source is compiled without proper #include

Question

I have a very simple c++ source like this:

#include <iostream> int main() { srand(time(NULL)); }

I am using g++ to compile like this :

g++ ./test.cpp

but it successfully compiles despite the fact that time() function is defined in ctime and it is not included with #include

my professor at university runs the code with visual studio (vc++) but he is unable to run the code without including ctime

Am I missing something here ?

by the way my g++ version is :

g++ (Ubuntu 11.2.0-7ubuntu2) 11.2.0

Standard headers are allowed to include other standard headers. Don't rely on this. Always include headers you need — Remy Lebeau
– Remy Lebeau, Commented Dec 14, 2021 at 6:41
on a side note for C++ random, have a look at the <random> header file. (srand is IMO a bit of a "C" left over) — Pepijn Kramer
– Pepijn Kramer, Commented Dec 14, 2021 at 6:44
@RemyLebeau yes.final conclusion is exactly what you said.but it can be misleading for a new c++ programmer! — Mohamadreza Nakhleh
– Mohamadreza Nakhleh, Commented Dec 14, 2021 at 7:27
@PepijnKramer yes you are right . actually this c++ code is translated from a c counterpart ! — Mohamadreza Nakhleh
– Mohamadreza Nakhleh, Commented Dec 14, 2021 at 7:28

nort3x · Accepted Answer · 2021-12-14 12:59:46Z

first of all explicitly include what you need,
(thanks to darkblueflow for pointing it out)
secondly #include order matters,

believe or not they can shadow declaration, just switch if first case doesn't work

#include <cstdlib> #include <ctime> // differs from #include <ctime> #include <cstdlib> // in some aspects

your best shot is to include headers explicitly and keep this in mind this good luck

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