Typedef'ing a subtype of a template

[Ney André de Mello Zunino]

Hello.

It's been about a year since I last did some C++ programming. I am now
paying the price as I try to catch up with it.

The following is the initial snippet of a simple program I am working
on. I don't know or remember how one goes about typedef'ing a subtype of
a template.

#include <string>
#include <map>

namespace {
using namespace std;

typedef map<string, int> Word_Map;
typedef Word_Map::value_type Word_Map_Pair;
}

The g++ compiler is not happy about the second typedef. I have tried
adding /typename/ to it, but the compiler then yells at me for using
/typename/ outside of a template.

Could anyone help a rusty fellow programmer?

Thank you,

 

[red floyd]

Hello.

It's been about a year since I last did some C++ programming. I am now
paying the price as I try to catch up with it.

The following is the initial snippet of a simple program I am working
on. I don't know or remember how one goes about typedef'ing a subtype of
a template.

#include <string>
#include <map>

namespace {
using namespace std;

typedef map<string, int> Word_Map;
typedef Word_Map::value_type Word_Map_Pair;
}

The g++ compiler is not happy about the second typedef. I have tried
adding /typename/ to it, but the compiler then yells at me for using
/typename/ outside of a template.

Could anyone help a rusty fellow programmer?

I see nothing wrong with your code. Is this the entire fragment? What
compiler are you using?

 

[Ian Collins]

Hello.

It's been about a year since I last did some C++ programming. I am now
paying the price as I try to catch up with it.

The following is the initial snippet of a simple program I am working
on. I don't know or remember how one goes about typedef'ing a subtype of
a template.

#include <string>
#include <map>

namespace {
using namespace std;

typedef map<string, int> Word_Map;
typedef Word_Map::value_type Word_Map_Pair;
}

The g++ compiler is not happy about the second typedef. I have tried
adding /typename/ to it, but the compiler then yells at me for using
/typename/ outside of a template.

How can it when you don't?

Post a complete example.

 

[Ney André de Mello Zunino]

I see nothing wrong with your code. Is this the entire fragment? What
compiler are you using?

The compiler is g++ (GCC) 4.1.3. And here's a complete program. In its
current form, it _will_ compile. However, substituting the second
typedef for the third one will cause the compilation error when the
priority_queue /Word_Queue/ is instantiated in main().

#include <string>
#include <map>
#include <queue>
#include <vector>

namespace {
using namespace std;

typedef map<string, int> Word_Map;
// typedef Word_Map::value_type Word_Map_Pair;
typedef pair<string, int> Word_Map_Pair;

struct Word_Map_Pair_Comparator {
bool operator()(const Word_Map_Pair& pair_1,
const Word_Map_Pair& pair_2) {
return pair_1.second >= pair_2.second;
}
};

typedef priority_queue<Word_Map_Pair, vector<Word_Map_Pair>,
Word_Map_Pair_Comparator> Word_Queue;
}

int main(int argc, char* argv[]) {
Word_Queue word_queue;
}

Thank you,

 

[Alf P. Steinbach]

* Ney André de Mello Zunino:

The compiler is g++ (GCC) 4.1.3. And here's a complete program. In its
current form, it _will_ compile. However, substituting the second
typedef for the third one will cause the compilation error when the
priority_queue /Word_Queue/ is instantiated in main().

#include <string>
#include <map>
#include <queue>
#include <vector>

namespace {
using namespace std;

typedef map<string, int> Word_Map;
// typedef Word_Map::value_type Word_Map_Pair;

The commented definition would effectively define (note the const, which
makes the type non-assignable)

typedef pair< string const, int > Word_Map_Pair;

By the way, note that your naming convention is formally invalid.

An underscore can't be followed by an uppercase letter, it is reserved
for implementation names.

typedef pair<string, int> Word_Map_Pair;

struct Word_Map_Pair_Comparator {
bool operator()(const Word_Map_Pair& pair_1,
const Word_Map_Pair& pair_2) {
return pair_1.second >= pair_2.second;
}
};

typedef priority_queue<Word_Map_Pair, vector<Word_Map_Pair>,
Word_Map_Pair_Comparator> Word_Queue;
}

int main(int argc, char* argv[]) {

Please don't include things such as unused arguments that cause warnings.

Aim for 100% clean compiles.

