关于C++：奇怪的重复模板模式——继承和朋友

Curiously Recurring Template Pattern — Inheritance and friends

简介

我正在尝试使用奇怪的重复模板模式，以使HDF5组中的内容可存储。但我有两个问题：

如何使成员实现私有化？

如何将CRTP与命名空间一起使用？

我想要什么

(代码如下)hdf5::Group代表一个hdf5组，可以存储数据集，hdf5::Storable是一个crtp模板类。函数hdf5::store应该取一个组和一个可存储对象，并调用该对象的实现。所有这些都应该在名称空间hdf5中，以保持事物的清洁。

A实现了一个可存储的。它位于hdf5名称空间(例如全局或其他名称空间)之外。实现方法A::store应该是私有的，以确保每个人都使用hdf5::store。

我所拥有的

有问题的部分有指出问题的评论。

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#include <string>
#include <iostream>

namespace hdf5 {

/// A group can contain other groups, and datasets.
class Group {
public:
/// Create a group under a parent group.
Group(Group* parent, const std::string &name)
: parent_(parent), name_(name)
{
std::cout <<"Creating group "" << name <<""";
if (parent != nullptr)
std::cout <<" under "" << parent->name_ <<""";
std::cout <<"." << std::endl;
};

/// Create a root group.
Group() : Group(nullptr,"root") { }

/// Create a dataset inside.
void create_dataset(const std::string &name)
{
std::cout <<"Creating dataset "" << name <<"""
<<" under "" << name_ <<""." << std::endl;
}

private:
Group *parent_;
std::string name_;
};

/** Abstraction of a storable class.
*
* Curiously recurring template pattern.
* Makes it possible to write
*
* store(grp, obj);
*
*/
template<class Derived>
class Storable {
friend void hdf5::store(hdf5::Group &grp, const Derived &obj) {
obj.store(grp);
}
};

} // namespace hdft

/// Some data class that should be storable.
class A : private hdf5::Storable<A> {
public:
A(const std::string &name) : name_(name) { }

/*
* Why can't I make it private? `store` should be friend.
*
* test.cc: In instantiation of ‘void hdf5::store(hdf5::Group&, const A&)’:
* test.cc:104:19: required from here
* test.cc:72:10: error: ‘void A::store(hdf5::Group&) const’ is private
* void store(hdf5::Group &grp) const {
* ^
* test.cc:45:9: error: within this context
* obj.store(grp);
* ^
*/
// private:
public:
/// Implementation of the storage
void store(hdf5::Group &grp) const {
grp.create_dataset(name_);
}

private:
std::string name_;
};

/// Demonstration.
int main(void) {
hdf5::Group root,
grpa(&root, std::string("group_a")),
grpb(&root, std::string("group_b"));
A a1(std::string("A1")), a2(std::string("A2"));

/*
* This is what I want, but it doesn't compile:
*
* test.cc: In function ‘int main()’:
* test.cc:96:5: error: ‘store’ is not a member of ‘hdf5’
* hdf5::store(root, a1);
* ^
*/
// hdf5::store(root, a1);
// hdf5::store(root, a2);
// hdf5::store(grpa, a1);
// hdf5::store(grpb, a2);

/*
* This OTOH compiles and runs.
*/
store(root, a1);
store(root, a2);
store(grpa, a1);
store(grpb, a2);
}

预期产量

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Creating group"root".
Creating group"group_a" under"root".
Creating group"group_b" under"root".
Creating dataset"A1" under"root".
Creating dataset"A2" under"root".
Creating dataset"A1" under"group_a".
Creating dataset"A2" under"group_b".