写一个简单的编译时间std::array
工厂从一个生成器函数,我偶然发现:铿锵++ 3.5.1和g ++ 4.9.2不同意函数是否是constexpr
或不。gcc和clang不同意constexpr函数
的代码(这是C++ 14!):
#include <array>
#include <utility>
template <class T, std::size_t N, class GenType, std::size_t... I>
constexpr std::array<T, N>
make_array_impl (GenType gen, std::index_sequence <I...>)
{
return {{ gen (I)... }};
}
template <class T, std::size_t N, class GenType>
constexpr std::array<T, N>
make_array (GenType gen)
{
return make_array_impl <T, N> (
gen,
std::make_index_sequence <N> {}
);
}
constexpr int
generator_const (std::size_t /* index */)
{
return 1;
}
constexpr auto
a = make_array <int, 3> (generator_const);
static_assert (a.size() == 3, "");
static_assert (a[0] == 1, "");
static_assert (a[1] == 1, "");
static_assert (a[2] == 1, "");
int main() {}
与克++编译:
migou ~ % g++ -std=c++14 ex.cpp
ex.cpp:28:41: in constexpr expansion of ‘make_array<int, 3ul, int (*)(long unsigned int)>(generator_const)’
ex.cpp:18:5: in constexpr expansion of ‘make_array_impl<int, 3ul, int (*)(long unsigned int), {0ul, 1ul, 2ul}>(gen, (std::make_index_sequence<3ul>{}, std::make_index_sequence<3ul>()))’
ex.cpp:8:21: error: expression ‘generator_const’ does not designate a constexpr function
return {{ gen (I)... }};
随着铛++它编译就好了。我可以继续考虑这个有效的g ++ 14(因此g ++ bug)吗?
'g ++'在['g ++'version __5__](https://gcc.gnu.org/projects/cxx1y.html)放宽了'constexpr'的要求,所以也许就是这个原因。 'clang'已经在3.4版本中实现了N3638。 – Zeta 2015-02-10 16:11:21