模板的模板参数

模板的模板参数(template template parameter)是指C++语言程序设计中,一个模板的参数是模板类型。只有类模板允许其模板参数是模板类型;函数模板不允许具有这样的模板参数(此处有争议[1])。

模板的模板参数可以有默认值。[2]例如:

template <class T = float> struct B {};
template <template <class TT = float> class T> struct A {
     inline void f();
     inline void g();
};
template <template <class TT> class T> void A<T>::f() {
      T<> t; // error - TT has no default template argument
}
template <template <class TT = char> class T> void A<T>::g() {
       T<> t; // OK - T<char>
}

模板的模板参数,其实参应当是类模板名字或者别名模板(alias template)。当模板的模板参数做“形参实参结合”时,仅考虑把实参的基本(即未特化)类模板与模板的模板形参做匹配;不考虑实参的偏特化类模板,即使偏特化后的参数列表与模板的模板形参匹配得上。[3] 例如:

template <typename T, template <typename ELEM, typename ALLOC = std::allocator<ELEM> > 
                         class CONT = std::deque>   //deque的基本型有2个模板参数 
class Stack { 
  private: 
    CONT<T> elems;         // OK!
    //… 
}; 

当模板的模板参数被实例化时,考虑采用该模板参数的偏特化版本。如果在实例化之处该偏特化版本仍不可见,且如果它是可见的则应该当采用,这时该程序为病态。[4] 例如:

template<class T> class A { // primary template
      int x;
};
template<class T> class A<T*> { // partial specialization
       long x;
};
template<template<class U> class V> class C {
        V<int> y;
        V<int*> z;
};
C<A> c; // V<int> within C<A> uses the primary template, so c.y.x has type int
        // V<int*> within C<A> uses the partial specialization, so c.z.x has type long

当一个模板的模板形参(称作P)与一个作为实参的模板(称作A)匹配时, 要求P与A各自的模板形参列表的对应成员满足:

  • 具有相同种类(类型参数、非类型参数、模板作为参数);
  • 如果二者是非类型的模板参数,则其具体类型应相等;
  • 如果二者是模板作为参数,则递归执行上述模板的模板参数形实匹配规则;
  • 如果P是一个可变参数模板,即P的模板形参列表包含了模板参数包(template parameter pack),则这个模板参数包应该匹配A中0个或者多个模板参数或者匹配A中的对应的模板参数包。[5]例如:
template<class T> class A { /* ... */ };
template<class T, class U = T> class B { /* ... */ };
template <class ... Types> class C { /* ... */ };
template<template<class> class P> class X { /* ... */ };
template<template<class ...> class Q> class Y { /* ... */ };
X<A> xa; // OK
X<B> xb; // ill-formed: default arguments for the parameters of a template argument are ignored
X<C> xc; // ill-formed: a template parameter pack does not match a template parameter
Y<A> ya; // OK
Y<B> yb; // OK
Y<C> yc; // OK  
template <class T> struct eval;
template <template <class, class...> class TT, class T1, class... Rest>
struct eval<TT<T1, Rest...>> { };
template <class T1> struct A;
template <class T1, class T2> struct B;
template <int N> struct C;
template <class T1, int N> struct D;
template <class T1, class T2, int N = 17> struct E;
eval<A<int>> eA; // OK: matches partial specialization of eval

参考文献

  1. (https://stackoverflow.com/questions/63280883/is-the-code-below-using-template-template-parameters#comment111899635_63280883)
  2. C++11语言标准,§14.1.14: A template-parameter of a template template-parameter is permitted to have a default template-argument.When such default arguments are specified, they apply to the template template-parameter in the scope of the template template-parameter.
  3. C++11语言标准,§14.3.3.1: When the template-argument names a class template, only primary class templates are considered when matching the template template argument with the corresponding parameter; partial specializations are not considered even if their parameter lists match that of the template template parameter.
  4. C++11语言标准,§14.3.3.2: If a specialization is not visible at the point of instantiation, and it would have been selected had it been visible, the program is ill-formed; no diagnostic is required.
  5. C++11语言标准,§14.3.3.3: When P’s template-parameter-list contains a template parameter pack (14.5.3), the template parameter pack will match zero or more template parameters or template parameter packs in the template-parameter-list of A with the same type and form as the template parameter pack in P (ignoring whether those template parameters are template parameter packs).
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