在返回shared_ptr的函数中返回新的东西

我有一个函数，它将shared_ptr返回给const对象。返回由操作符new返回的指针构造的shared_ptr，但直接返回该指针会导致编译错误:

Error   3   error C2664: 'std::shared_ptr<_Ty>::shared_ptr(std::nullptr_t)' : cannot convert parameter 1 from 'script::float_data *' to 'std::nullptr_t'    c:xxxvalue.cpp    59

下面是导致错误的代码:

shared_ptr< const data > float_data::operator + ( shared_ptr< const data > rhs ) const
{
    int rhs_as_int; float rhs_as_float;
    switch( rhs->to_numeric( rhs_as_int, rhs_as_float ) )
    {
    case E_INT:
        return new float_data( val + rhs_as_int );
    case E_FLOAT:
        return new float_data( val + rhs_as_float );
    }
}

类是:

class data
{
public:
    enum type
    {
        E_INT,
        E_FLOAT,
        E_STRING
    };
public:
    virtual ~data() { }
public:
    virtual std::shared_ptr< const data > operator + ( std::shared_ptr< const data > rhs ) const = 0;
    virtual std::shared_ptr< const data > operator - ( std::shared_ptr< const data > rhs ) const = 0;
    virtual std::shared_ptr< const data > operator * ( std::shared_ptr< const data > rhs ) const = 0;
    virtual std::shared_ptr< const data > operator / ( std::shared_ptr< const data > rhs ) const = 0;
    virtual std::shared_ptr< data > operator = ( std::shared_ptr< const data > rhs ) = 0;
public:
    virtual type to_numeric( int & as_int, float & as_float ) const = 0;
};
class int_data : public data
{
private:
    int val;
public:
    int_data( int i );
public:
    virtual std::shared_ptr< const data > operator + ( std::shared_ptr< const data > rhs ) const;
    virtual std::shared_ptr< const data > operator - ( std::shared_ptr< const data > rhs ) const;
    virtual std::shared_ptr< const data > operator * ( std::shared_ptr< const data > rhs ) const;
    virtual std::shared_ptr< const data > operator / ( std::shared_ptr< const data > rhs ) const;
    virtual std::shared_ptr< data > operator = ( std::shared_ptr< const data > rhs );
public:
    virtual type to_numeric( int & as_int, float & as_float ) const;
};
class float_data : public data
{
private:
    float val;
public:
    float_data( float f );
public:
    virtual std::shared_ptr< const data > operator + ( std::shared_ptr< const data > rhs ) const;
    virtual std::shared_ptr< const data > operator - ( std::shared_ptr< const data > rhs ) const;
    virtual std::shared_ptr< const data > operator * ( std::shared_ptr< const data > rhs ) const;
    virtual std::shared_ptr< const data > operator / ( std::shared_ptr< const data > rhs ) const;
    virtual std::shared_ptr< data > operator = ( std::shared_ptr< const data > rhs );
public:
    virtual type to_numeric( int & as_int, float & as_float ) const;
};
class string_data : public data
{
private:
    std::string val;
public:
    string_data( const char * s );
public:
    virtual std::shared_ptr< const data > operator + ( std::shared_ptr< const data > rhs ) const;
    virtual std::shared_ptr< const data > operator - ( std::shared_ptr< const data > rhs ) const;
    virtual std::shared_ptr< const data > operator * ( std::shared_ptr< const data > rhs ) const;
    virtual std::shared_ptr< const data > operator / ( std::shared_ptr< const data > rhs ) const;
    virtual std::shared_ptr< data > operator = ( std::shared_ptr< const data > rhs );
public:
    virtual type to_numeric( int & as_int, float & as_float ) const;
};

我不认为这是c++ 11特有的东西，但我是c++ 11的新手，所以我不确定。我不明白为什么不是两种方式都返回指针工作，为什么编译器自动选择期望nullptr_t的构造函数。