错误:无法将“ std :: basic_ostream< char>”左值绑定到“ std :: basic_ostream< char>&&” [英] error: cannot bind ‘std::basic_ostream<char>’ lvalue to ‘std::basic_ostream<char>&&’
问题描述
我已经看过几个问题,特别是重载运算符<< ;:不能将左值绑定到'std :: basic_ostream< char>&'
非常有用。它让我知道我的问题是我在做c ++ 11无法推断出类型的事情。
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.
我认为我的问题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stackoverflow.com问题中建议的有关如何将模板类对象放入STL容器的建议。
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.
我的类:
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;
}
}
并且,代码段的编译错误 database.cc:530:90:错误:无法将'std :: basic_ostream< char>'左值绑定到'std :: basic_ostream< char>&&'发生:
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:
//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;
}
如果我的输出尝试打印 i->,第二
,它会编译并运行,我看到了指针。如果我尝试输出 *(i-> second)
,则会收到编译错误。当单步执行gdb时,似乎仍然知道 i->秒
的类型正确
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 seems to still know that i->second
is of the correct type
(gdb) p i->second
$2 = (DbValueBase *) 0x680900
(gdb) p *(i->second)
warning: RTTI symbol not found for class 'DbValue<std::string>'
$3 = warning: RTTI symbol not found for class 'DbValue<std::string>'
{_vptr.DbValueBase = 0x4377e0 <vtable for DbValue<std::string>+16>}
(gdb) quit
我希望自己做错了什么。但是,它比我自己能够解决的复杂得多。其他人看到我做错了或不完整的事情是什么?
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?
编辑:
@PiotrNycz确实为我下面提出的问题提供了很好的解决方案。但是,尽管当前在进行开发时会打印值,但对于这些 DbValue<>
对象的真正需求是使它们返回正确类型的值,然后我可以将其输入数据库操作方法。我应该在最初的问题中提到打印是有价值的,但不是目标的终点。
@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.
推荐答案
尽管调试器正确地将 *(i-> second)
标识为 DbValue< std :: string>
,则使用仅在运行时可用的信息进行确定。
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.
编译器只知道它与 DbValueBase& 一起使用。
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 DbValueBase
or subclass.
用于获取 DbValueBase&
> DbValue<> 对象,您可能想循环进入访客设计模式。
For obtaining the contents of a DbValue<>
object through a DbValueBase&
, you might want to loop into the Visitor design pattern.
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;
};
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