Why does this template code compile?

[Peter]

In "C++ Templates The Complete Guide" book by Vandevoorde
and Josuttis I found the following template definition
near the end of chapter 8.2 ("Template Arguments"):

template<template<typename T, T*> class Buf>
class Lexer
{
static char storage[5];
Buf<char, &Lexer<Buf>::storage> buf;
};

Buf is a template template argument whose template
arguments are: type parameter T and unnamed non-type
parameter - a pointer to T. In the definition of Lexer,
Buf is instantiated so that T = char and second argument
(of type T*) is &Lexer<Buf>::storage, but why is this
second substitution correct?
How does &Lexer<Buf>::storage resolve to char*?

Here's my reasoning.

Lex<Buf>::storage is of type char[5],
&Lex<Buf>::storage is of type char(*)[5].

If we had:

Buf<char, Lexer<Buf>::storage> buf;

instead of:

Buf<char, &Lexer<Buf>::storage> buf;

in the definition of Lexer template then
Lexer<Buf>::storage would decay to char*, but why
can the template be instantiated with
&Lexer<Buf>::storage as well? I tested both definitions
here:

http://www.compileonline.com/compile_cpp11_online.php

and they both compile.

However, if I try to instantiate Lexer with
&Lexer<Buf>::storage as second argument to Buf, I get
a compilation error as expected. Here's a complete example:


template<template<typename T, T*> class Buf>
class Lexer
{
static char storage[5];
Buf<char, &Lexer<Buf>::storage> buf;
};

template<typename T, T*>
class Foo
{
};

int main()
{
Lexer<Foo> lex;
return 0;
}

Now:

1) if I try to compile the above I get the following error:

main.cpp: In instantiation of 'class Lexer<Foo>':
main.cpp:15:15: required from here
main.cpp:5:37: error: could not convert template argument
'& Lexer<Foo>::storage' to 'char*'

2) if I change "&Lexer<Buf>::storage" to
"Lexer<Buf>::storage" the code compiles

3) if I comment out the instantiation of Lexer from main()
the code compiles with either &Lexer<Buf>::storage or
Lexer<Buf>::storage as second argument to Buf.

Can you explain what happens here and why Buf template can
be instantiated with either of the two arguments?

[Victor Bazarov]

On 1/3/2014 7:25 PM, Peter wrote:
> In "C++ Templates The Complete Guide" book by Vandevoorde
> and Josuttis I found the following template definition
> near the end of chapter 8.2 ("Template Arguments"):
>
> template<template<typename T, T*> class Buf>
> class Lexer
> {
> static char storage[5];
> Buf<char, &Lexer<Buf>::storage> buf;
> };
>
> Buf is a template template argument whose template
> arguments are: type parameter T and unnamed non-type
> parameter - a pointer to T. In the definition of Lexer,
> Buf is instantiated so that T = char and second argument
> (of type T*) is &Lexer<Buf>::storage, but why is this
> second substitution correct?
> How does &Lexer<Buf>::storage resolve to char*?

I think it doesn't "resolve". Not until you actually instantiate the
template.

> Here's my reasoning.
>
> Lex<Buf>::storage is of type char[5],
> &Lex<Buf>::storage is of type char(*)[5].

No. It's not. &(Lex<Buf>::storage) would be a pointer to an array.
But since the parentheses are missing (see 5.3.1), the expression is a
pointer-to-member.

> If we had:
>
> Buf<char, Lexer<Buf>::storage> buf;
>
> instead of:
>
> Buf<char, &Lexer<Buf>::storage> buf;
>
> in the definition of Lexer template then
> Lexer<Buf>::storage would decay to char*, but why
> can the template be instantiated with
> &Lexer<Buf>::storage as well? I tested both definitions
> here:
>
> http://www.compileonline.com/compile_cpp11_online.php
>
> and they both compile.

Unless you try to instantiate the 'Lexer' template, the code only
undergoes syntax check and not the actual compilation (no conversions
are attempted).

