[英]Can I declare and use a static-non-template member of a template class in C++?
let's say I have:假设我有:
namespace name{
template< typename T >
class Example
{
....
};
}
I want to have a single instance of a map that I'll use across all the classes that inherit from Example
.我想要一个地图实例,我将在所有继承自Example
的类中使用它。 The first thing that came into my mind was having a static
member of Example
but it will force me to have a unique member for each class that will use it - and I don't want that due to memory usage restrictions (Am I wrong here?).我想到的第一件事是拥有Example
的static
成员,但它会迫使我为每个将使用它的类拥有一个唯一的成员 - 由于内存使用限制,我不希望这样(我在这里错了吗?)。
Declaring it in the namespace
will also create an instance for each compilation unit.在namespace
中声明它也会为每个编译单元创建一个实例。
What can I do to overcome this?我该怎么做才能克服这个问题?
BTW my initial solution was to [use a const extern map][1] but I can't understand the error I'm getting there.顺便说一句,我最初的解决方案是 [使用 const extern map][1],但我无法理解我遇到的错误。
EDIT:编辑:
I've followed @SamVarshavchik answer, and now I'm getting the same error as in the previous question.我遵循了@SamVarshavchik 的回答,现在我遇到了与上一个问题相同的错误。
namespace name{
struct Example_base {
protected:
static std::map<std::string, int> the_same_map;
};
template< typename T >
class Example : public Example_base
{
....
};
}
Results结果
R_X86_64_PC32 relocation at offset 0xd3 against symbol `name::Example_base::map' can not be used;不能使用针对符号“name::Example_base::map”的偏移量 0xd3 处的 R_X86_64_PC32 重定位; recompile with -fPIC使用 -fPIC 重新编译
For each usage in every compilation unit [1]: R_X86_64_PC32 relocation when using a const extern map对于每个编译单元 [1] 中的每个用法: 使用 const extern 映射时的 R_X86_64_PC32 重定位
Note that I've multiple instances of the classes that implement Example
with the same template and that I can't use this flag请注意,我有多个使用相同模板实现Example
的类的实例,并且我不能使用此标志
This calls for inheritance.这就需要继承。
namespace name{
struct Example_base {
protected:
static std::map<std::string, int> the_same_map;
};
template< typename T >
class Example : public Example_base
{
....
};
}
Now, all subclasses of any instance of the Example
template share the_same_map
.现在, Example
模板的任何实例的所有子类都共享the_same_map
。
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