Word_Queue word_queue;
}

This is OK.

However, using the non-assignable version of Word_Map_Pair as a
container element type is formally Undefined Behavior, and a good
compiler + library implementation will then issue diagnostics.


Cheers, & hth.,

- Alf

 

[Ney André de Mello Zunino]

Alf P. Steinbach escreveu:

The commented definition would effectively define (note the const, which
makes the type non-assignable)

typedef pair< string const, int > Word_Map_Pair;

I couldn't realize (remember) that the /map::value_type/ typedef is of

By the way, note that your naming convention is formally invalid.

An underscore can't be followed by an uppercase letter, it is reserved
for implementation names.

You know, I've always been a "camelCaser", of the kind who thought
underscores were really ugly. It was only *very* recently that I allowed
myself to have a different appreciation of underscores. The
experimentation is still in progress.

As for the implementation-reserved forms, I thought they concerned only
the cases where an uppercase letter follows an underscore at the
beginning of an identifier. But anyway, rusty as I am now, it's very
likely that I am missing something.

int main(int argc, char* argv[]) {

Please don't include things such as unused arguments that cause warnings.

Aim for 100% clean compiles.

Sorry about that. The code posted is a trimmed-down version of a
slightly larger program where command line arguments are indeed used. I
just forgot to remove them here.

However, using the non-assignable version of Word_Map_Pair as a
container element type is formally Undefined Behavior, and a good
compiler + library implementation will then issue diagnostics.

Thanks, Alf. Got it now.

Cheers,

 

[Bo Persson]

Ney André de Mello Zunino wrote:
:: Alf P. Steinbach escreveu:
::
:::: namespace {
:::: using namespace std;
::::
:::: typedef map<string, int> Word_Map;
:::: // typedef Word_Map::value_type Word_Map_Pair;
:::
::: The commented definition would effectively define (note the
::: const, which makes the type non-assignable)
:::
::: typedef pair< string const, int > Word_Map_Pair;
::
:: I couldn't realize (remember) that the /map::value_type/ typedef
:: is of the form pair<const Key, T>. Thanks for pointing it out.
::
::: By the way, note that your naming convention is formally invalid.
:::
::: An underscore can't be followed by an uppercase letter, it is
::: reserved for implementation names.
::
:: You know, I've always been a "camelCaser", of the kind who thought
:: underscores were really ugly. It was only *very* recently that I
:: allowed myself to have a different appreciation of underscores. The
:: experimentation is still in progress.
::
:: As for the implementation-reserved forms, I thought they concerned
:: only the cases where an uppercase letter follows an underscore at
:: the beginning of an identifier.

You are correct.

Alf is probably mixing the rules up - a double underscore is reserved
anywhere in the identifier. Underscore uppercase only at the
beginning.

_Reserved_identifier
Reserved__identifier
Non_Reserved_Identifier


Bo Persson

 

[Alf P. Steinbach]

* Bo Persson:

Ney André de Mello Zunino wrote:
:: Alf P. Steinbach escreveu:
::
:::: namespace {
:::: using namespace std;
::::
:::: typedef map<string, int> Word_Map;
:::: // typedef Word_Map::value_type Word_Map_Pair;
:::
::: The commented definition would effectively define (note the
::: const, which makes the type non-assignable)
:::
::: typedef pair< string const, int > Word_Map_Pair;
::
:: I couldn't realize (remember) that the /map::value_type/ typedef
:: is of the form pair<const Key, T>. Thanks for pointing it out.
::
::: By the way, note that your naming convention is formally invalid.
:::
::: An underscore can't be followed by an uppercase letter, it is
::: reserved for implementation names.
::
:: You know, I've always been a "camelCaser", of the kind who thought
:: underscores were really ugly. It was only *very* recently that I
:: allowed myself to have a different appreciation of underscores. The
:: experimentation is still in progress.
::
:: As for the implementation-reserved forms, I thought they concerned
:: only the cases where an uppercase letter follows an underscore at
:: the beginning of an identifier.

You are correct.

Alf is probably mixing the rules up - a double underscore is reserved
anywhere in the identifier. Underscore uppercase only at the
beginning.

_Reserved_identifier
Reserved__identifier
Non_Reserved_Identifier

Well, yeah, you're right. Additionally identifiers starting with
underscore are reserved in the global namespace.

Cheers,

- Alf