> However, if I try to instantiate Lexer with
> &Lexer<Buf>::storage as second argument to Buf, I get
> a compilation error as expected. Here's a complete example:
>
>
> template<template<typename T, T*> class Buf>
> class Lexer
> {
> static char storage[5];
> Buf<char, &Lexer<Buf>::storage> buf;
> };
>
> template<typename T, T*>
> class Foo
> {
> };
>
> int main()
> {
> Lexer<Foo> lex;
> return 0;
> }
>
> Now:
>
> 1) if I try to compile the above I get the following error:
>
> main.cpp: In instantiation of 'class Lexer<Foo>':
> main.cpp:15:15: required from here
> main.cpp:5:37: error: could not convert template argument
> '& Lexer<Foo>::storage' to 'char*'
>
> 2) if I change "&Lexer<Buf>::storage" to
> "Lexer<Buf>::storage" the code compiles
>
> 3) if I comment out the instantiation of Lexer from main()
> the code compiles with either &Lexer<Buf>::storage or
> Lexer<Buf>::storage as second argument to Buf.

Which most likely means that the conversion is never attempted.

> Can you explain what happens here and why Buf template can
> be instantiated with either of the two arguments?

I can't explain how 'Buf' can be instantiated if it acutally can't be
instantiated (you get the error when you try to instantiate it).

V
--
I do not respond to top-posted replies, please don't ask

[Alf P. Steinbach]

On 04.01.2014 01:25, Peter wrote:
> In "C++ Templates The Complete Guide" book by Vandevoorde
> and Josuttis I found the following template definition
> near the end of chapter 8.2 ("Template Arguments"):
>
> template<template<typename T, T*> class Buf>
> class Lexer
> {
> static char storage[5];
> Buf<char, &Lexer<Buf>::storage> buf;
> };
>
> Buf is a template template argument whose template
> arguments are: type parameter T and unnamed non-type
> parameter - a pointer to T. In the definition of Lexer,
> Buf is instantiated so that T = char and second argument
> (of type T*) is &Lexer<Buf>::storage, but why is this
> second substitution correct?

It isn't.

> How does &Lexer<Buf>::storage resolve to char*?

It doesn't. The code is ungood.

> Here's my reasoning.
>
> Lex<Buf>::storage is of type char[5],
> &Lex<Buf>::storage is of type char(*)[5].

Yep.

> If we had:
>
> Buf<char, Lexer<Buf>::storage> buf;
>
> instead of:
>
> Buf<char, &Lexer<Buf>::storage> buf;
>
> in the definition of Lexer template then
> Lexer<Buf>::storage would decay to char*, but why
> can the template be instantiated with
> &Lexer<Buf>::storage as well?

It can't.

> I tested both definitions here:
>
> http://www.compileonline.com/compile_cpp11_online.php
>
> and they both compile.

Strange, it uses g++ 4.7.2 and that template definition (when
instantiated) does not compile with g++ 4.7.2 on my system.

Wait, lemme test that with the online compiler...

Nope, it failed:

[quote]

main.cpp: In instantiation of 'class Lexer<Foo>':
main.cpp:15:15: required from here

main.cpp:5:36: error: could not convert template argument '&

Lexer<Foo>::storage' to 'char*'

Buf<char, &Lexer<Buf>::storage> buf;

[/quote]

> However, if I try to instantiate Lexer with
> &Lexer<Buf>::storage as second argument to Buf, I get
> a compilation error as expected.

This doesn't make sense. It's the &Lexer<Buf>::storage that you claimed
(apparently incorrectly) compiled fine.

> Here's a complete example:
>
>
> template<template<typename T, T*> class Buf>
> class Lexer
> {
> static char storage[5];
> Buf<char, &Lexer<Buf>::storage> buf;
> };
>
> template<typename T, T*>
> class Foo
> {
> };
>
> int main()
> {
> Lexer<Foo> lex;
> return 0;
> }
>
> Now:
>
> 1) if I try to compile the above I get the following error:
>
> main.cpp: In instantiation of 'class Lexer<Foo>':
> main.cpp:15:15: required from here
> main.cpp:5:37: error: could not convert template argument
> '& Lexer<Foo>::storage' to 'char*'
>
> 2) if I change "&Lexer<Buf>::storage" to
> "Lexer<Buf>::storage" the code compiles

Yes, of course.

> 3) if I comment out the instantiation of Lexer from main()
> the code compiles with either &Lexer<Buf>::storage or
> Lexer<Buf>::storage as second argument to Buf.

Template definitions get only superficial analysis until they're
instantiated.

> Can you explain what happens here

Yes, the claim at the start, of the code compiling with the online
compiler, appears to be incorrect.

> and why Buf template can
> be instantiated with either of the two arguments?

It can't, it's a bug (of the typo kind) in the code.

I suggest reporting it to the book's errata list (check first if it's
there already):

http://www.josuttis.com/tmplbook/errata.html


Cheers & hth.,

- Alf