使用boost::shared_ptr时出错

Question

我正在学习boost和smart pointers。在编译过程中，我遇到了一个错误，我不知道是怎么回事。我不明白我做错了什么。问题在构造函数中:

DefaultCreature(const Creature& def) : def_(def) {}

下面是我的代码:

#include <iostream>
#include <boost/smart_ptr.hpp>
using namespace std;
using namespace boost;
class Creature;
typedef shared_ptr<Creature> PCreature;
class Creature {
public:
    Creature(const string& name) : name_(name) {}
    const string& getName() const { return name_; }
private:
    string name_;
};
class DefaultCreature {
public:
    DefaultCreature(const Creature& def) : def_(def) {}
private:
    PCreature def_;
};
int main() {
    DefaultCreature factory(Creature("lion"));
    return 0;
}

和一个错误:

exercise1.cpp: In constructor ‘DefaultCreature::DefaultCreature(const Creature&)’:
exercise1.cpp:20:52: error: no matching function for call to ‘boost::shared_ptr<Creature>::shared_ptr(const Creature&)’
exercise1.cpp:20:52: note: candidates are:
In file included from /usr/local/include/boost/shared_ptr.hpp:17:0,
                 from /usr/local/include/boost/smart_ptr.hpp:21,
                 from exercise1.cpp:2:
/usr/local/include/boost/smart_ptr/shared_ptr.hpp:472:14: note: template<class Ap> boost::shared_ptr::shared_ptr(Ap, typename boost::detail::sp_enable_if_auto_ptr<Ap, int>::type)
/usr/local/include/boost/smart_ptr/shared_ptr.hpp:472:14: note:   template argument deduction/substitution failed:
/usr/local/include/boost/smart_ptr/shared_ptr.hpp: In substitution of ‘template<class Ap> boost::shared_ptr::shared_ptr(Ap, typename boost::detail::sp_enable_if_auto_ptr<Ap, int>::type) [with Ap = Creature]’:
exercise1.cpp:20:52:   required from here
/usr/local/include/boost/smart_ptr/shared_ptr.hpp:472:14: error: no type named ‘type’ in ‘struct boost::detail::sp_enable_if_auto_ptr<Creature, int>’
/usr/local/include/boost/smart_ptr/shared_ptr.hpp:446:14: note: template<class Y> boost::shared_ptr::shared_ptr(std::auto_ptr<_Tp1>&)
/usr/local/include/boost/smart_ptr/shared_ptr.hpp:446:14: note:   template argument deduction/substitution failed:
exercise1.cpp:20:52: note:   types ‘std::auto_ptr<T>’ and ‘const Creature’ have incompatible cv-qualifiers
(...)
/usr/local/include/boost/smart_ptr/shared_ptr.hpp:339:5: note: boost::shared_ptr<T>::shared_ptr() [with T = Creature]
/usr/local/include/boost/smart_ptr/shared_ptr.hpp:339:5: note:   candidate expects 0 arguments, 1 provided
/usr/local/include/boost/smart_ptr/shared_ptr.hpp:328:25: note: boost::shared_ptr<Creature>::shared_ptr(const boost::shared_ptr<Creature>&)
/usr/local/include/boost/smart_ptr/shared_ptr.hpp:328:25: note:   no known conversion for argument 1 from ‘const Creature’ to ‘const boost::shared_ptr<Creature>&’

Answer 1

shared_ptr的参数必须是动态分配对象的地址，但代码传递的是引用。更改为，例如:

class DefaultCreature {
public:
    DefaultCreature(const Creature& def) : def_(new Creature(def)) {}
private:
    PCreature def_;
};

或使用boost::make_shared:

class DefaultCreature {
public:
    DefaultCreature(const Creature& def) :
        def_(boost::make_shared<Creature>(def)) {}
private:
    PCreature def_;
};

如果DefaultCreature的实例是唯一可以访问def_所指向的对象的对象，那么它没有理由是boost::shared_ptr:使用boost::scoped_ptr代替。参见哪些c++智能指针实现可用?关于智能指针的一个非常有用的概述。

---

然而，从发布的代码来看，似乎没有理由使用任何性质的指针。只需在DefaultCreature中存储Creature实例(Creature是可复制的，没有多态要求，基于发布的代码)。