Will gcc remove unused class field, in the final executable file?

Question

For example, will gcc/g++(version 9.4, or 7.3) remove unused c and pc in the output file, because this piece of code access only C::i and C::j, but not C::c andC::pc.

#include <iostream> struct C { // i, j is used below int i; int j; // c, pc is not used char c; char *pc; C() : i(2) {} }; int main() { C c; c.j = 9; std::cout << c.j << c.i << sizeof(c) << std::endl; }

The problem I am confused with is almost solved, inspired by the site.

Now I understand that the field is accessed as a pointer plus an offset, thus without an access, there will be no code representing as such a pointer plus an offset, in the final executable file.

Read about padding. Also see dupe: How to get rid of padding bytes between data members of a struct — Richard
– Richard, Commented May 31, 2023 at 9:22
The size of a structure is a multiple of the alignment requirement of a data member with the largest alignment requirement. — Richard
– Richard, Commented May 31, 2023 at 9:24
What would it mean to "remove unused class fields"? How would you know whether gcc "removed" them? At least the value of sizeof(c) must be computed as if the members are present. Do the fields now count as used or as unused in your book? — j6t
– j6t, Commented May 31, 2023 at 10:29
in the code you posted c and pc are not "unused". Its just comments. Comments are only relevant for human readers but not for anything else — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented May 31, 2023 at 12:23
Not sure this is really a duplicate or even about padding. Are you saying that the sizeof(c) gives the same output with or without the char *pc; line commented-out? — Adrian Mole
– Adrian Mole, Commented Jun 1, 2023 at 5:57

n. m. could be an AI · Accepted Answer · 2023-06-05 05:28:50Z

Compilers will not optimize out unused struct fields. In theory they might be allowed (but not required) to do so, but in practice it never happens. One reason is that the compiler normally sees only one translation unit at a time, and can never be sure that the field is not accessed in other translation units.

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