我有一些代码,我多年来一直成功使用它来实现“变体类型对象”;也就是说,一个C ++对象可以保存各种类型的值,但只使用(大约)尽可能多的内存作为最大的可能类型。该代码在精神上类似于tagged-union,但它也支持非POD数据类型。它通过使用char缓冲区,放置new / delete和reinterpret_cast<>来实现这种魔力。
我最近尝试在gcc 4.4.3(使用-O3和-Wall)下编译此代码,并收到许多类似的警告:
warning: dereferencing type-punned pointer will break strict-aliasing rules
据我所知,这表明gcc的新优化器可能会生成'buggy'代码,我显然希望避免这种代码。
我在下面粘贴了我的代码的“玩具版”;我可以对我的代码做些什么来使它在gcc 4.4.3下更安全,同时仍然支持非POD数据类型?我知道,作为最后的手段,我总是可以使用-fno-strict-aliasing编译代码,但是如果代码在优化下没有中断会更好,所以我宁愿不这样做。
(请注意,我希望避免在代码库中引入boost或C ++ 0X依赖,因此虽然boost / C ++ 0X解决方案很有趣,但我更喜欢更老式的东西)< / p>
#include <new>
class Duck
{
public:
Duck() : _speed(0.0f), _quacking(false) {/* empty */}
virtual ~Duck() {/* empty */} // virtual only to demonstrate that this may not be a POD type
float _speed;
bool _quacking;
};
class Soup
{
public:
Soup() : _size(0), _temperature(0.0f) {/* empty */}
virtual ~Soup() {/* empty */} // virtual only to demonstrate that this may not be a POD type
int _size;
float _temperature;
};
enum {
TYPE_UNSET = 0,
TYPE_DUCK,
TYPE_SOUP
};
/** Tagged-union style variant class, can hold either one Duck or one Soup, but not both at once. */
class DuckOrSoup
{
public:
DuckOrSoup() : _type(TYPE_UNSET) {/* empty*/}
~DuckOrSoup() {Unset();}
void Unset() {ChangeType(TYPE_UNSET);}
void SetValueDuck(const Duck & duck) {ChangeType(TYPE_DUCK); reinterpret_cast<Duck*>(_data)[0] = duck;}
void SetValueSoup(const Soup & soup) {ChangeType(TYPE_SOUP); reinterpret_cast<Soup*>(_data)[0] = soup;}
private:
void ChangeType(int newType);
template <int S1, int S2> struct _maxx {enum {sz = (S1>S2)?S1:S2};};
#define compile_time_max(a,b) (_maxx< (a), (b) >::sz)
enum {STORAGE_SIZE = compile_time_max(sizeof(Duck), sizeof(Soup))};
char _data[STORAGE_SIZE];
int _type; // a TYPE_* indicating what type of data we currently hold
};
void DuckOrSoup :: ChangeType(int newType)
{
if (newType != _type)
{
switch(_type)
{
case TYPE_DUCK: (reinterpret_cast<Duck*>(_data))->~Duck(); break;
case TYPE_SOUP: (reinterpret_cast<Soup*>(_data))->~Soup(); break;
}
_type = newType;
switch(_type)
{
case TYPE_DUCK: (void) new (_data) Duck(); break;
case TYPE_SOUP: (void) new (_data) Soup(); break;
}
}
}
int main(int argc, char ** argv)
{
DuckOrSoup dos;
dos.SetValueDuck(Duck());
dos.SetValueSoup(Soup());
return 0;
}
答案 0 :(得分:1)
好的,如果你愿意存储额外的空白*,你可以这样做。我重新格式化了你的样本,所以我更容易使用。看看这个,看看它是否符合您的需求。另外,请注意我提供了一些示例,因此您可以添加一些有助于实现的模板。它们可以扩展得更多,但这应该会给你一个好主意。
还有一些输出内容可以帮助您了解正在发生的事情。
还有一件事,我假设你知道你需要提供适当的copy-ctor和赋值操作符,但这不是这个问题的关键。
我的g ++版本信息:
g ++ --version g ++(SUSE Linux)4.5.0 20100604 [gcc-4_5-branch revision 160292]
#include <new>
#include <iostream>
class Duck
{
public:
Duck(float s = 0.0f, bool q = false) : _speed(s), _quacking(q)
