C++ 具有依赖范围的嵌套模板
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Nested templates with dependent scope
提问by user383352
What is dependent scope and what is the meaning of typename in the context of the following error?
在以下错误的上下文中，什么是依赖范围以及 typename 的含义是什么？
$ make
g++ -std=gnu++0x main.cpp
main.cpp:18:10: error: need 'typename' before 'ptrModel<std::vector<Data> >::Type' because 'ptrModel<std::vector<Data> >' is a dependent scope
make: *** [all] Error 1
/*
* main.cpp
*/
#include <vector>
#include <memory>
template<typename T>
struct ptrModel
{
typedef std::unique_ptr<T> Type;
};
template<typename Data>
struct ptrType
{
typedef ptrModel< std::vector<Data> >::Type Type;
};
int main()
{
return 0;
}
回答by Tyler McHenry
The compiler told you exactly what to do. Write typenamebefore ptrModel<std::vector<Data> >::Type, like so:
编译器会准确地告诉您要做什么。写typename之前ptrModel<std::vector<Data> >::Type，像这样：
typedef typename ptrModel<std::vector<Data> >::Type Type;
The reasonfor this requirement is that the compiler doesn't at this point know whether ptrModel<std::vector<Data> >::Typedescribes a member variable or a nested type. It can't even figure that out by looking at the definition of ptrModelbecause there might be a specialization of ptrModelfor std::vector<Data>somewhere else in the program that it hasn't gotten to yet which changes which of these things ::Typerefers to. So you need to tell it explicitly.
这个要求的原因是编译器此时不知道是ptrModel<std::vector<Data> >::Type描述成员变量还是嵌套类型。它甚至无法通过查看 的定义来弄清楚，ptrModel因为可能在程序中的其他地方有一个专门化的ptrModelforstd::vector<Data>尚未到达哪些更改这些事物中的哪一个::Type。所以你需要明确地告诉它。
The name ptrModel<std::vector<Data> >::Typehas a "dependent scope" because it is in a scope that dependson the instantiation of a template.
该名称ptrModel<std::vector<Data> >::Type具有“依赖范围”，因为它位于依赖于模板实例化的范围内。
