结合使用好奇重复模板模式(CRTP)和其他类型参数 [英] Use Curiously Recurring Template Pattern (CRTP) with additional type parameters

问题描述

我尝试使用好奇重复模板模式(CRTP)并提供其他类型参数:

I try to use the Curiously Recurring Template Pattern (CRTP) and provide additional type parameters:

template <typename Subclass, typename Int, typename Float>
class Base {
    Int *i;
    Float *f;
};
...

class A : public Base<A, double, int> {
};

这可能是一个错误，更合适的超类应该是 Base< A，double，int> -尽管这种参数顺序的不匹配并不那么明显.如果我可以使用typedef中的参数含义，将更容易看到此错误:

This is probably a bug, the more appropriate superclass would be Base<A, double, int> -- although this argument order mismatch is not so obvious to spot. This bug would be easier to see if I could use name the meaning of the parameters in a typedef:

template <typename Subclass>
class Base {
    typename Subclass::Int_t *i;  // error: invalid use of incomplete type ‘class A’
    typename Subclass::Float_t *f;
};

class A : public Base<A> {
    typedef double Int_t;         // error: forward declaration of ‘class A’
    typedef int Double_t;
};

但是，它不能在gcc 4.4上编译，所报告的错误已在上面的注释中给出-我认为原因是在创建A之前，它需要实例化Base模板，但这又需要知道A

However, this does not compile on gcc 4.4, the reported errors are given as comments above -- I think the reason is that before creating A, it needs to instantiate the Base template, but this in turn would need to know A.

在使用CRTP时是否有传入命名"模板参数的好方法?

Is there a good way of passing in "named" template parameters while using CRTP?

推荐答案

您可以使用特征类:

// Must be specialized for any type used as TDerived in Base<TDerived>.
// Each specialization must provide an IntType typedef and a FloatType typedef.
template <typename TDerived>
struct BaseTraits;

template <typename TDerived>
struct Base 
{
    typename BaseTraits<TDerived>::IntType *i;
    typename BaseTraits<TDerived>::FloatType *f;
};

struct Derived;

template <>
struct BaseTraits<Derived> 
{
    typedef int IntType;
    typedef float FloatType;
};

struct Derived : Base<Derived> 
{
};

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