getmax(x,2);
cout << y << endl;
cout << getmax(7,x) << endl;
return 0;
}

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Defined macros are not affected by block structure. A macro lasts until it is undefined with the #undef preprocessor directive:

#define TABLE_SIZE 100
int table1[TABLE_SIZE];
#undef TABLE_SIZE
#define TABLE_SIZE 200
int table2[TABLE_SIZE];

This would generate the same code as:

int table1[100];
int table2[200];

Function macro definitions accept two special operators (# and ##) in the replacement sequence:
If the operator # is used before a parameter is used in the replacement sequence, that parameter is replaced by a string literal (as if it were enclosed between double quotes)

#define str(x) #x
cout << str(test);

This would be translated into:

cout << "test";

The operator ## concatenates two arguments leaving no blank spaces between them:

#define glue(a,b) a ## b
glue(c,out) << "test";

This would also be translated into:

cout << "test";

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 may result obscure to other programmers, since the syntax they expect is on many occasions different from the regular expressions programmers expect in C++.

NEXT >> Conditional inclusions

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