gcc 7.2.0无法识别std :: expf和std :: logf [英] std::expf and std::logf not recognized by gcc 7.2.0

问题描述

似乎gcc（尝试7.2.0和5.4.0）没有 std :: expf 和 std :: logf -请参阅coliru 样本。由于cppreference表示它们是在C ++ 11中添加的，我是否缺少某些gcc特定宏，或者它们在gcc中总体上还是缺失的？

It seems that gcc (tried 7.2.0 and 5.4.0) does not have std::expf and std::logf - see coliru sample. Since cppreference says they were added in C++11 is there some gcc specific macro I am missing or are they in general missing in gcc?

推荐答案

这是libstdc ++中的错误，该错误与GCC一起提供。它不完全符合C ++ 17（ as于2019年6月的v9.1.0版本）。 Ubuntu默认使用的< cmath> 版本的版权声明说它的最新更新时间是2016年。版本9.1.0的确有 #if __cplusplus> 201402L 部分，但未声明C ++ 17所需的标识符。 有未解决的错误报告。

This is a bug in libstdc++, which ships with GCC. It does not fully conform to C++17 (as of v9.1.0 in June 2019). The copyright notice on the version of <cmath> that Ubuntu uses by default says it was last updated in 2016. Version 9.1.0 does have a #if __cplusplus > 201402L section, but it doesn’t declare the identifiers required by C++17. There is an open bug report.

它从不声明 expf 或 logf （也不声明 cosf ， sinf 等），即使 C ++ 17表示应 C ++ 11 standard 说，在C中定义为函数的名称应在C ++标准库中定义为函数，标准C库中使用外部链接声明的每个名称都保留给实现，以用作在命名空间std 和全局命名空间中都具有外部 C 链接的名称。但是， std :: expf 等。从< cmath> 提供的功能表中丢失，直到 P0175r1在2016年6月。这显然是一个疏忽，但GCC始终只在全局命名空间中提供它们。

It never declares expf or logf (nor cosf, sinf, etc.) within the std:: namespace, even though C++17 says it shall. The C++11 standard says, "Names that are defined as functions in C shall be defined as functions in the C++ standard library," and "Each name from the Standard C library declared with external linkage is reserved to the implementation for use as a name with extern "C" linkage, both in namespace std and in the global namespace." However, std::expf et al. are missing from the table of functions provided by <cmath> until P0175r1 in June 2016. This was apparently an oversight, but GCC has always made them available only in the global namespace.

libc ++ 库确实声明了它们，因此可以使用 clang ++ -std = c ++ 17 -stdlib = libc ++ 进行编译。您还可以 #include< math.h> 在全局名称空间中使用它们，或使用重载的 exp（）， log（），等等。

The libc++ library does declare them, so compiling with clang++ -std=c++17 -stdlib=libc++ should work. You can also #include <math.h> to use them in the global namespace, or use the overloaded exp(), log(), etc. on float arguments.

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