`cosf`、`sinf` 等不在 `std` 中

Question

根据此处的讨论，我已 reported a bug 致 Ubuntu 开发人员。

编译以下示例 C++ 程序时：

#include <cmath>
#include <stdio.h>

int main()
{
    printf("%f\n", std::cosf(0.0f));
}

我收到以下错误消息：error: ‘cosf’ is not a member of ‘std’

包括 math.h 并使用非命名空间版本工作正常。怎么回事？

我在 Ubuntu 19.04 上使用 g++ 8.3.0-6ubuntu1。

我正在用 g++ --std=c++17 test.cpp

构建

Answer 1

那个版本的库（libstdc++8) is not fully conforming to C++17. The copyright notice says it was last updated in 2016. As of June 2019, the latest upstream release is bugged. It does have a #if __cplusplus > 201402L section, but it doesn’t declare the identifiers required by C++17. There is an open bug report.

查看 Ubuntu 上的 /usr/include/c++/8/cmath，它包括 <math.h>，取消定义其函数的一系列宏（C 标准库要求）以访问它们的名称，导入 cos、acos 等进入 std:: 命名空间，然后将重载的 float 和 long double 重载声明为 inline.

它从未在 std:: 命名空间内声明 cosf，尽管 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, it does not explicitly state that std::expf et al. must be supported until P0175r1 in June 2016. 这显然是一个疏忽。

libc++ 库确实声明了它们，因此使用 clang++ -std=c++17 -stdlib=libc++ 编译应该可以。

`cosf`、`sinf` 等不在 `std` 中

`cosf`, `sinf`, etc. are not in `std`

c++

gcc

libstdc++