Why clang on Mac automatically includes some missing headers?

Question

I noticed that clang++ includes a missing header - <limits> on Mac, while g++ shows errors about it on Linux. Now I wonder why clang does it, and gcc not. And how I can force clang to not do it.

Here is a sample code which compiles by clang on Mac, but not by gcc on Linux:

#include <iostream>
using namespace std;

int main()
{
    cout << "int max: " << numeric_limits<int>::max() << endl;
}

UPD

I looked into libraries and here is what I found.

Internally <iostream> includes <istream>, which defines >> operator for different types. <istream> wants to know limits for short, int and streamsize types.

clang++ uses libc++ standard library, which uses std::numeric_limits class template from <limits> in <istream> for this purpose. That's why this header is included automatically when <iostream> is included.

g++ uses libstdc++ standard library, which uses __gnu_cxx::__numeric_traits class template from <ext/numeric_traits.h> instead of using <limits> in <istream> (<bits/istream.tcc>). There is also a comment in that header which explains why they don't use <limits>:

<limits> is big and we avoid including it

Used compilers:

> clang++ --version
Apple LLVM version 8.0.0 (clang-800.0.42.1)

$ g++ --version
g++ (Debian 4.9.2-10) 4.9.2

Pete Becker Pete Becker · Accepted Answer · 2016-11-26T15:30:05

In C++, unlike C, standard headers are allowed to #include other standard headers. That sometimes leads to mysterious errors like the ones you're seeing: one compiler's <iostream> includes <limits> and the other doesn't. The solution is to always include the headers needed for whatever names you use. In this case that means to #include <limits> in your code, even though it compiles okay as is with one compiler. There's no harm in #include a header that's already been pulled in, so that's okay with both compilers. It's annoying sometimes, but that's just the way things are.

Why clang on Mac automatically includes some missing headers?

2 Answers