Serialization template classes

[Johannes Bauer]

Hello group,

I've written a serialization class which serializes arbitrary objects to
by "Block" representation. A template exists for POD objects, as do some
specializations, like:

template<> class Serializer<std::string> {
public:
static void Serialize(const std::string &Input, Block &Output) {
Serializer<unsigned int>::Serialize(Input.size(), Output);
Output.Increment(Input.size());
std::copy(Input.begin(), Input.end(), Output.endptr() - Input.size());
}
[...]

Those all work fine. Now I wanted to introduce a template that could
reduce a map which contains arbitrary types:

template<typename T1, typename T2> class Serializer< std::map<T1, T2> > {
public:
static void Serialize(const std::map<T1, T2> &Input, Block &Output) {
Serializer<unsigned int>::Serialize(Input.size(), Output);
std::map<T1, T2>::const_iterator i;
for (i = Input.begin(); i != Input.end(); i++) {
Serializer<T1>::Serialize(i->first, Output);
Serializer<T2>::Serialize(i->second, Output);
}
}

The compiler however now complains with a *very* weird error:

Common/Serializer.hpp: In static member function ‘static void
Serializer<std::map<T1, T2, std::less<_Key>,
std::allocator<std:air<const _Key, _Tp> > > >::Serialize(const
std::map<T1, T2, std::less<_Key>, std::allocator<std:air<const _Key,
_Tp> > >&, Block&)’:
Common/Serializer.hpp:61: error: expected `;' before ‘i’
Common/Serializer.hpp:62: error: ‘i’ was not declared in this scope

What am I doing wrong?

Kind regards,
Johannes

 

[Ron AF Greve]

Hi,

I usually try putting typename in front of declararions with the g++
compiler

typename std::map<T1, T2>::const_iterator i;

Not sure if that is the problem but you could give it a try.

Regards, Ron AF Greve

http://informationsuperhighway.eu

 

[Johannes Bauer]

Hi Ron,

I usually try putting typename in front of declararions with the g++
compiler

typename std::map<T1, T2>::const_iterator i;

Not sure if that is the problem but you could give it a try.

Indeed! Now what the heck did just happen there? Why do I need the
"typename" keyword - is this a compiler problem or am I misunderstanding
part of the language?

Kind regards,
Johannes

 

[Maxim Yegorushkin]

Hi Ron,




Indeed! Now what the heck did just happen there? Why do I need the
"typename" keyword - is this a compiler problem or am I misunderstanding
part of the language?

Please see here:
http://www.comeaucomputing.com/techtalk/#typename
http://ricin.lfsnal.org/books/oreilly/books/cplsian/cplsian-CHP-7-SECT-8.html

However, your code is still not quite right.

The reason is that std::map<> has three template arguments, not two.
Some (older) compilers may accept your code and assume the default
value for the third template argument.

 

[Bart van Ingen Schenau]

Hi Ron,


Indeed! Now what the heck did just happen there? Why do I need the
"typename" keyword - is this a compiler problem or am I
misunderstanding part of the language?

This is a part of the language.
The name "std::map<T1, T2>::const_iterator" is a so-called dependent
name, because the exact meaning of std::map<T1, T2>::const_iterator
depends on the values of the template parameters T1 and T2.
The problem with dependent names is that the compiler can't decide if it
is supposed to refer to a type or to a variable, and the default choice
then become to interpret the name as referring to a variable.
If the default choice is wrong, you must instruct the compiler to assume
it will be a type name with the typename keyword.

Kind regards,
Johannes

Bart v Ingen Schenau

 

[coal]

Hello group,

I've written a serialization class which serializes arbitrary objects to
by "Block" representation. A template exists for POD objects, as do some
specializations, like:

template<> class Serializer<std::string> {
  public:
    static void Serialize(const std::string &Input, Block &Output) {
      Serializer<unsigned int>::Serialize(Input.size(), Output);
      Output.Increment(Input.size());
      std::copy(Input.begin(), Input.end(), Output.endptr() - Input..size());
    }
[...]

Those all work fine. Now I wanted to introduce a template that could
reduce a map which contains arbitrary types:

template<typename T1, typename T2> class Serializer< std::map<T1, T2> > {
  public:
    static void Serialize(const std::map<T1, T2> &Input, Block &Output) {
      Serializer<unsigned int>::Serialize(Input.size(), Output);
      std::map<T1, T2>::const_iterator i;
      for (i = Input.begin(); i != Input.end(); i++) {
        Serializer<T1>::Serialize(i->first, Output);
        Serializer<T2>::Serialize(i->second, Output);
      }
    }

The compiler however now complains with a *very* weird error:

Common/Serializer.hpp: In static member function ‘static void
Serializer<std::map<T1, T2, std::less<_Key>,
std::allocator<std:air<const _Key, _Tp> > > >::Serialize(const
std::map<T1, T2, std::less<_Key>, std::allocator<std:air<const _Key,
_Tp> > >&, Block&)’:
Common/Serializer.hpp:61: error: expected `;' before ‘i’
Common/Serializer.hpp:62: error: ‘i’ was not declared in this scope

What am I doing wrong?

Wikipedia has a page on serialization that has a C++
section -- http://en.wikipedia.org/wiki/Serialization.

There's a comparison between Boost Serialization and
the C++ Middleware Writer here --
http://webEbenezer.net/comparison.html.

The C++ Middleware Writer, on line since 2002,
was the first on line, C++ code generator.


Brian Wood
Ebenezer Enterprises
www.webEbenezer.net

I recommend the articles on social topics by professor
Jonathan Katz -- http://wuphys.wustl.edu/~katz.