在下面的代码中,我尝试构建类型网格。例如,之间float
and int
,将结果推广为float
:
float join(float f, int) { return f; }
float join(float f, float) { return f; }
然后我介绍一个wrapper
type:
template <typename Inner>
struct wrapper
{
using inner_t = Inner;
inner_t value;
};
谁的行为与join
操作相当自然:
template <typename Inner1, typename Inner2>
auto
join(const wrapper<Inner1>& w1, const wrapper<Inner2>& w2)
-> wrapper<decltype(join(w1.value, w2.value))>
{
return {join(w1.value, w2.value)};
}
也可以是join
ed 为“标量”类型:
template <typename Inner1, typename T2>
auto
join(const wrapper<Inner1>& w1, const T2& value2)
-> wrapper<decltype(join(w1.value, value2))>
{
return {join(w1.value, value2)};
}
到目前为止,效果很好,有效。但是,因为在实际情况中我实际上有更多这样的规则,所以我想避免重复规则的数量来表达join
操作,因此,我表示join(scalar, wrapper) := join(wrapper, scalar)
(事实上,我更喜欢类似的东西join(v1, v2) := join(v2, v1)
,但让我们从更具体的事情开始。):
template <typename T1, typename Inner2>
auto
join(const T1& value1, const wrapper<Inner2>& w2)
-> decltype(join(w2, value1))
{
return join(w2, value1);
}
这适用于join(scalar, scalar)
, join(wrapper, scalar)
and join(scalar, wrapper)
。但是之后join(wrapper, wrapper)
导致 G++ 4.9 和 Clang++ 3.5 的模板函数无限扩展,我不明白。
int main()
{
int i;
float f;
wrapper<float> fr;
join(f, i);
join(fr, i);
join(i, fr);
join(fr, fr); // Loops.
}
Clang:
clang++-mp-3.5 -std=c++11 bar.cc
bar.cc:21:5: fatal error: recursive template instantiation exceeded maximum depth of
256
join(const wrapper<Inner1>& w1, const T2& value2)
^
bar.cc:29:5: note: while substituting deduced template arguments into function
template 'join' [with T1 = wrapper<float>, Inner2 = float]
join(const T1& value1, const wrapper<Inner2>& w2)
^
GCC:
g++-mp-4.9 -std=c++11 bar.cc
bar.cc:30:34: error: template instantiation depth exceeds maximum of 900 (use -ftemplate-depth= to increase the maximum) substituting 'template<class T1, class Inner2> decltype (join(w2, value1)) join(const T1&, const wrapper<Inner2>&) [with T1 = <missing>; Inner2 = <missing>]'
-> decltype(join(w2, value1))
^
bar.cc:30:34: recursively required by substitution of 'template<class T1, class Inner2> decltype (join(w2, value1)) join(const T1&, const wrapper<Inner2>&) [with T1 = wrapper<float>; Inner2 = float]'
bar.cc:30:34: required by substitution of 'template<class T1, class Inner2> decltype (join(w2, value1)) join(const T1&, const wrapper<Inner2>&) [with T1 = wrapper<float>; Inner2 = float]'
bar.cc:43:18: required from here
我不明白为什么重载不会减少递归。到底是怎么回事?可能有一种可能的替代实现(类)模板专门化,但我并不是在寻找替代实现:我想了解这个实现有什么问题。提前致谢。