C++ - Can a template template parameter be of a variable or function?

问题

I'm still trying to fully understand templates. I see them as special types.

Recently I was reading about template template parameters of classes and I'm wondering if it it is possible to have a template template parameter of function or variable and not only of class? Something like this:

template<typename T> void func(T); //template of function 'func'

template<int a> double var = a; //template of variable 'var'

template<template<typename> void templ_param() = func, //parameter template of function

template<int> double templ_param_0 = var //parameter template of variable

> void func1();

EDIT: If not why and what is the alternative?

回答1:

I'm wondering if it it is possible to have a template template parameter of function

No, you cannot. Your example code snippet won't work.

template <template <typename T>> void templ_param() = func,

A template template parameter must be a class template or an alias template.

From the C++ standard:

14.3.3 Template template arugments

1 A template-argument for a template template-parameter shall be the name of a class template or an alias template, expressed as id-expression.

回答2:

No, template template parameters may be only types.

[temp.param]/1 describes template parameter syntax as follows:

template-parameter:

• type-parameter
• parameter-declaration

type-parameter:

• type-parameter-key ..._opt identifier_opt
• type-parameter-key identifier_opt= type-id
• template < template-parameter-list > type-parameter-key ..._opt identifier_opt
• template < template-parameter-list > type-parameter-key identifier_opt= id-expression

type-parameter-key:

• class
• typename

So template template parameters are put into category of type-parameters, and indeed, their declaration must contain either class or typename after template<...>

As an alternative you can wrap template functions and variables into classes:

template <typename T>
struct FuncWrapper {
    static void func(T t) {
        std::cout << "in func " << t << std::endl;
    }
};

template <int a>
struct VarWrapper {
    static constexpr double var = a;
};

template<
    template <typename T> class FW = FuncWrapper,
    template <int> class VW = VarWrapper> 
void func1() {
    FW<int>::func(VW<42>::var);
}

int main() {
    func1<>();
}

来源：https://stackoverflow.com/questions/29219576/c-can-a-template-template-parameter-be-of-a-variable-or-function