typename "dependent scope"

最新推荐文章于 2024-03-30 18:26:46 发布

转载最新推荐文章于 2024-03-30 18:26:46 发布 · 2.4k 阅读

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#c++ #compiler #gcc template

本文解决了一个C++编译错误，该错误出现在尝试访问模板内的类型时。通过添加'typename'关键字解决了问题，并解释了为何需要这样做。

【转】 http://hi.baidu.com/xgekhggjdibatxq/item/5ce5f6812121afcbb17154dc

$ 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]Error1

* main.cpp

#include<vector>

#include<memory>

template<typename T>

struct ptrModel

{

typedef std::unique_ptr<T>Type;

};

template<typenameData>

struct ptrType

{

typedef ptrModel< std::vector<Data>>::TypeType;

};

int main()

{

return0;

}

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;

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 ofptrModel 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.

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.