[英]How to get the value of a template class?
I'm trying to get the value of a template class. To get the value of a class, I can easily do like:我正在尝试获取模板 class 的值。要获取 class 的值,我可以轻松地执行以下操作:
int get_value()
{
return *this;
}
But I want to create a class, and extend it, and don't make get_value()
in all classes again.但我想创建一个 class,并扩展它,并且不要在所有类中再次创建
get_value()
。 So, I did that:所以,我这样做了:
template<typename T>
class _extend : public T
{
public:
auto _get_value()
{
return *this;
}
};
template<typename T>
class extend : public _extend<T>
{
public:
T get_value()
{
auto value = this->_get_value(); /* It was `_get_value()`, changed to `this->_get_value()` due to the comments */
T output = value;
return output;
}
};
But it's not working: output is nothing.但它不起作用:output 什么都不是。
Edit编辑
Sample program:示例程序:
#include <iostream>
namespace kc
{
template<typename T>
class _extend : public T
{
public:
auto _get_value()
{
return *this;
}
};
template<typename T>
class extend : public _extend<T>
{
public:
T get_value()
{
auto value = this->_get_value();
T output = value;
return output;
}
};
class A : public std::string, public extend<std::string>
{
public:
using std::string::string;
};
}
int main()
{
kc::A a("a");
std::cout << a.get_value() << std::endl;
}
The problem with the code in the question is the multiple inheritance.问题中代码的问题是多个inheritance。 The
*this
that _extend<string>::get_value()
will be called with is not the string that has a value. _extend<string>::get_value()
将被调用的*this
不是具有值的字符串。
Although I agree with @NicolBolas in comments in that I don't understand why you want this, but you can make a class template that will return the value of what it derives from with only one level of inheritance.尽管我同意@NicolBolas 在评论中的观点,因为我不明白您为什么要这样做,但是您可以制作一个 class 模板,该模板将仅返回一层 inheritance 的值。 You just need to give it a perfect forwarding constructor and have
get_value()
cast to the base type, ie您只需要给它一个完美的转发构造函数并将
get_value()
转换为基本类型,即
#include <iostream>
#include <string>
namespace kc
{
template<typename T>
class extend : public T
{
public:
template<typename... Args>
extend(Args&&... args) : T(std::forward<Args>(args)...)
{}
T get_value()
{
return *static_cast<T*>(this);
}
};
}
int main()
{
kc::extend<std::string> a("foo");
std::cout << a.get_value() << std::endl;
}
Basically:基本上:
template<class C>
class extend : public C
{
public:
using C::C;
auto get()
{
return *this;
}
};
And a full example:一个完整的例子:
#include <iostream>
template<class C>
class extend : public C
{
public:
using C::C;
auto get()
{
return *this;
}
};
class e_string : public extend<std::string>
{
public:
using extend<std::string>::extend;
};
int main()
{
e_string s = "Test";
std::cout << s.get() << std::endl;
}
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