CODEWITHSUNDEEP
c++cperformancegcccompilation
user
user

Reputation: 25738

Difference between “static const”, “#define”, and “enum” in performance and memory usage aspects

There might be any because of inlining of #define statements.

I understand that answer may be compiler dependent, lets asume GCC then.

There already are similar questions about C and about C++, but they are more about usage aspects.

Upvotes: 4

Views: 840

Answers (2)

Leeor
Leeor

Reputation: 19706

The compiler would treat them the same given basic optimization.
It's fairly easy to check - consider the following c code :

#define a 1
static const int b = 2;
typedef enum {FOUR = 4} enum_t;

int main() {

    enum_t c = FOUR;

    printf("%d\n",a);
    printf("%d\n",b);
    printf("%d\n",c);

    return 0;
}

compiled with gcc -O3:

0000000000400410 <main>:
  400410:       48 83 ec 08             sub    $0x8,%rsp
  400414:       be 01 00 00 00          mov    $0x1,%esi
  400419:       bf 2c 06 40 00          mov    $0x40062c,%edi
  40041e:       31 c0                   xor    %eax,%eax
  400420:       e8 cb ff ff ff          callq  4003f0 <printf@plt>
  400425:       be 02 00 00 00          mov    $0x2,%esi
  40042a:       bf 2c 06 40 00          mov    $0x40062c,%edi
  40042f:       31 c0                   xor    %eax,%eax
  400431:       e8 ba ff ff ff          callq  4003f0 <printf@plt>
  400436:       be 04 00 00 00          mov    $0x4,%esi
  40043b:       bf 2c 06 40 00          mov    $0x40062c,%edi
  400440:       31 c0                   xor    %eax,%eax
  400442:       e8 a9 ff ff ff          callq  4003f0 <printf@plt>

Absolutely identical assembly code, hence - the exact same performance and memory usage.

Edit: As Damon stated in the comments, there may be some corner cases such as complicated non literals, but that goes a bit beyond the question.

Upvotes: 6

David Rodr&#237;guez - dribeas
David Rodr&#237;guez - dribeas

Reputation: 208353

When used as a constant expression there will be no difference in performance. If used as an lvalue, the static const will need to be defined (memory) and accessed (cpu).

Upvotes: 3

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