Introduction to Variable Parameter ellipsis, variable parameter ellipsis

Introduction to Variable Parameter ellipsis, variable parameter ellipsis

Introduction to Variable Parameter ellipsis

C allows you to define the number of parameters and functions with uncertain types. For example, the standard function printf in C uses this mechanism. When declaring a function with an uncertain parameter, you can use the ellipsis (...) in the parameter section. . "…" Tells the compiler to check whether the parameter type is the same as the real parameter type or the number of parameters when calling a function.

For example:

[Root @ centos-64-min exercise] # vi chang. c
# Include <stdio. h>
Int mult (int x, int y, const char * fmt ,...)
{
Int result = x * y;
If (fmt)
{
Printf ("% s:", fmt );
}
Return result;
}

The compiler only checks whether the first parameter and the second parameter are integers and whether the third parameter is a string. It processes all the subsequent parameters, no matter what they are, as ellipsis, no matter.

[Root @ centos-64-min exercise] # vi chang. c
# Include <stdio. h>
Int mult (int x, int y, const char * fmt ,...)
{
Int result = x * y;
If (fmt)
{
Printf ("% s", fmt );
}
Return result;
}
Int main (void)
{
Int s = 0;
S = mult (1, 2, "1*2 = ");
Printf ("% d \ n", s );

S = mult (2, 3, "2*3 =", "hahaha", 12345,666 66); // ignore the following parameters: "Hahahaha", 12345, and 66666
Printf ("% d \ n", s );
Return 0;
}

"Chang. c" 20L, 314C written
[Root @ centos-64-min exercise] # gcc-o chang. c
[Root @ centos-64-min exercise] #./chang
1*2 = 2
2*3 = 6

Use of variable parameters in macro definition

Example:

[Root @ centos-64-min exercise] # vi chang. c
S = rectangle_area (3, 4, "this rectangle's area is ");
# Include <stdio. h>
Int mult (int x, int y, const char * fmt ,...)
{
Int result = x * y;
If (fmt)
{
Printf ("% s", fmt );
}
Return result;
}

# Define rectangle_area (x, y, args...) mult (x, y, # args)
# Define square_area (x, args...) rectangle_area (x, x, # args)
# Define myprintf (args...) square_area (1, # args)

Int main (void)
{
Int s = 0;
S = rectangle_area (3, 4, "this rectangle's area is ");
Printf ("% d \ n", s );

S = rectangle_area (3, 4, "this rectangle's area is", 1, "hahaha ");
Printf ("% d \ n", s );

S = square_area (3, "this square's area is ");
Printf ("% d \ n", s );

S = square_area (3, "this squre's area is", "66666", "Hahahaha", 12345 );
Printf ("% d \ n", s );

Myprintf ("6666666666666666666666666 \ n ");
Myprintf ("hahahahahahahahaha \ n", "66666666666666", 12345 );
Return 0;
}
~
~
~
~
"Chang. c" 34L, 819C written
[Root @ centos-64-min exercise] # gcc-o chang. c
[Root @ centos-64-min exercise] #./chang
This rectangle's area is 12
This rectangle's area is 12
This square's area is 9
This squre's area is 9
6666666666666666666666666
Hahahahahahahahahaha

 

Simple analysis:

Myprintf ("hahahahahahahahaha \ n", "66666666666666", 12345 );

Equivalent:

Square_area (1, "hahahahahahahahaha \ n", "66666666666666", 12345 );

Equivalent:

Rectangle_area (1, 1, "hahahahahahahahaha \ n", "66666666666666", 12345 );

Equivalent:

Mult (1, 1, "hahahahahahahahaha \ n", "66666666666666", 12345 );

Mult only reads mult (1, 1, "hahahahahahahahaha \ n ");

Here: The # args values defined by define at each layer are passed to args ..., top-level # args <=> const char * fmt ,... so in the end, mult only reads the variable of the first string of args, followed by the variable... processed