In the C programming language the effect of call be reference can be achieved by passing a reference as a value. Knowledge of this reference can then be used to alter the value held in it. This mechanism thus only simulates call by reference and should (more accurately) be referred to as call be reference value.

C Example:

#include <stdio.h>

void meanValue(float *, float);

void main(void)
{
float value_1, value_2;

scanf("%d %s\n",&value_1,&value_2):

meanValue(&value_1, value_2);

printf("Mean value_1 = %f\n",value_1);
}

void meanValue(float *num_1, float mum_2)
{
printf("num_1 = %f, num_2 = %f\n",*num_1,num_2);
*num_1 = (*num_1+num_2)/2.0;
}

Remember that the operator & is interpreted as "the address of ...". And the operator * when followed by a pointer is interpreted as "the variable pointed at by ...".

Call by result is used in Ada to implement out mode parameter passing. The formal parameter acts as an uninitialised local variable which is given a value during execution of the procedure. The value of the formal parameter is then assigned to the actual parameter on returning from the routine.

Ada Example:

with CS_IO; use CS_IO;

procedure OUT_MODE_EXAMPLE is
        VALUE_1, VALUE_2: float;
        MEAN: float;

        procedure MEAN_VALUE (NUM_1, NUM_2: in float; NUM_3: out float) is
        begin
                NUM_3:= (NUM_1+NUM_2)/2.0;
        end MEAN_VALUE;

begin
        MEAN_VALUE(VALUE_1, VALUE_2, MEAN);
        put("The mean is ");
        put(MEAN);new_line;
        new_line;
end OUT_MODE_EXAMPLE;

Call by copy restore (also known as call by value result) is an amalgamation of call by value and call by result. The formal parameter acts as a local variable which is initialised to the value of the actual parameter. Within the routine, changes to the formal parameter only affect the local copy. On returning from the routine the final value of the formal parameter is assigned to the actual parameter. Call by copy restore is supported by Ada to achieve "in-out" parameter operation. It has the same disadvantages as those associated with call by value.

Ada Example:

with CS_IO; use CS_IO;

procedure IN_OUT_MODE_EXAMPLE is
        VALUE_1, VALUE_2: float;

        procedure MEAN_VALUE (NUM_1: in out float; NUM_2: in float) is
        begin
                NUM_1:= (NUM_1+NUM_2)/2.0;
        end MEAN_VALUE;

begin
        get(VALUE_1);get(VALUE_2);
        put("BEFORE MEAN VALUE: VALUE_1 = ");
        put(VALUE_1);put(", VALUE_2 = ");put(VALUE_2);new_line;
        MEAN_VALUE(VALUE_1, VALUE_2);
        put("The mean is ");
        put(VALUE_1);new_line;
        put("AFTER MEAN VALUE: VALUE_1 = ");
        put(VALUE_1);put(", VALUE_2 = ");put(VALUE_2);new_line;
end IN_OUT_MODE_EXAMPLE;

The above all assumes that we wish to pass data items. Some languages support passing of procedures (e.g. Pascal). Example: in numerical analysis, where methods for finding (say) roots of functions etc. can be written to be independent of any particular routine, it useful to be able to pass appropriate functions to solving routine.

Remember to switch of the hot water: return to imperative home page.