我有一个模板类,在其中我使用typedef来声明一个映射,如下所示:
#include <map>
template <typename T> class LocalStub {
typedef std::map<T, T> QueryMap;
typedef std::pair<T, T> QueryPair;
typedef QueryMap::iterator QueryMapIterator;
public:
LocalStub();
~LocalStub();
void AddQuery(const T &, const T &);
const T &Answer(const T &) const;
private:
QueryMap _queryMap;
};
编译错误
../src/LocalStub/include/localstub.hpp:12: error: declaration of 'class T'
../src/LocalStub/include/localstub.hpp:11: error: shadows template parm 'class T'
../src/LocalStub/include/localstub.hpp:13: error: template declaration of 'typedef'
../src/LocalStub/include/localstub.hpp:14: error: declaration of 'class T'
../src/LocalStub/include/localstub.hpp:11: error: shadows template parm 'class T'
../src/LocalStub/include/localstub.hpp:15: error: template declaration of 'typedef'
../src/LocalStub/include/localstub.hpp:16: error: declaration of 'class T'
../src/LocalStub/include/localstub.hpp:11: error: shadows template parm 'class T'
../src/LocalStub/include/localstub.hpp:17: error: template declaration of 'typedef'
../src/LocalStub/include/localstub.hpp:26: error: 'QueryMap' does not name a type
我做错了什么?我不明白为什么我会犯那个错误。
您忘记用QueryMapIterator
编写关键字typename
。这是更新版本:
template <typename T> class LocalStub {
typedef std::map<T, T> QueryMap;
typedef std::pair<T, T> QueryPair;
typedef typename QueryMap::iterator QueryMapIterator;
public:
LocalStub();
~LocalStub();
void AddQuery(const T &, const T &);
const T &Answer(const T &) const;
private:
QueryMap _queryMap;
};
这个要求的原因是编译器在这一点上不知道QueryMapIterator
描述的是成员变量还是嵌套类型
TL;DR-typedef typename QueryMap::iterator QueryMapIterator;
加长版:QueryMap::iterator
是一个依赖名称,因此需要在使用typedef
之前使用typename
。依赖名称问题来自于这样一个事实,即在模板构造type::something
中,something
可能指类型、值或函数,如下例所示:
template<class T>
struct foo{
void bar(){ T::something; }
};
struct baz{
using something = int;
};
struct bez{
static const int something = 0;
};
因此,如果您传递的名称引用了一个类型,则需要typename
向编译器提供额外信息。