我不确定我在这里看到了什么。下面的测试结果来自运行OS X 10.5.8的旧PowerMac G5。它仍然在大端PowerPC cpu下进行测试。编译器是GCC 5.4，由MacPorts提供。
测试程序如下。编译它会导致错误 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。
为什么程序无法编译？
$ /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 );
^
这可能与旧硬件相关：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"
答案 0 :(得分：1)
为什么程序无法编译？
海湾合作委员会在Issue 79017: Old PowerMac G5, MacPorts GCC 5.4, C++11 and "std::log2 has not been declared"的报告得到了海湾合作委员会开发者的一些好评。
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);由于缺少这六个功能，我们会对所有C99数学进行处理 功能缺失。我们可以把支票分成两部分，所以我们 分别检查这些，这将允许其余的C99数学 要导入到名称空间std。
的库
#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;
}