我一直在努力寻找如何使用boost::any 创建一个可以先使用模板打印任何类型的打印功能。
template <typename T>
struct printer {
void print(ostream& os, const boost::any& a);
};
我需要先定义print()。
我希望任何人都拥有真正的operator <<，这个想法很简单：为每个任何对象附加一个类的实例
printer<T> 与合适的 T 并在 any 的值类型更改时更改此对象。
第一个技术问题是打印机对象依赖于 T 而 any 不是（也不应该是）类模板。
我真的需要今晚或明天的帮助。我有明天的最后期限，但我希望今晚继续工作。
【问题讨论】：
有很简单的方法可以做到这一点，在“Beyond the C++ Standard Library: An Introduction to Boost”中描述：
struct streamer {
virtual void print(ostream &o, const boost::any &a) const =0;
virtual streamer * clone() const = 0;
virtual ~streamer() {}
};
template <class T>
struct streamer_impl: streamer{
void print(ostream &o, const boost::any &a) const { o << *boost::any_cast<T>(a); }
streamer *clone() const { return new streamer_impl<T>(); }
};
class any_out {
streamer *streamer_;
boost::any o_;
void swap(any_out & r){
std::swap(streamer_, r.streamer_);
std::swap(o_, r.o_);
}
public:
any_out(): streamer_(0) {}
template<class T> any_out(const T& value)
: streamer_(new streamer_impl<T>()), o_(value) {}
any_out(const any_out& a)
: streamer_(a.streamer_ ? a.streamer_->clone() : 0), o_(a.o_) {}
template <class T>
any_out & operator=(const T& r) {
any_out(r).swap(*this);
return *this;
}
~any_out() { delete streamer_; }
friend std::ostream &operator<<(std::ostream& o, const any_out & a) {
if(a.streamer_)
a.streamer_->print(o, a);
return o;
}
};
然后你使用any_out 而不是boost::any。
【讨论】：
我是这样做的，我觉得干净又安全：
any_extension.hpp：
namespace cpputil
{
struct AnyWriter
{
/// Register a type with the AnyWriter.
/// @pre T must have an ostream << operator available somewhere
template<class T> static bool registerType()
{
return registeredTypes().emplace(std::type_index(typeid(T)),
std::bind(&AnyWriter::write<T>,
std::placeholders::_1,
std::placeholders::_2)).second;
}
/// Write any registred object to a stream
/// @pre Underlying type must have been registered with a call to AnyWriter::registerType<T>
/// @param os is reference to a std::ostream
/// @param anyObject is a reference to a boost::any
static void writeAny(std::ostream& os, const boost::any& anyObject);
private:
// A function object that converts an any to a type and streams it to an ostream
using WriteFunction = std::function<void (std::ostream&, const boost::any&)>;
// a map of typeinfo to WriteFunction
using RegisteredTypes = std::unordered_map<std::type_index, WriteFunction >;
// retrieve the WriteFunction map in a safe way
static RegisteredTypes& registeredTypes();
// Convert an any to a type, and write it to a stream
template<class T> static void write(std::ostream& os, const boost::any& anyObject) {
try {
const T& typedObject = boost::any_cast<const T&>(anyObject);
os << typedObject;
}
catch(boost::bad_any_cast& e) {
os << "<exception in conversion: " << e.what() << ">";
}
}
};
}
namespace std {
ostream& operator<<(ostream& os, const ::boost::any& anyObject);
}
any_extension.cpp：
#include "any_extension.h"
#include <string>
namespace cpputil {
namespace AnyWriterRegistration {
const bool stringRegistered = AnyWriter::registerType<std::string>();
const bool intRegistered = AnyWriter::registerType<int>();
const bool doubleRegistered = AnyWriter::registerType<double>();
}
AnyWriter::RegisteredTypes& AnyWriter::registeredTypes()
{
static RegisteredTypes _registrationMap;
return _registrationMap;
}
void AnyWriter::writeAny(std::ostream &os, const boost::any &anyObject)
{
auto registered = registeredTypes();
auto iFind = registered.find(anyObject.type());
if(iFind == registered.end()) {
os << "<unregistered type: " << anyObject.type().name() << ">";
}
else {
iFind->second(os, anyObject);
}
}
}
namespace std {
ostream& operator<<(ostream& os, const ::boost::any& anyObject)
{
if(anyObject.empty()) {
os << "<empty>";
}
else {
cpputil::AnyWriter::writeAny(os, anyObject);
}
return os;
}
}
对于您希望支持的任何类型，只需确保已为其类型调用 AnyWriter::register() 并且操作符
例如：
any_test.cpp：
struct chicken {};
std::operator<<(std::ostream& os, const chicken& aChicken) {
os << "cluck!";
return os;
}
namespace {
const bool chickenRegistered = AnyWriter::register<Chicken>();
}
void chickenTest() {
boost::any animal = chicken();
std::cout << animal << std::endl;
}
输出： 咯咯！
【讨论】：
Pawel Zubrycki 的回答非常好（提到 Björn Karlsson 的书）。
但代码在以下几行中有一些错误：
// ...
