Function
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A function is a routine that, in contrast to procedures, returns a value.
A call of a function is virtually substituted by its return value.
If the {$extendedSyntax} compiler switch state is off, function calls can not appear as non-productive statements, but have to be or be part of an expression.
The word function is a reserved word.
Return value
In addition to a normal procedure, a function’s formal signature contains a return type:
The formal parameter list has to be succeeded by a colon and return type.
For instance the following function returns a Boolean.
function myFunction(const firstParameter: real): Boolean;
When implementing functions there are several ways to define the function’s return value.
program functionDemo(input, output, stderr);
{$mode objFPC}
// traditional syntax:
// the result is stored in the variable
// its name is the same as the function's
function myLine(const x: real): real;
begin
myLine := 0.5 * x + 2;
end;
If {$modeswitch result+}, which is set by {$mode objFPC} and {$mode Delphi}, inside the implementation block the special identifier result is available, too:
// using special `result` identifier
function myParabola(const x: real): real;
begin
result := sqr(x) - 1;
end;
Additionally, in {$mode objFPC} the routine exit will set the return value, too, and leave the stack frame.
In the previous two examples further statements could have appeared, and they would have been executed, whilst after an exit the routine is done.
This is the behavior a return statement in C or other programming languages has.
// using exit routine
function even(const x: longint): Boolean;
begin
exit(not odd(x));
end;
In assembly language other rules apply. If the return type is an integral value, the accumulator register is used, provided it fits in there:
// in assembly language:
// return type fits into a single register => use accumulator register
function zero(const x: int64): Boolean;
{$ifDef CPUx86_64}
assembler; register;
{$asmMode intel}
asm
// xor modifies flags => put it in front of test
xor rax, rax // rax := 0 (remove residue)
test x, x // x = 0 ?
setz al // rax := ZF
// When you examine the assembler output
// you will notice the compiler automatically inserts code
// that moves the contents of rax to the right spot on the stack,
// unless the noStackFrame hint
// (automatically set by some optimization levels)
// instructs the compiler to omit the stack frame if possible.
end;
{$else}
begin
result := x = 0;
end;
{$endIf}
Otherwise, depending on which {$asmMode} is active, the @result (Intel) or __result (AT&T) macro can be used.
type
bodyAttributes = record
surfaceArea: real;
volume: real;
end;
// in assembly language:
// return type doesn't fit into accumulator => @result macro gives address
function sphere(const radius: real): bodyAttributes;
{$ifDef CPUx86_64}
assembler;
{$asmMode intel}
const
three: longint = 3;
four: longint = 4;
var
r: real;
asm
pextrq r, radius, 0 // r := (@radius+0)^
lea rax, @result // rax := @result
fld r // radius
fld st(0) // radius radius
fild four // 4 radius radius
fldpi // pi 4 radius radius
fmul // 4*pi radius radius
fxch // radius 4*pi radius
fld st(0) // radius radius 4*pi radius
fmul // radius^2 4*pi radius
fmul // 4*pi*radius^2 radius
fst [rax].bodyAttributes.surfaceArea
fmul // 4*pi*radius^3
fild three // 3 4*pi*radius^3
fdivp // 4/3*pi*radius^3
fst [rax].bodyAttributes.volume
end;
{$else}
begin
sphere.surfaceArea := 4 * pi() * sqr(radius);
sphere.volume := 4 / 3 * pi() * sqr(radius) * abs(radius);
end;
{$endIf}
Originally Pascal expected exact one assignment to the result variable (whichever is used). FPC however does not prohibit multiple assignments. It will emit a warning if none of the possible result identifiers were used or the exit routine is not written.
- Warning
- Function result does not seem to be set
- You can get this warning if the compiler thinks that a function return value is not set. This will not be displayed for assembler procedures, or procedures that contain assembler blocks.
begin
writeLn(sphere(2.0).surfaceArea);
end.
Note, in the case of operator overloads, a special form of a function, a result variable identifier has to be declared in the formal signature. See the article for details.
Remarks
Pure functions are functions that do not depend on an external state.