{
std::cout << "Duck::Duck()" << std::endl;
}
virtual ~Duck() // virtual only to demonstrate that this may not be a POD type
{
std::cout << "Duck::~Duck()" << std::endl;
}
float _speed;
bool _quacking;
};
class Soup
{
public:
Soup(int s = 0, float t = 0.0f) : _size(s), _temperature(t)
{
std::cout << "Soup::Soup()" << std::endl;
}
virtual ~Soup() // virtual only to demonstrate that this may not be a POD type
{
std::cout << "Soup::~Soup()" << std::endl;
}
int _size;
float _temperature;
};
enum TypeEnum {
TYPE_UNSET = 0,
TYPE_DUCK,
TYPE_SOUP
};
template < class T > TypeEnum type_enum_for();
template < > TypeEnum type_enum_for< Duck >() { return TYPE_DUCK; }
template < > TypeEnum type_enum_for< Soup >() { return TYPE_SOUP; }
/** Tagged-union style variant class, can hold either one Duck or one Soup, but not both at once. */
class DuckOrSoup
{
public:
DuckOrSoup() : _type(TYPE_UNSET), _data_ptr(_data) {/* empty*/}
~DuckOrSoup() {Unset();}
void Unset() {ChangeType(TYPE_UNSET);}
void SetValueDuck(const Duck & duck)
{
ChangeType(TYPE_DUCK);
reinterpret_cast<Duck*>(_data_ptr)[0] = duck;
}
void SetValueSoup(const Soup & soup)
{
ChangeType(TYPE_SOUP);
reinterpret_cast<Soup*>(_data_ptr)[0] = soup;
}
template < class T >
void set(T const & t)
{
ChangeType(type_enum_for< T >());
reinterpret_cast< T * >(_data_ptr)[0] = t;
}
template < class T >
T & get()
{
ChangeType(type_enum_for< T >());
return reinterpret_cast< T * >(_data_ptr)[0];
}
template < class T >
T const & get_const()
{
ChangeType(type_enum_for< T >());
return reinterpret_cast< T const * >(_data_ptr)[0];
}
private:
void ChangeType(int newType);
template <int S1, int S2> struct _maxx {enum {sz = (S1>S2)?S1:S2};};
#define compile_time_max(a,b) (_maxx< (a), (b) >::sz)
enum {STORAGE_SIZE = compile_time_max(sizeof(Duck), sizeof(Soup))};
char _data[STORAGE_SIZE];
int _type; // a TYPE_* indicating what type of data we currently hold
void * _data_ptr;
};
void DuckOrSoup :: ChangeType(int newType)
{
if (newType != _type)
{
switch(_type)
{
case TYPE_DUCK: (reinterpret_cast<Duck*>(_data_ptr))->~Duck(); break;
case TYPE_SOUP: (reinterpret_cast<Soup*>(_data_ptr))->~Soup(); break;
}
_type = newType;
switch(_type)
{
case TYPE_DUCK: (void) new (_data) Duck(); break;
case TYPE_SOUP: (void) new (_data) Soup(); break;
}
}
}
int main(int argc, char ** argv)
{
Duck sample_duck; sample_duck._speed = 23.23;
Soup sample_soup; sample_soup._temperature = 98.6;
std::cout << "Just saw sample constructors" << std::endl;
{
DuckOrSoup dos;
std::cout << "Setting to Duck" << std::endl;
dos.SetValueDuck(sample_duck);
std::cout << "Setting to Soup" << std::endl;
