Initializing a map...

[barcaroller]

Is there a way in C++ to initialize an STL map in one statement (the way
arrays can be initialized in C)?

For example, instead of using:

map<type1,type2mymap;
mymap[key1] = value1;
mymap[key2] = value2;
I would like to use something like:

// wrong syntax!
map<type1,type2mymap = { (key1, value1), (key2, value2) };

[Jeff Schwab]

barcaroller wrote:

Is there a way in C++ to initialize an STL map in one statement (the way
arrays can be initialized in C)?

For example, instead of using:

map<type1,type2mymap;
mymap[key1] = value1;
mymap[key2] = value2;
I would like to use something like:

// wrong syntax!
map<type1,type2mymap = { (key1, value1), (key2, value2) };

There's no special syntax for maps. You do have a few options, though.
One is to initialize an array with the nicer syntax, then initialize
the map from the array.

typedef std::map<type1, type2map_type;
typedef map_type::value_type pair_type;

template<typename T, std::size_t z>
std::size_t size(T const (&a)[z]) {
return z;
}

int main() {
pair_type initializers[] =
{ pair_type(key1, value1), pair_type(key2, value2) };
map_type m(initializers, initializers + size(initializers));
}

Another option is to create the map within a function, then return it by
value.

map_type create_map() {
map_type result;
result.insert(pair_type(key1, value1));
result.insert(pair_type(key2, value2));
return result;
}

int main() {
map_type map = create_map();
}

A third option is to let the map start out empty, then use a function to
populate it.

void populate(map_type& m) {
m.insert(pair_type(key1, value1));
m.insert(pair_type(key2, value2));
}

int main() {
map_type m;
populate(m);
}

[Jim Langston]

Your messages always have the text as an attachment instead of in the body
of the message. Just thought you might want to know.

--
Jim Langston
ta*******@rocketmail.com
"Sam" <sa*@email-scan.comwrote in message
news:co*****************************@commodore.ema il-scan.com...

[James Kanze]

On Feb 21, 4:22 am, "Alf P. Steinbach" <[e-mail]:

* Jim Langston:

Your messages always have the text as an attachment instead
of in the body of the message. Just thought you might want
to know.

Only for Outlook Express users... ;-)

Or Google groups.

It seems Outlook Express is the only commonly used newsreader
that doesn't handle digitally signed messages correctly.

For what definition of "correctly"? According to the RFC,
attachments are not allowed in news, so arguably, "correctly"
would mean not displaying or propagating the message:-).
(Realistically, of course: the first rule is always "be liberal
in what you accept, and conservative in what you send". No good
newsreader would ever append an attachment, but all good
newsreaders would accept it. And allowing at least a digital
signature does seem a reasonable extension to me.)

--
James Kanze (GABI Software) email:ja*********@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[Sam]

Jeff Schwab writes:

>>
#include <iostream>
#include <ostream>

int main()
{
std::map<int, intm(map_initializer(3,4)(5,6)(7,8));

...

> return 0;
}

That's a neat idea. It could probably be made a little more efficient
by replacing the calls to map::operator[] with calls to map::find and

operator[] takes only one argument. You can use a std::pair, but it'll make
this even more ugly.

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[Jeff Schwab]

James Kanze wrote:

On Feb 21, 1:22 am, Jeff Schwab <[e-mail]:

>barcaroller wrote:

>>Is there a way in C++ to initialize an STL map in one
statement (the way arrays can be initialized in C)?

>>For example, instead of using:

>> map<type1,type2mymap;
mymap[key1] = value1;
mymap[key2] = value2;

>>I would like to use something like:

>> // wrong syntax!
map<type1,type2mymap = { (key1, value1), (key2, value2) };

>There's no special syntax for maps.

I think there will be in the next version of the standard. (I
know that there was a proposal for extended initializers, but
I'm not sure what the current status of the proposal was.)

The proposal:
http://www.open-std.org/jtc1/sc22/wg...2007/n2215.pdf

I didn't know about that. Very neat. I'm not thrilled with g++ warning
me about:

std::tr1::array<int, 42= { 0 }; // g++ wants { { 0 } }

It would bother me less if there were consistent syntax for primitive
and UD types.

>You do have a few options, though.

> One is to initialize an array with the nicer syntax, then initialize
the map from the array.

This is the only way to create a const map.

That's a very good point, and is the reason my "not really
initialization" solutions are inferior.

> int main() {
pair_type initializers[] =
{ pair_type(key1, value1), pair_type(key2, value2) };
map_type m(initializers, initializers + size(initializers));
}

I often find it worthwhile to define a special structure for
this, something along the lines of:

typedef std::map< std::string, double Map ;
struct MapInit
{
char const* key ;
double value ;
operator Map::value_type() const
{
return Map::value_type( std::string( key ), value ) ;
}
} ;

Is the key stored as a char const* so that construction of the
initializers does not require any run-time overhead? Does MapInit count
as a POD type, and is there benefit to using POD initializers?