Chapter 7: Portability - TOC - 7.3 STORAGE CONTROL

7.3.2 Access-to-Subprogram Values

guideline

Do not compare access-to-subprogram values.

rationale

The Ada Reference Manual (1995, §3.10.2) explains that an "implementation may consider two access-to-subprogram values to be unequal, even though they designate the same subprogram. This might be because one points directly to the subprogram, while the other points to a special prologue that performs an Elaboration_Check and then jumps to the subprogram." The Ada Reference Manual (1995, §4.5.2) states that it is "unspecified whether two access values that designate the same subprogram but are the result of distinct evaluations of Access attribute references are equal or unequal."

See also Guideline 5.3.4.

exceptions

If you must compare an access-to-subprogram value, you should define a constant using the access-to-subprogram value and make all future comparisons against the constant. However, if you attempt to compare access-to-subprogram values with different levels of indirection, the values might still be unequal, even if designating the same subprogram.