[英]Partial specialization of type parameter in template class with type & template template parameters
[英]Partial template specialization of a function for a type which needs additional template parameters
我有一個內部包含數組的類型。 我需要一個函數,該函數返回A或1(如果類型不是A)中的數組成員數。
這是代碼:
#include <cstdio>
template<typename T, unsigned n>
struct A
{
T values[n];
};
template<typename T>
unsigned count_components()
{
return 1;//all types except 'A' should have 1 component
}
template<typename T, unsigned n>
unsigned count_components<A<T, n> >()//specialize count_components for 'A'
{
return n;
}
int main()
{
printf("%d\n", count_components<A<float, 4> >());//should be 4
printf("%d\n", count_components<float>());//should be 1
return 0;
}
G ++錯誤:
test.cpp:13:37: error: function template partial specialization ”count_components<A<T, n> >” is not allowed
unsigned count_components<A<T, n> >()//specialize count_components for 'A'
^
功能不能部分專門化。 相反,您可以使用可以部分專門化的類:
#include <cstdio>
template<typename T, unsigned n>
struct A
{
T values[n];
};
template<typename T>
struct component_counter
{
static unsigned count()
{
return 1;
}
};
template<typename T, unsigned n>
struct component_counter<A<T, n> >
{
static unsigned count()
{
return n;
}
};
int main()
{
printf("%d\n", component_counter<A<float, 4> >::count());//should be 4
printf("%d\n", component_counter<float>::count());//should be 1
return 0;
}
在這種情況下,count()實際上根本不必是一個函數! 您可以這樣做:
#include <cstdio>
template<typename T, unsigned n>
struct A
{
T values[n];
};
template<typename T>
struct component_counter
{
static const unsigned count=1;
};
template<typename T, unsigned n>
struct component_counter<A<T, n> >
{
static const unsigned count=n;
};
int main()
{
printf("%d\n", component_counter<A<float, 4> >::count);//should be 4
printf("%d\n", component_counter<float>::count);//should be 1
return 0;
}
更少的代碼。 需要注意的是,計數必須是不可或缺的類型才能起作用。
當我有函數時,我更喜歡保留函數(這對成員函數更有利,因為您仍然可以訪問*this
)。
template<typename T, unsigned n>
unsigned count_components_switch(boost::mpl::identity<A<T, n>>)
{
return n;
}
template<typename T>
unsigned count_components_switch(boost::mpl::identity<T>)
{
return 1;
}
template<typename T>
unsigned count_components()
{
return (count_components_switch)(boost::mpl::identity<T>());
}
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