c – 私有类专门化错误

在编译期间,clang中的代码我有警告(在vc中它工作正常):

warning : explicit specialization of ‘Helper’ within class scope is a
Microsoft extension [-Wmicrosoft]

#include <stdio.h>
#include <string>

enum class Car { BMW };

class C
{
    static void Method() { puts("inner");}
};

template<typename T>
class BaseClass
{
private:
    template<typename V> 
    struct Helper;

    template<>
    struct Helper<Car>
    {
        typedef C InnerType;
        static const char* Name() { return "Car"; }
    };

    typedef Helper<T> Info;
    typedef typename Info::InnerType InnerType;

private:
    T v;

protected:
    BaseClass()
    { }

public:
    T Value() const { return v; }
    std::string Name() const { return Info::Name(); }
    static void Print() { InnerType::Method(); }
};


class MyCar : public BaseClass<Car>
{
public:
    MyCar() : BaseClass() {}
};

int main()
{
    MyCar a;
    printf("%s\n", a.Name().c_str());
//  a.Print();
}

我试图在BaseClass之外移动Helper类的特化以与标准兼容:

template<> template<>
struct BaseClass<Car>::Helper<Car>
{
    typedef C InnerType;
    static const char* Name() { return "Car"; }
};

但是现在我有编译错误:

error: implicit instantiation of undefined template ‘BaseClass::Helper’

如果我删除该行:typedef typename Info :: InnerType InnerType; (以及功能Print中的相关用法)然后一切正常.

是否可以修复此错误?我想把我的助手课程保密.

最佳答案 你可以这样做:

#include <stdio.h>
#include <string>

enum class Car { BMW };

class C
{
    static void Method() { puts("inner");}
};

template<typename T>
class BaseClass
{
private:
    template<typename V> 
    struct Helper;

    template<typename V>
    using Info = Helper<V>;

    template<typename V>
    using InnerType = typename Info<V>::InnerType;


private:
    T v;

protected:
    BaseClass()
    { }

public:
    T Value() const { return v; }
    std::string Name() const { return Info<T>::Name(); }
    static void Print() { InnerType<T>::Method(); }
};

template<> template<>
struct BaseClass<Car>::Helper<Car>
{
    typedef C InnerType;
    static const char* Name() { return "Car"; }
};


class MyCar : public BaseClass<Car>
{
public:
    MyCar() : BaseClass() {}
};

int main()
{
    MyCar a;
    printf("%s\n", a.Name().c_str());
    //a.Print();
}

(gcc 5.1 / clang 3.6,-std = C 11)

该程序打印“汽车”.

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