[英]Calling member function template of class template
I have the following program (sorry, it is rather complex, but already a boiled-down version from a larger program): 我有以下程序(对不起,它相当复杂,但已经是一个较大程序的煮沸版本):
#include <stdio.h>
// two different vector types
template <typename T, int N>
struct Vec1
{
Vec1() { puts("Vec1()"); }
};
template <typename T, int N>
struct Vec2
{
Vec2() { puts("Vec2()"); }
};
// a function wrapper
template <typename T, int N>
struct MyFct
{
template <template <typename, int> class VEC>
static inline VEC<T,N>
apply()
{
puts("MyFct::apply()");
return VEC<T,N>();
}
};
// tester
#if 0
template <typename T, int N, template <typename, int> class FCT>
struct Tester
{
static inline void test()
{
puts("Tester::test");
Vec1<T,N> v1;
v1 = FCT<T,N>::apply<Vec1>();
Vec2<T,N> v2;
v2 = FCT<T,N>::apply<Vec2>();
}
};
#endif
int
main()
{
MyFct<float,16>::apply<Vec1>();
MyFct<int,32>::apply<Vec2>();
// Tester<float,16,MyFct>::test();
return 0;
}
The program compiles with #if 0
(without Tester
), but with #if 1
(with Tester
) I get the error message 该程序使用#if 0
(不带Tester
)编译,但使用#if 1
(使用Tester
)我收到错误消息
g++ -Wall -o templatetemplate2a templatetemplate2a.C
templatetemplate2a.C: In static member function 'static void Tester<T, N, FCT>::test()':
templatetemplate2a.C:41:30: error: missing template arguments before '>' token
v1 = FCT<T,N>::apply<Vec1>();
^
templatetemplate2a.C:41:32: error: expected primary-expression before ')' token
v1 = FCT<T,N>::apply<Vec1>();
^
templatetemplate2a.C:43:30: error: missing template arguments before '>' token
v2 = FCT<T,N>::apply<Vec2>();
^
templatetemplate2a.C:43:32: error: expected primary-expression before ')' token
v2 = FCT<T,N>::apply<Vec2>();
This is surprising since the direct application (see main()
) of apply<Vec1>()
and apply<Vec2>()
works fine. 这是令人惊讶的,因为直接应用(参见main()
) apply<Vec1>()
和apply<Vec2>()
工作正常。 However, if I do the same inside of Tester::test()
, I get the error messages. 但是,如果我在Tester::test()
执行相同操作,则会收到错误消息。 Is this some name-scope issue? 这是一些名称范围问题吗? Are the template classes Vec1
and Vec2
unknown inside of Tester
? Tester
的模板类Vec1
和Vec2
未知的吗? How can I make them known? 我怎样才能让他们知道? Or is there some other problem? 还是有其他问题吗?
apply()
is a member function template, you need to use the keyword template to tell the compiler that it's a template, when calling it with dependent name. apply()
是一个成员函数模板,当使用依赖名称调用它时,需要使用关键字模板告诉编译器它是一个模板。 Note the difference between FCT<T,N>
and MyFct<float,16>
, the former depends on template parameters FCT
, T
and N
, while the later doesn't. 注意FCT<T,N>
和MyFct<float,16>
之间的区别,前者取决于模板参数FCT
, T
和N
,而后者则不然。
Inside a template definition, template can be used to declare that a dependent name is a template. 在模板定义中,模板可用于声明从属名称是模板。
eg 例如
v1 = FCT<T,N>::template apply<Vec1>();
v2 = FCT<T,N>::template apply<Vec2>();
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