C++ 错误：无法将“std::basic_ostream<char>”左值绑定到“std::basic_ostream<char>&&”

error: cannot bind ‘std::basic_ostream<char>’ lvalue to ‘std::basic_ostream<char>&&’

提问by cross

I have already looked at a couple questions on this, specifically Overloading operator<<: cannot bind lvalue to ‘std::basic_ostream<char>&&'was very helpful. It let me know that my problem is I'm doing something that c++11 can't deduce the type from.

我已经看过几个关于此的问题，特别是 重载运算符<<：无法将左值绑定到 'std::basic_ostream<char>&&'非常有帮助。它让我知道我的问题是我正在做一些 c++11 无法从中推断出类型的事情。

I think a big part of my problem is that the instantiated class I'm working with, is templated, but originally obtained from a pointer to a non-template base class. This is something I did advised from another stackoverflow.com question about how to put template class objects into an STL container.

我认为我的问题的很大一部分是我正在使用的实例化类是模板化的，但最初是从指向非模板基类的指针获得的。这是我从另一个关于如何将模板类对象放入 STL 容器的 stackoverflow.com 问题中提出的建议。

My classes:

我的课程：

class DbValueBase {
  protected:
    virtual void *null() { return NULL; }   // Needed to make class polymorphic
};

template <typename T>
class DbValue : public DbValueBase {
  public:
    DbValue(const T&val)  { data = new T(val); }
    ~DbValue() { if (data) delete data; }

    T   *data;

    const T&    dataref() const { return *data; } 

    friend std::ostream& operator<<(std::ostream& out, const DbValue<T>& val)
    {
        out << val.dataref();
        return out;
    }
}

And, the code snippet where the compile error database.cc:530:90: error: cannot bind ‘std::basic_ostream<char>' lvalue to ‘std::basic_ostream<char>&&'occurs:

并且，database.cc:530:90: error: cannot bind ‘std::basic_ostream<char>' lvalue to ‘std::basic_ostream<char>&&'发生编译错误的代码片段：

//nb:  typedef std::map<std::string,DbValueBase*>  DbValueMap;
    const CommPoint::DbValueMap&    db_values = cp.value_map();
    for (auto i = db_values.cbegin() ; i != db_values.cend() ; i++) {
        // TODO: Need to implement an ostream operator, and conversion
        // operators, for DbValueBase and DbValue<>
        // TODO: Figure out how to get a templated output operator to
        // work... 
 //     DbValue<std::string> *k = dynamic_cast<DbValue<std::string>*>(i->second);
        std::cerr << "  Database field " << i->first << " should have value " << *(i->second) << endl;
    }

If my output tries to print i->second, it compiles and runs, and I see the pointer. If I try to output *(i->second), I get the compile error. When single-stepping in gdb, it seemsto still know that i->secondis of the correct type

如果我的输出尝试 print i->second，它会编译并运行，并且我会看到指针。如果我尝试输出*(i->second)，我会收到编译错误。在gdb中单步执行时，似乎仍然知道i->second是正确的类型

(gdb) p i->second
 = (DbValueBase *) 0x680900
(gdb) p *(i->second)
warning: RTTI symbol not found for class 'DbValue<std::string>'
 = warning: RTTI symbol not found for class 'DbValue<std::string>'
{_vptr.DbValueBase = 0x4377e0 <vtable for DbValue<std::string>+16>}
(gdb) quit

I'm hoping that I'm doing something subtly wrong. But, it's more complicated than I seem to be able to figure it out on my own. Anyone else see what thing(s) I've done wrong or incompletely?

我希望我做错了一些微妙的事情。但是，它比我自己似乎能够弄清楚的要复杂得多。其他人看到我做错了什么或不完整的事情了吗？

Edit:

编辑：

@PiotrNycz did give a good solution for my proposed problem below. However, despite currently printing values while doing development, the real need for these DbValue<>objects is to have them return a value of the correct type which I can then feed to database operation methods. I should've mentioned that in my original question, that printing is of value, but not the end of my goal.

@PiotrNycz 确实为我在下面提出的问题提供了一个很好的解决方案。然而，尽管当前在进行开发时打印值，但对这些DbValue<>对象的真正需要是让它们返回正确类型的值，然后我可以将其提供给数据库操作方法。我应该在我最初的问题中提到，印刷是有价值的，但不是我目标的终点。

采纳答案by Bart van Ingen Schenau

Although the debugger correctly identifies *(i->second)as being of the type DbValue<std::string>, that determination is made using information that is only available at runtime.

尽管调试器正确识别*(i->second)为 类型DbValue<std::string>，但该确定是使用仅在运行时可用的信息进行的。

The compiler only knows that it is working with a DbValueBase&and has to generate its code on that basis. Therefore, it can't use the operator<<(std::ostream&, const DbValue<T>&)as that does not accept a DbValueBaseor subclass.

编译器只知道它正在使用 aDbValueBase&并且必须在此基础上生成其代码。因此，它不能使用operator<<(std::ostream&, const DbValue<T>&)不接受 aDbValueBase或子类的 。

---

---

For obtaining the contents of a DbValue<>object through a DbValueBase&, you might want to loop into the Visitor design pattern.

为了DbValue<>通过 a获取对象的内容DbValueBase&，您可能需要循环访问访问者设计模式。

Some example code:

一些示例代码：

class Visitor {
public:
    template <typename T>
    void useValue(const T& value);
};

class DbValueBase {
public:
    virtual void visit(Visitor&) = 0;
};

template <class T>
class DbValue : public DbValueBase {
pblic:
    void visit(Visitor& v) {
        v.useValue(m_val);
    }
private:
    T m_val;
};

回答by PiotrNycz

If you want to print object by base pointer - make the ostream operator in base class:

如果要通过基指针打印对象 - 在基类中创建 ostream 运算符：

class DbValueBase {
  protected:
    virtual ~DbValueBase() {}
    virtual void print(std::ostream&) const = 0;
    friend std::ostream& operator << (std::ostream& os, const DbValueBase & obj)
    {
       obj.print(os); return os;
    }
};

template <typename T>
class DbValue : public DbValueBase {
  public:
    void print(std::ostream& os) const 
    {
        out << dataref();
    }
};