使用Boost.TypeErasure模拟纯虚模板成员函数编译报错求助

参考Boost.TypeErasure文档中的“polymorphic range formatter”示例（可模拟纯虚模板成员函数），尝试简化实现一个仅接收值并输出到std::cout的示例，编写代码如下：

#include <boost/type_erasure/any.hpp>
#include <boost/type_erasure/iterator.hpp>
#include <boost/type_erasure/operators.hpp>
#include <boost/type_erasure/tuple.hpp>
#include <boost/type_erasure/same_type.hpp>
#include <iostream>

using namespace boost::type_erasure;

struct _x : placeholder {};

class abstract_printer2 {
public:
    template<class X>
    void print(X x) const {
        tuple<requirements, _x> args(x);
        do_print(get<0>(args));
    }
    virtual ~abstract_printer2() {}
protected:
    typedef boost::mpl::vector<
        ostreamable<std::ostream, _x>
    > requirements;
    typedef boost::type_erasure::any<requirements, _x> x_type;
    virtual void do_print(x_type x) const = 0;
};

class separator_printer2 : public abstract_printer2 {
protected:
    virtual void do_print(x_type x) const {
        std::cout << x << std::endl;
    }
};

int main() {
    separator_printer2 p4;
    int q = 5;
    p4.print(q);
}

在Visual Studio 2022中编译时出现以下错误：

Build started at 14:02...
1>------ Build started: Project: test, Configuration: Debug x64 ------
1>test.cpp
1>C:\bob\projects\test\test.cpp(17,17): error C2280: 'boost::type_erasure::any<abstract_printer2::requirements,_x>::any(const boost::type_erasure::any<abstract_printer2::requirements,_x> &)': attempting to reference a deleted function
1>C:\bob\projects\boost\boost\type_erasure\any.hpp(1928,1):
1>compiler has generated 'boost::type_erasure::any<abstract_printer2::requirements,_x>::any' here
1>C:\bob\projects\boost\boost\type_erasure\any.hpp(1928,1):
1>'boost::type_erasure::any<abstract_printer2::requirements,_x>::any(const boost::type_erasure::any<abstract_printer2::requirements,_x> &)': function was implicitly deleted because a base class invokes a deleted or inaccessible function 'boost::type_erasure::any_constructor_control<boost::type_erasure::any_constructor_impl<Concept,T>,void>::any_constructor_control(const boost::type_erasure::any_constructor_control<boost::type_erasure::any_constructor_impl<Concept,T>,void> &)'
1>        with
1>        [
1>            Concept=abstract_printer2::requirements,
1>            T=_x
1>        ]
1>C:\bob\projects\boost\boost\type_erasure\any.hpp(376,5):
1>'boost::type_erasure::any_constructor_control<boost::type_erasure::any_constructor_impl<Concept,T>,void>::any_constructor_control(const boost::type_erasure::any_constructor_control<boost::type_erasure::any_constructor_impl<Concept,T>,void> &)': function was explicitly deleted
1>        with
1>        [
1>            Concept=abstract_printer2::requirements,
1>            T=_x
1>        ]
1>C:\bob\projects\test\test.cpp(17,17):
1>the template instantiation context (the oldest one first) is
1>  C:\bob\projects\test\test.06.factory\test.cpp(38,7):
1>  see reference to function template instantiation 'void abstract_printer2::print<int>(X) const' being compiled
1>        with
1>        [
1>            X=int
1>        ]
1>Done building project "test.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========
========== Build completed at 14:02 and took 01.517 seconds ==========

错误核心是boost::type_erasure::any的拷贝构造函数被删除，原因如下：

1. 定义x_type时的requirements仅包含ostreamable<std::ostream, _x>，未添加copy_constructible<_x>约束，导致any默认禁用拷贝构造。
2. do_print函数的参数是x_type x（值传递），调用时需要拷贝构造any对象，触发了被删除的拷贝构造函数。

有两种修复方式：

方式一：添加拷贝构造约束 #

在requirements中加入copy_constructible<_x>：

typedef boost::mpl::vector<
    ostreamable<std::ostream, _x>,
    copy_constructible<_x>  // 添加拷贝构造约束
> requirements;

方式二：改用引用传递参数 #

将do_print的参数改为const x_type&，避免拷贝操作：

// abstract_printer2中修改函数声明
virtual void do_print(const x_type& x) const = 0;

// separator_printer2中修改函数实现
virtual void do_print(const x_type& x) const {
    std::cout << x << std::endl;
}

两种方式任选其一即可解决编译错误，推荐方式二，无需额外添加拷贝约束，同时避免不必要的对象拷贝。

内容的提问来源于stack exchange，提问作者Bob