使用boost :: shared_ptr時出錯

Question

我正在學習加速和智能指針。 在編譯期間，我遇到了一個錯誤，並且我無法弄清楚這是怎么回事。 我不明白我在做什么錯。 問題出在構造函數中：

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

這是我的代碼：

#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;
}

還有一個錯誤：

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的參數必須是動態分配的對象的地址，但是代碼正在傳遞引用。 更改為，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的實例是唯一可以訪問def_指向的對象的對象，則沒有理由將其設置為boost::shared_ptr ：請使用boost::scoped_ptr 。 請參閱可用的C ++智能指針實現？ 有關智能指針的非常有用的概述。

---

但是，從發布的代碼來看，似乎沒有理由使用任何性質的指針。 只需將一個Creature實例存儲在DefaultCreature （根據發布的代碼， Creature是可復制的，並且不需要多態）。