error: 'log2' is not a member of 'std'

Question

我不确定我在这里看到了什么。下面的测试结果来自旧的 PowerMac G5 运行 OS X 10.5.8。它仍然在 big-endian PowerPC cpu 下进行测试。编译器为GCC 5.4，由MacPorts提供。

测试程序如下。编译会报错error: 'log2' is not a member of 'std'.

$ cat test.cxx
#include <cmath>
int main(int argc, char* argv[])
{
    double d = std::log2(2.3456789f);
    return 0;
}

根据 Is std::log2() an extension or C++ standard?，std::log2 应该可用。

为什么程序编译失败？

$ /opt/local/bin/g++-mp-5 --version
g++-mp-5 (MacPorts gcc5 5.4.0_0) 5.4.0
Copyright (C) 2015 Free Software Foundation, Inc.

$ /opt/local/bin/g++-mp-5 -std=c++11 test.cxx -o test.exe
test.cxx: In function 'int main(int, char**)':
test.cxx:4:16: error: 'log2' is not a member of 'std'
     double d = std::log2(2.3456789f);
                ^
test.cxx:4:16: note: suggested alternative:
In file included from /usr/include/math.h:26:0,
                 from /opt/local/include/gcc5/c++/cmath:44,
                 from test.cxx:1:
/usr/include/architecture/ppc/math.h:431:15: note:   'log2'
 extern double log2( double );
               ^

这可能与旧硬件有关：C++11 cmath functions not in std namespace for android NDK w/gcc-4.8 or clang 3.4。我意识到 Android 是一个不同的平台。

现在在 MacPorts 问题跟踪器中打开：Issue 53226: PowerMac, GCC 5.4 and "error: 'std::log2' has not been declared"。

现在在 GCC 问题跟踪器中打开：Issue 79017: Old PowerMac G5, MacPorts GCC 5.4, C++11 and "std::log2 has not been declared"

Answer 1

Why is the program failing to compile?

Issue 79017: Old PowerMac G5, MacPorts GCC 5.4, C++11 and "std::log2 has not been declared" 的 GCC 报告得到了 GCC 开发人员的一些好评。

根据Jonathan Wakely and the results of the program below：

test.cxx:53:21: error: 'llrint' was not declared in this scope
llrint(0.0);

test.cxx:54:23: error: 'llrintf' was not declared in this scope
llrintf(0.0f);

test.cxx:55:23: error: 'llrintl' was not declared in this scope
llrintl(0.0l);

test.cxx:56:22: error: 'llround' was not declared in this scope
llround(0.0);

test.cxx:57:24: error: 'llroundf' was not declared in this scope
llroundf(0.0f);

test.cxx:58:24: error: 'llroundl' was not declared in this scope
llroundl(0.0l);   
Because these six functions are missing we treat all C99 math functions as missing. We could split the checks into two pieces, so we check for these separately, which would allow the rest of the C99 math library to be imported into namespace std.

#include <math.h>
int main() {
          typedef double_t  my_double_t;
          typedef float_t   my_float_t;
          acosh(0.0);
          acoshf(0.0f);
          acoshl(0.0l);
          asinh(0.0);
          asinhf(0.0f);
          asinhl(0.0l);
          atanh(0.0);
          atanhf(0.0f);
          atanhl(0.0l);
          cbrt(0.0);
          cbrtf(0.0f);
          cbrtl(0.0l);
          copysign(0.0, 0.0);
          copysignf(0.0f, 0.0f);
          copysignl(0.0l, 0.0l);
          erf(0.0);
          erff(0.0f);
          erfl(0.0l);
          erfc(0.0);
          erfcf(0.0f);
          erfcl(0.0l);
          exp2(0.0);
          exp2f(0.0f);
          exp2l(0.0l);
          expm1(0.0);
          expm1f(0.0f);
          expm1l(0.0l);
          fdim(0.0, 0.0);
          fdimf(0.0f, 0.0f);
          fdiml(0.0l, 0.0l);
          fma(0.0, 0.0, 0.0);
          fmaf(0.0f, 0.0f, 0.0f);
          fmal(0.0l, 0.0l, 0.0l);
          fmax(0.0, 0.0);
          fmaxf(0.0f, 0.0f);
          fmaxl(0.0l, 0.0l);
          fmin(0.0, 0.0);
          fminf(0.0f, 0.0f);
          fminl(0.0l, 0.0l);
          hypot(0.0, 0.0);
          hypotf(0.0f, 0.0f);
          hypotl(0.0l, 0.0l);
          ilogb(0.0);
          ilogbf(0.0f);
          ilogbl(0.0l);
          lgamma(0.0);
          lgammaf(0.0f);
          lgammal(0.0l);
          llrint(0.0);
          llrintf(0.0f);
          llrintl(0.0l);
          llround(0.0);
          llroundf(0.0f);
          llroundl(0.0l);
          log1p(0.0);
          log1pf(0.0f);
          log1pl(0.0l);
          log2(0.0);
          log2f(0.0f);
          log2l(0.0l);
          logb(0.0);
          logbf(0.0f);
          logbl(0.0l);
          lrint(0.0);
          lrintf(0.0f);
          lrintl(0.0l);
          lround(0.0);
          lroundf(0.0f);
          lroundl(0.0l);
          nan(0);
          nanf(0);
          nanl(0);
          nearbyint(0.0);
          nearbyintf(0.0f);
          nearbyintl(0.0l);
          nextafter(0.0, 0.0);
          nextafterf(0.0f, 0.0f);
          nextafterl(0.0l, 0.0l);
          nexttoward(0.0, 0.0);
          nexttowardf(0.0f, 0.0f);
          nexttowardl(0.0l, 0.0l);
          remainder(0.0, 0.0);
          remainderf(0.0f, 0.0f);
          remainderl(0.0l, 0.0l);
          remquo(0.0, 0.0, 0);
          remquof(0.0f, 0.0f, 0);
          remquol(0.0l, 0.0l, 0);
          rint(0.0);
          rintf(0.0f);
          rintl(0.0l);
          round(0.0);
          roundf(0.0f);
          roundl(0.0l);
          scalbln(0.0, 0l);
          scalblnf(0.0f, 0l);
          scalblnl(0.0l, 0l);
          scalbn(0.0, 0);
          scalbnf(0.0f, 0);
          scalbnl(0.0l, 0);
          tgamma(0.0);
          tgammaf(0.0f);
          tgammal(0.0l);
          trunc(0.0);
          truncf(0.0f);
          truncl(0.0l);
          return 0;
}

error: 'log2' is not a member of 'std'

error: 'log2' is not a member of 'std'

c++

c++11

macports

gcc5