Compiler workarounds

This page tracks the workarounds for the various compiler issues that we encountered in the development. This is mostly of interest for developers interested in contributing to xtensor.

Visual Studio 2015 and std::enable_if

With Visual Studio, std::enable_if evaluates its second argument, even if the condition is false. This is the reason for the presence of the indirection in the implementation of the xfunction_type_t meta-function.

GCC-4.9 and Clang < 3.8 and constexpr std::min and std::max

std::min and std::max are not constexpr in these compilers. In xio.hpp, we locally define a XTENSOR_MIN macro used instead of std::min. The macro is undefined right after it is used.

Clang < 3.8 matching initializer_list with static arrays

Old versions of Clang don’t handle overload resolution with braced initializer lists correctly: braced initializer lists are not properly matched to static arrays. This prevent compile-time detection of the length of a braced initializer list.

A consequence is that we need to use stack-allocated shape types in these cases. Workarounds for this compiler bug arise in various files of the code base. Everywhere, the handling of Clang < 3.8 is wrapped with checks for the X_OLD_CLANG macro.

GCC < 5.1 and std::is_trivially_default_constructible

The version of the STL shipped with versions of GCC older than 5.1 are missing a number of type traits, such as std::is_trivially_default_constructible. However, for some of them, equivalent type traits with different names are provided, such as std::has_trivial_default_constructor.

In this case, we polyfill the proper standard names using the deprecated std::has_trivial_default_constructor. This must also be done when the compiler is clang when it makes use of the GCC implementation of the STL, which is the default behavior on linux. Properly detecting the version of the GCC STL used by clang cannot be done with the __GNUC__ macro, which are overridden by clang. Instead, we check for the definition of the macro _GLIBCXX_USE_CXX11_ABI which is only defined with GCC versions greater than 5.

GCC-6 and the signature of std::isnan and std::isinf

We are not directly using std::isnan or std::isinf for the implementation of xt::isnan and xt::isinf, as a workaround to the following bug in GCC-6 for the following reason.

  • C++11 requires that the <cmath> header declares bool std::isnan(double) and bool std::isinf(double).
  • C99 requires that the <math.h> header declares int ::isnan(double) and int ::isinf(double).

These two definitions would clash when importing both headers and using namespace std.

As of version 6, GCC detects whether the obsolete functions are present in the C <math.h> header and uses them if they are, avoiding the clash. However, this means that the function might return int instead of bool as C++11 requires, which is a bug.