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 libstdc++ shipped with GCC older than 5.1 (and also used by Clang on linux) does not implement std::is_trivially_default_constructible but std::has_trivial_default_constructor instead. With GCC, this is done with a simple check of the version of GCC. In the case of the clang - linux combination, libstdc++ may be used. Since clang overrides the __GNUC__ macro, the version of libstdc++ used cannot be retrived at runtime and some meta-programming techniques are used to determine which function is available.

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.