C++ STL and multiple definitions

Question

While searching the doc on cppreference.com, I've seen that some features are defined multiple times in different headers...

For example: std::move (in <algorithm> and <utility>), std::size_t, etc (see below).

The page about std::size_t starts with this precision:

Defined in header <cstddef>
Defined in header <cstdio>
Defined in header <cstdlib>
Defined in header <cstring>
Defined in header <ctime>
Defined in header <cuchar>                (since C++17)
Defined in header <cwchar>

Why is this the case? And which header should one choose rather than any other?

score 2 · Accepted Answer · answered May 13 '21 at 11:37

Every standard library header needs to be self contained. If we want to use it, we are not to be forced into including anything else. Since all of the headers in the list you cite end up using size_t in some capacity, including them must also make size_t available. So it's standard mandated behavior.

Mind you, that the wording in cppreference is a bit misleading. It's not that every header always defines the type alias. It's more than likely that the actual definition is in some internal implementation specific header, that all of those public headers include it themselves.

So the behavior in essence, is that there is only ever one "true" definition of size_t. And you can get it by including any of the above headers.

As for which one to choose? You can examine the synopsis of each header. If all you need is size_t then cstddef is the most minimal header that includes it.

I don't have enough reputation to upvate, but I'd like to let you know that I would have upvoted your answer! Thank a lot, really helpful! — stephane, May 14 '21 at 04:59

score 1 · Answer 2 · answered May 13 '21 at 11:30

Why is this the case?

2 reasons I can think of.

The function declarations and anything else in the headers use size_t and therefore need to declare it (same with any other thing).
(Maybe) Convenience. size_t for example is used in a lot of cases so it's simpler to have size_t just from including, for example <iostream> rather than having to include <cstddef> too.

And which header should one choose rather than any other?

It appears someone else had the same question. Though I'd say take advantage of size_t being in multiple headers. If you only need <iostream> for a program, no need to include <cstddef> as well just for the sake of it.

Thanks a lot for your answer, it helps! I don't have enough reputation to upvote, sorry. — stephane, May 14 '21 at 05:02

score -2 · Answer 3 · answered May 13 '21 at 11:56

-2

You can simply add #include<bits/stdc++.h> that will import all the necessary libraries for you, But sometimes it is not advisable to include all the libraries then you have to include specific libraries.

answered May 13 '21 at 11:56

Mohsin Mehmood

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1

Is that a standard header? – Surt May 13 '21 at 12:07
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[It's *never* advisable](https://stackoverflow.com/questions/31816095/why-should-i-not-include-bits-stdc-h). – mediocrevegetable1 May 13 '21 at 12:14
Yeah, most of the time it is not advisable just like not to add using namespace std but if someone is doing competitive coding it is advisable to just put this library to avoid wasting any time. – Mohsin Mehmood May 13 '21 at 12:52
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This doesn't answer the question, also it's not a standard header – al3c May 13 '21 at 12:52
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@Surt -- no. It comes from one specific compiler. – Pete Becker May 13 '21 at 13:31
@MohsinMehmood -- if you're going to recommend compiler-specific approaches to problems you should say that that's the case. – Pete Becker May 13 '21 at 13:32

C++ STL and multiple definitions

3 Answers3