Template template parameter and typename

[I wish]

I wrote some test code

template <typename T>
class A {};

class B{};

template < template <typename T> typename U >
class C {};
int main( void )
{
typedef C< A > my;
return 0;
}

But I get compile error
t.cc:6: error: parse error before `typename'
t.cc:9: error: cannot declare `::main' to be a template
t.cc:9: error: too many template parameter lists in declaration of `int main()'
t.cc:9: error: syntax error before `{' token

So I chenge the second typename in line 6 to class, then the code

can compile correctly.

Why I can't use typename in original code? I think typename in template

parameter can replace class.

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[Denis Remezov]

I wish wrote:

I wrote some test code

template <typename T>
class A {};

class B{};

template < template <typename T> typename U >
class C {};
int main( void )
{
typedef C< A > my;
return 0;
}

But I get compile error
t.cc:6: error: parse error before `typename'
t.cc:9: error: cannot declare `::main' to be a template
t.cc:9: error: too many template parameter lists in declaration of `int main()'
t.cc:9: error: syntax error before `{' token

So I chenge the second typename in line 6 to class, then the code

can compile correctly.

Why I can't use typename in original code? I think typename in template

parameter can replace class.

"typename" and "class" are interchangeable when declaring a type-name (T).
U is not a type-name but a template-name, and must be declared with "class".
Think of it as in a declaration

template <typename T> class U; //cannot say "... typename U"

Denis