问题描述
是否无法从模板类的对象访问模板类的类型别名(或 typedef
)?例如,为什么以下情况是不可能的:
template <typename TKey,typename TData>
class MyClass
{
public:
using key_t = TKey;
using data_t = TData;
TKey key;
TData data;
MyClass(TKey _key,TData _data) : key(_key),data(_data) { }
};
int main() {
MyClass<int,float> mc{1,1.0f};
using DataType = typename mc.data_t; //error: expected ';' after alias declaration
mc.data_t newData = 2.0f; //error: Cannot refer to type member in 'MyClass<int,float> with '.'
return 0;
}
还有其他方法可以做这样的事情吗?
解决方法
你的语法错误。改为:
int main() {
MyClass<int,float> mc{1,1.0f};
using DataType = typename MyClass<int,float>::data_t;
MyClass<int,float>::data_t newData = 2.0f;
return 0;
}
如果您想直接从 mc
获取别名,您可以 using DataType = decltype(mc)::data_t;
。您实际上可以避免任何重复的 MyClass<int,float>
:
int main() {
MyClass<int,1.0f};
// suppose we cannot or do not want to spell out MyClass<int,float> again...
using my_class_t = decltype(mc);
using data_t = my_class_t::data_t;
data_t newData = 2.0f;
}