o << *boost::any_cast<T>(a); // should be: o << *boost::any_cast<T>(&a);
// ...
a.streamer_->print(o, a); // should be: a.streamer_->print(o, a.o_);
这是 Pawel Zubrycki 答案的更正版本（部分...）
#ifndef ANY_OUT_H
#define ANY_OUT_H
#include <iostream>
#include <boost/any.hpp>
struct streamer {
virtual void print(std::ostream &o, const boost::any &a) const =0;
virtual streamer * clone() const = 0;
virtual ~streamer() {}
};
template <class T>
struct streamer_impl: streamer{
void print(std::ostream &o, const boost::any &a) const { o << *boost::any_cast<T>(&a); }
streamer *clone() const { return new streamer_impl<T>(); }
};
class any_out {
streamer *streamer_;
boost::any o_;
void swap(any_out & r){
std::swap(streamer_, r.streamer_);
std::swap(o_, r.o_);
}
public:
any_out(): streamer_(0) {}
template<class T> any_out(const T& value)
: streamer_(new streamer_impl<T>()), o_(value) {}
any_out(const any_out& a)
: streamer_(a.streamer_ ? a.streamer_->clone() : 0), o_(a.o_) {}
template <class T>
any_out & operator=(const T& r) {
any_out(r).swap(*this);
return *this;
}
~any_out() { delete streamer_; }
friend std::ostream &operator<<(std::ostream& o, const any_out & a);
};
std::ostream &operator<<(std::ostream& o, const any_out & a) {
if(a.streamer_)
a.streamer_->print(o, a.o_);
return o;
}
#endif
此测试代码有效：
{
any_out a = 5;
std::cout << a << std::endl;
}
但是！！！！
以下操作不起作用：
int main()
{
char str[] = "mystring";
any_out a = str;
std::cout << a << std::endl;
a = "myconststring";
std::cout << a << std::endl;
}
这里什么都没有被打印出来。
为什么？？
类型搞砸了，在下面的构造函数中
any_out(const T& value)
如果我们将流媒体实例化为
new streamer_impl<T>()
从构造函数中移除引用，即
any_out(const T value)
... 是一种解决方案。
另一种解决方案是保留引用并调整streamer_impl 的模板实例化。见下文
以下推荐的解决方案是：
#ifndef ANY_OUT_H
#define ANY_OUT_H
#include <iostream>
#include <boost/any.hpp>
struct streamer {
virtual void print(std::ostream &o, const boost::any &a) const =0;
virtual streamer * clone() const = 0;
virtual ~streamer() {}
};
template <class T>
struct streamer_impl: streamer{
void print(std::ostream &o, const boost::any &a) const { o << boost::any_cast<T>(a); }
streamer *clone() const { return new streamer_impl<T>(); }
};
class any_out {
boost::any o_;
streamer *streamer_;
void swap(any_out & r){
std::swap(streamer_, r.streamer_);
std::swap(o_, r.o_);
}
public:
any_out(): streamer_(0) {}
template<class T> any_out(const T& value)
: o_(value),
#if 1
streamer_(new streamer_impl<typename std::decay<decltype(value)>::type>)
#else
streamer_((o_.type() == typeid(const char *))
? static_cast<streamer *>(new streamer_impl<const char *>)
: static_cast<streamer *>(new streamer_impl<T>))
#endif
{
}
// template<class T> any_out(const T value)
// : o_(value),
// streamer_(new streamer_impl<T>)
// {
// }
any_out(const any_out& a)
: o_(a.o_), streamer_(a.streamer_ ? a.streamer_->clone() : 0) {}
template <class T>
any_out & operator=(const T& r) {
any_out(r).swap(*this);
return *this;
}
~any_out() { delete streamer_; }
friend std::ostream &operator<<(std::ostream& o, const any_out & a);
};
std::ostream &operator<<(std::ostream& o, const any_out & a) {
if(a.streamer_)
a.streamer_->print(o, a.o_);
return o;
}
#endif
上面给一些麻烦的测试代码现在可以很好地工作（使用“推荐的解决方案”）：
int main()
{
char str[] = "mystring";
any_out a = str;
std::cout << a << std::endl;
a = "myconststring";
std::cout << a << std::endl;
}
【讨论】：
在 Boost 邮件列表中查看此主题： http://lists.boost.org/Archives/boost/2005/01/79232.php
它有一些想法，其中一些看起来不错，而另一些则不行（对我来说）。但总的来说，这似乎是一项很难以一般方式完成的任务，因为（如该线程中所述），某些类型永远不会是 ostream'able，但可以包含在 boost::any 对象中。
【讨论】：