dos.SetValueSoup(sample_soup);
std::cout << "Should see DuckOrSoup destruct which will dtor a Soup"
<< std::endl;
}
{
std::cout << "Do it again with the templates" << std::endl;
DuckOrSoup dos;
std::cout << "Setting to Duck" << std::endl;
dos.set(sample_duck);
std::cout << "duck speed: " << dos.get_const<Duck>()._speed << std::endl;
std::cout << "Setting to Soup" << std::endl;
dos.set(sample_soup);
std::cout << "soup temp: " << dos.get_const<Soup>()._temperature << std::endl;
std::cout << "Should see DuckOrSoup destruct which will dtor a Soup"
<< std::endl;
}
{
std::cout << "Do it again with only template get" << std::endl;
DuckOrSoup dos;
std::cout << "Setting to Duck" << std::endl;
dos.get<Duck>() = Duck(42.42);
std::cout << "duck speed: " << dos.get_const<Duck>()._speed << std::endl;
std::cout << "Setting to Soup" << std::endl;
dos.get<Soup>() = Soup(0, 32);
std::cout << "soup temp: " << dos.get_const<Soup>()._temperature << std::endl;
std::cout << "Should see DuckOrSoup destruct which will dtor a Soup"
<< std::endl;
}
std::cout << "Get ready to see sample destructors" << std::endl;
return 0;
}
答案 1 :(得分:1)
我会像这样编写代码:
typedef boost::variant<Duck, Soup> DuckOrSoup;
但我想每个人都有自己的品味。
顺便说一下,你的代码是错误的,你没有处理可能的对齐问题,你不能只是把一个对象放在内存中的任何一点,有一个尊重的约束,它会随着每种类型而改变。在C ++ 0x中,有alignof
关键字可以获取它,还有一些其他实用程序可以获得对齐存储。
答案 2 :(得分:0)
我设法说服GCC(4.2.4,与-Wstrict-aliasing=2
一起运行)不要抱怨使用void *
临时,即。
void SetValueDuck(const Duck & duck) {ChangeType(TYPE_DUCK); void *t=&_data; reinterpret_cast<Duck*>(t)[0] = duck;}
答案 3 :(得分:0)
我仍然无法理解这种需要或用法但是 带有-O3 -Wall的g ++ 4.4.3可以使用以下补丁。 如果它有效,你可以分享用例,为什么需要它?
class DuckOrSoup
{
public:
DuckOrSoup() : _type(TYPE_UNSET) {_duck = NULL; _soup = NULL;/* empty*/}
~DuckOrSoup() {Unset();}
void Unset() {ChangeType(TYPE_UNSET);}
void SetValueDuck(const Duck & duck) {ChangeType(TYPE_DUCK); _duck = new (&_data[0])Duck (duck); }
void SetValueSoup(const Soup & soup) { ChangeType(TYPE_SOUP); _soup = new (&_data[0])Soup (soup); }
private:
void ChangeType(int newType);
template <int S1, int S2> struct _maxx {enum {sz = (S1>S2)?S1:S2};};
#define compile_time_max(a,b) (_maxx< (a), (b) >::sz)
enum {STORAGE_SIZE = compile_time_max(sizeof(Duck), sizeof(Soup))};
char _data[STORAGE_SIZE];
int _type; // a TYPE_* indicating what type of data we currently hold
Duck* _duck;
Soup* _soup;
};
void DuckOrSoup :: ChangeType(int newType)
{
if (newType != _type)
{
switch(_type)
{
case TYPE_DUCK:
_duck->~Duck();
_duck = NULL;
break;
case TYPE_SOUP:
_soup->~Soup();
_soup = NULL;
break;
}
_type = newType;
switch(_type)
{
case TYPE_DUCK: _duck = new (&_data[0]) Duck(); break;
case TYPE_SOUP: _soup = new (&_data[0]) Soup(); break;
}
}
}