How to correctly cast time_t to long int?

Question

I'm still learning about type casting in C++ and I'm currently doing this

long int t = time(NULL);

I'm using VS2013 and noticed the conversion from 'time_t' to 'long' warning so I thought I would type cast it to look like;

long int t = static_cast<long int> time(NULL);

However this doesn't work yet combining a static cast and a C-style cast works

long int t = static_cast<long int> (time(NULL));

I was just wondering if anyone could help shed some light on this?

That's not "combining a static cast and a C-style cast", it's just the correct syntax for a static cast, which requires `()` around its argument. — Mike Seymour, Jan 20 '15 at 12:23
Thank you, I completely missed the requirement to put a static cast value in the parenthesis. I've learn't something new. — Ryanas, Jan 20 '15 at 12:29
Instead of merely restricting the result to the range of `long`, which still (as of 2014) leaves you many years of future dates, you could cast to `unsigned char`, shortening that interval to just 255 seconds into 1970. — Cheers and hth. - Alf, Jan 20 '15 at 12:31
http://stackoverflow.com/questions/9483974/converting-time-t-to-int — Ciro Santilli OurBigBook.com, Jun 29 '15 at 15:15

Sebastian · Accepted Answer · 2015-01-20T12:31:19.477

7

time(NULL) is not a cast but a function call which returns time_t. Since time_t is not exactly the same type as long int, you see the warning.

Furthermore, static_cast<T>(value) requires the parenthesis, that is why your first version does not work.

edited Jan 20 '15 at 12:31

answered Jan 20 '15 at 12:19

Sebastian

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Thank you, I missed that. Marking as answer when I can. – Ryanas Jan 20 '15 at 12:27
It says I can accept an answer in two minutes. I will do so after those two minutes. – Ryanas Jan 20 '15 at 12:29

score 2 · Answer 2 · answered Jan 20 '15 at 12:33

Your question contains the answer. The static_cast generic method in the code you provide takes the time_t type as its input and converts it to a long int as its return value. This code does not contain a C-style type-cast.

long int t = static_cast<long int> (time(NULL));

Type-casting should also work too, because time_t is an arithmetic type and the C cast operator will perform the promotion to the long int type.

long int t = (long int)time(NULL);

This casting tutorial might be an interesting read for you.

score 0 · Answer 3 · answered Jan 20 '15 at 12:25

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A time_t value is the number of seconds since the start of Jan 1 1970. Casting that to 32-bit long you therefore restrict yourself to values representing time values before the year 2038, roughly. That's not a good idea, and the ungoodness of it is the reason for your warning.

The attempted expression

static_cast<long int> time(NULL)

is just invalid syntax. A static_cast requires a parenthesis with the value.

answered Jan 20 '15 at 12:25

Cheers and hth. - Alf

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Is there no big/little endian matter related to this, which needs to use `ntohl` to be compatible everywhere? – Sandburg Jul 09 '19 at 15:57

How to correctly cast time_t to long int?

3 Answers3