The assignment operator defines the action of a assignent of one type of variable to another. The
result type must match the type of the variable at the left of the assignment statement, the single
parameter to the assignment operator must have the same type as the expression at the right of
the assignment operator.
This system can be used to declare a new type, and define an assignment for that type. For
instance, to be able to assign a newly defined type ’Complex’
Var
C,Z : Complex; // New type complex
begin
Z:=C; // assignments between complex types.
end;
|
The following assignment operator would have to be defined:
Operator := (C : Complex) z : complex;
|
To be able to assign a real type to a complex type as follows:
var
R : real;
C : complex;
begin
C:=R;
end;
|
the following assignment operator must be defined:
Operator := (r : real) z : complex;
|
As can be seen from this statement, it defines the action of the operator := with at the right a real
expression, and at the left a complex expression.
an example implementation of this could be as follows:
operator := (r : real) z : complex;
begin
z.re:=r;
z.im:=0.0;
end;
|
As can be seen in the example, the result identifier (z in this case) is used to store the result of the
assignment. When compiling in Delphi mode or objfpc mode, the use of the special identifier
Result is also allowed, and can be substituted for the z, so the above would be equivalent
to
operator := (r : real) z : complex;
begin
Result.re:=r;
Result.im:=0.0;
end;
|
The assignment operator is also used to convert types from one type to another. The compiler will
consider all overloaded assignment operators till it finds one that matches the types of the left
hand and right hand expressions. If no such operator is found, a ’type mismatch’ error is
given.
Remark: The assignment operator is not commutative; the compiler will never reverse the role of the two
arguments. in other words, given the above definition of the assignment operator, the following is
not possible:
var
R : real;
C : complex;
begin
R:=C;
end;
|
if the reverse assignment should be possible (this is not so for reals and complex numbers) then the
assigment operator must be defined for that as well.
Remark: The assignment operator is also used in implicit type conversions. This can have unwanted effects.
Consider the following definitions:
operator := (r : real) z : complex;
function exp(c : complex) : complex;
|
then the following assignment will give a type mismatch:
Var
r1,r2 : real;
begin
r1:=exp(r2);
end;
|
because the compiler will encounter the definition of the exp function with the complex argument.
It implicitly converts r2 to a complex, so it can use the above exp function. The result of this
function is a complex, which cannot be assigned to r1, so the compiler will give a ’type
mismatch’ error. The compiler will not look further for another exp which has the correct
arguments.
It is possible to avoid this particular problem by specifying
An experimental solution for this problem exists in the compiler, but is not enabled by default.
Maybe someday it will be.