具有从属作用域的嵌套模板 [英] Nested templates with dependent scope

问题描述

什么是依赖作用域，在以下错误的上下文中typename的含义是什么？

What is dependent scope and what is the meaning of typename in the context of the following error?

$ 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;
}

推荐答案

去做。在之前写入 typename > ptrModel< std :: vector< Data> > :: Type ，如下所示：

The compiler told you exactly what to do. Write typename before ptrModel<std::vector<Data> >::Type, like so:

 typedef typename ptrModel<std::vector<Data> >::Type Type;

此要求的 reason 是编译器不在此点知道 ptrModel< std :: vector< Data> > :: Type 描述了成员变量或嵌套类型。它甚至不能通过查看 ptrModel 的定义，因为可能有一个专门化 ptrModel for std :: vector< Data> 在程序中的其他地方，它改变了这些东西 :: Type 是指。因此，您需要明确告诉它。

The reason for this requirement is that the compiler doesn't at this point know whether ptrModel<std::vector<Data> >::Type describes a member variable or a nested type. It can't even figure that out by looking at the definition of ptrModel because there might be a specialization of ptrModel for std::vector<Data> somewhere else in the program that it hasn't gotten to yet which changes which of these things ::Type refers to. So you need to tell it explicitly.

名称 ptrModel< std :: vector< Data& > :: Type 有一个dependent scope，因为它在依赖于模板的实例化的范围

The name ptrModel<std::vector<Data> >::Type has a "dependent scope" because it is in a scope that depends on the instantiation of a template.

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