错误C2899:typename不能在模板声明之外使用

Question

我在MSV2010中尝试以下内容

namespace statismo {
template<>
struct RepresenterTraits<itk::Image<itk::Vector<float, 3u>, 3u> > {

    typedef itk::Image<itk::Vector<float, 3u>, 3u> VectorImageType;
    typedef VectorImageType::Pointer DatasetPointerType;
    typedef VectorImageType::Pointer DatasetConstPointerType;
    typedef typename VectorImageType::PointType PointType;
    typedef typename VectorImageType::PixelType ValueType;
};

我收到以下错误:

错误C2899:typename不能在模板声明之外使用

我们将非常感谢您提供解决方案的帮助.

Answer 1

namespace statismo {
template<>
struct RepresenterTraits<itk::Image<itk::Vector<float, 3u>, 3u> > {

    // bla

    typedef typename VectorImageType::PointType PointType;
            ^^^^^^^^

    typedef typename VectorImageType::PixelType ValueType;
            ^^^^^^^^
};

您的typename关键字放在RepresenterTraits<T>for 的明确specalization中itk::Image<itk::Vector<float, 3u>, 3u>.但是,这是一个常规类,而不是类模板.这意味着它VectorImageType不是依赖名称,编译器知道它PixelType是嵌套类型.这就是为什么它不允许使用typename.请参阅此问答.

请注意,在C++ 11,该限制已经解除,并且使用的typename是允许的,但不是必需的非模板上下文.参见例如这个例子

#include <iostream>

template<class T>
struct V 
{ 
    typedef T type; 
};

template<class T>
struct S
{
    // typename required in C++98/C++11
    typedef typename V<T>::type type;    
};

template<>
struct S<int>
{
    // typename not allowed in C++98, allowed in C++11
    // accepted by g++/Clang in C++98 mode as well (not by MSVC2010)
    typedef typename V<int>::type type;    
};

struct R
{
    // typename not allowed in C++98, allowed in C++11
    // accepted by g++ in C++98 mode as well (not by Clang/MSVC2010)
    typedef typename V<int>::type type;    
};

int main()
{
}

实例(只需使用g ++/clang和std = c ++ 98/std = c ++ 11命令行选项).