#if
预处理器指令计算"常量"的表达式。预处理器已知。是否有类似的东西可以评估模板参数的表达式?
在实践中,我有类似的东西:
template<int i>
class Elem{ /*...*/};
#ifndef NUM_ELEM
#define NUM_ELEM 2
#endif
class MyClass
{
#if NUM_ELEM >= 1
Elem<1> e_1;
#endif
#if NUM_ELEM >= 2
Elem<2> e_2;
#endif
#if NUM_ELEM >= 3
Elem<3> e_3;
#endif
/*...*/
}
但我真的很想将MyClass变成模板:
template<int num_elem>
MyClass{
#if num_elem >= 1 //but #if can't understand num_elem
Elem<1> e_1;
#endif
#if num_elem >= 2
Elem<2> e_2;
#endif
#if num_elem >= 3
Elem<3> e_3;
#endif
/*...*/
};
答案 0 :(得分:7)
简短的回答是否定的,但如果您愿意稍微改变一下您的要求,可以采取以下措施:
// A run of elements - Elem<n>, ..., Elem<2>, Elem<1>
template <int n> class NumElems
{
template <int u, int v> friend class ElemGetter;
NumElems<n-1> before;
public:
Elem<n> e;
// method to retrieve an element by number
template <int m> Elem<m> &getElem();
};
// helper class to retrieve an element.
// 'n' is the element number to retrieve
// 'm' is the number of elements
// by default, ElemGetter<n,m> defers to ElemGetter<n,m-1>.
template <int n, int m> class ElemGetter
{
public:
static Elem<n> &getElem(NumElems<m> &numElems)
{
return ElemGetter<n,m-1>::getElem(numElems.before);
}
};
// specialisation of ElemGetter: if the element to get is the same as the
// number of elements (i.e. is the last element) then return it
// immediately.
template <int n> class ElemGetter<n,n>
{
public:
static Elem<n> &getElem(NumElems<n> &numElems)
{
return numElems.e;
}
};
// get an element by number; defers to the ElemGetter helper.
template <int n> template <int m> Elem<m> &NumElems<n>::getElem()
{
return ElemGetter<m,n>::getElem(*this);
}
template <> class NumElems<0>
{
};
...然后你可以用:
声明你的Elem成员集NumElems<NUM_ELEM> elems;
您可以使用以下方式访问它们:
Elem<2> &e = elems.getElem<2>();
原始建议代码
我提议的原始代码实际上并没有编译,但是我会在这里包含它,因为它更好地演示了上面的 intent :
// Original, doesn't compile - but it would be nice if it did :/
template <int n> class NumElems : private NumElems<n-1>
{
Elem<n> e;
template <int m> Elem<m> &getElem()
{
return NumElems<n-1>::getElem<m>();
}
template <> Elem<n> &getElem<n>()
{
return e;
}
};
template <> class NumElems<0>
{
};
不幸的是,C ++不允许以这种方式对成员模板函数进行专门化,虽然它不清楚(对我而言)为什么不 - 代码肯定更简单,而不必创建一个帮助类,就像在工作中一样上面的代码。