Template Template Parameters

[* Tong *]

First of all, thanks mlimber for answering my previous question. Now...

I'm following the example in "C++ Templates: The Complete Guide",
section 5.4 Template Template Parameters, and I'm wondering if it's
basics/stack8.hpp example can be compiled under g++ 4.

First a bit background about template template:

5.4 Template Template Parameters

It can be useful to allow a template parameter itself to be a class
template. Again, our stack class template can be used as an example. To
use a different internal container for stacks, the application programmer
has to specify the element type twice. Thus, to specify the type of the
internal container, you have to pass the type of the container and the
type of its elements again:

Stack<int,std::vector<int vStack; // integer stack that uses a vector

Using template template parameters allows you to declare the Stack class
template by specifying the type of the container without respecifying the
type of its elements:

stack<int,std::vectorvStack; // integer stack that uses a vector

To do this you must specify the second template parameter as a template
template parameter...

To solve it, the book proposed the following solution (extracted to
preserve problem only):

$ cat -n templ-templ.cc
1 #include <memory>
2
3 template< typename ALLOC = std::allocator<int
4 class c {};
5
6 template <typename T,
7 template <typename ELEM,
8 typename ALLOC = std::allocator<ELEM
9 class CONT = std::deque>
10 class Stack { };
11
12 int main()
13 {
14 }
However, it doesn't compile under my g++:

$ g++ --version
g++ (GCC) 4.2.3 (Debian 4.2.3-1)

$ g++ -g templ-templ.cc -o templ-templ
templ-templ.cc:9: error: invalid default argument for a template template parameter

Anyone can help make it compile?

Thanks

tong

[Bo Persson]

* Tong * wrote:

To solve it, the book proposed the following solution (extracted to
preserve problem only):

$ cat -n templ-templ.cc
1 #include <memory>
2
3 template< typename ALLOC = std::allocator<int
4 class c {};
5
6 template <typename T,
7 template <typename ELEM,
8 typename ALLOC = std::allocator<ELEM
9 class CONT = std::deque>
10 class Stack { };
11
12 int main()
13 {
14 }
However, it doesn't compile under my g++:

$ g++ --version
g++ (GCC) 4.2.3 (Debian 4.2.3-1)

$ g++ -g templ-templ.cc -o templ-templ
templ-templ.cc:9: error: invalid default argument for a template
template parameter

Anyone can help make it compile?

What about adding an

#include <deque>

to tell the compiler what the default parameter is?
You might also want to check Josuttis' list of known typos in the
book, like <memoryinstead of <allocator>, at this address:

http://www.josuttis.com/tmplbook/
Bo Persson

[* Tong *]

On Wed, 13 Aug 2008 01:32:51 +0200, Bo Persson wrote:

>templ-templ.cc:9: error: invalid default argument for a template
template parameter

Anyone can help make it compile?

What about adding an

#include <deque>

to tell the compiler what the default parameter is?

yes, yes, silly me.

Thanks.

[PeterAPIIT]

Invalid default argument for a template template is caused by these
two lines as below:

template <typename ALLOC = std::alocator<int
class C;

This default template argument need <memoryheader file
template <typename T,

template <
typename ELEM,
typename ALLOC = std::allocator<ELEM>

>,

class CONT = std::deque

>

I wonder how this can compile under standard C++ compiler.

A template inside a template parameter.

How to achieve this.

Thanks.

[red floyd]

Pe********@gmail.com wrote:

Invalid default argument for a template template is caused by these
two lines as below:

template <typename ALLOC = std::alocator<int
class C;

This default template argument need <memoryheader file
template <typename T,

template <
typename ELEM,
typename ALLOC = std::allocator<ELEM>

>,

class CONT = std::deque

>

I wonder how this can compile under standard C++ compiler.

A template inside a template parameter.

How to achieve this.

That's the whole point of a template template parameter.