[英]Specialization of member function of class template
I have a Base
class which I use as a Tree. 我有一个用作树的
Base
类。 From this Base class I derived a template Container
class which should be able to hold various types. 从这个基类中,我派生了一个模板
Container
类,该类应该能够容纳各种类型。 I would like to give the Container class a toString
function, which converts its values and the values of all of its children to a string. 我想给Container类一个
toString
函数,该函数将其值及其所有子级的值转换为字符串。 A Container<A>
can have a child of a different type Container<B>
. Container<A>
可以具有另一个类型Container<B>
。
I don't know how to do this. 我不知道该怎么做。 Below is my code.
下面是我的代码。
// File base.h:
class Base {
public:
virtual string toString();
protected:
vector<Base *> _children
}
// File base.cpp:
string Base::toString() {
string a="something"
return a;
}
Then I have a templated derived class: 然后,我有一个模板化的派生类:
// File container.h:
template<class T>
class Container: public Base {
public:
string toString() override;
private:
T _data;
}
I would like to specialize the toString function, so it can deal with different types: 我想专门研究toString函数,以便它可以处理不同的类型:
File container.cpp:
template <class T>
string Container<T>::toString() {
string valueString = "not sure how to handle this type";
for(int i=0;i<_children.size;i++) {
valueString+=" "+_children[i]->toString();
}
return valueString;
}
template <>
string Container<int>::toString() {
string valueString = std::to_string(_data);
for(int i=0;i<_children.size;i++) {
valueString+=" "+_children[i]->toString();
}
return valueString;
}
I gave the Base
class also a toString
function, as I don't know how I would cast the _children
to an unspecified Container class so I can access its toString
function. 我还给
Base
类提供了toString
函数,因为我不知道如何将_children
转换为未指定的Container类,以便可以访问其toString
函数。
If I use the above approach, I get an error at linking: 如果使用上述方法,则在链接时会出现错误:
undefined reference to Container<void*>::toString()
undefined reference to Container<bool*>::toString()
and for all the other types I ever used. 以及我曾经使用过的所有其他类型。 However, I would like to avoid specializing all possible types.
但是,我想避免专门化所有可能的类型。
Edit: As it has been suggested to move the content of container.cpp to the header file: If I do this, I get errors like this: 编辑:由于建议将container.cpp的内容移动到头文件中:如果执行此操作,则会出现如下错误:
Multiple definition of Container<int>::toString().
First defined here
It seems basically that whereever I include Container.h
I get such a multiple definition. 基本上,无论我在哪里包含
Container.h
我都会得到这样的多重定义。 This although I have a 尽管我有一个
#ifndef CONTAINER
#define CONTAINER
include guard? 包括警卫?
In C++ template only compiles when substituted. 在C ++模板中,仅在替换时才编译。
In your situation, Container<T>::toString
are defined in side container.cpp
, but no substituted, so they won't be compiled. 在您的情况下,
Container<T>::toString
是在端container.cpp
中定义的,但没有替代值,因此它们将不会被编译。
When you reference to Container<T>::toString
somewhere, the definition of the function is not visible to the compile unit, the compile will generate a relocation slot, hoping the definition will be find at link stage. 当您在某处引用
Container<T>::toString
,该函数的定义对于编译单元不可见,编译将生成一个重定位槽,希望在链接阶段找到该定义。 But the function is never defined anywhere, so you got link error. 但是该函数从未在任何地方定义,因此您遇到了链接错误。
Solution: put the function definition to header file. 解决方案:将函数定义放在头文件中。
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