“未在此范围中声明”模板和继承的错误 [英] "not declared in this scope" error with templates and inheritance

问题描述

以下代码示例再现了我的问题：

Here is code sample which reproduces my problem:

template <typename myType>
class Base {
public:
    Base() {}
    virtual ~Base() {}
protected:
    int myOption;
    virtual void set() = 0;
};

template <typename InterfaceType>
class ChildClass : public Base < std::vector<InterfaceType> >
{
public:
    ChildClass() {}
    virtual ~ChildClass() {}
 protected:
    virtual void set();
};

template <typename InterfaceType>
void ChildClass<InterfaceType>::set()
{
     myOption = 10;
}

我的用法在 main（）：

ChildClass<int> myObject;

我得到以下错误（ubuntu上的gcc 4.4.3）：

I get the following error (gcc 4.4.3 on ubuntu):

'myOption'未在此范围内声明

‘myOption’ was not declared in this scope

如果我的ChildClass将没有新的模板参数这将工作正常，即：

If my ChildClass would be without new template parameter this would work fine, i.e.:

class ChildClass : public Base < std::vector<SomeConcreteType> >

---

编辑

我设法解决它，如果我的设置方法看起来像：

---

Edit

I've managed to solve it, if my set method looks like:

Base<std::vector<InterfaceType> >::myOption = 10;

它工作正常。仍然不清楚为什么我需要指定所有模板参数。

It works fine. Still though not sure why I need to specify all template parameters.

推荐答案

myOption 不是依赖的名称，即它不依赖于模板参数，因此编译器尝试提前查找它。您必须使其成为依赖名称：

myOption is not a dependent name, i.e. it doesn't depend on the template arguments explicitly so the compiler tries to look it up early. You must make it a dependent name:

template <typename InterfaceType>
void ChildClass<InterfaceType>::set()
{
     this->myOption = 10;
}

现在取决于 code>，因此取决于模板参数。因此，编译器将在实例化时绑定它。

Now it depends on the type of this and thus on the template arguments. Therefore the compiler will bind it at the time of instantiation.

这称为两阶段名称查找。

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