Creating a stringstream in a parameter

[anongroupaccount@googlemail.com]

I have a base class that takes a string to its constructor. I want to
construct a concrete class that does this:

class concrete: public base
{
public:
concrete() : base( /* create a stringstream here and convert to
string */ ){}
};

However, I can't get it to work. I can pass this:

ostringstream(" String").str()

but not this:

(ostringstream( "String") << 3).str()

or this:

((ostringstream )(ostringstream ("String") << 3)).str()

which is what I want to do (as I need to build the string on the fly.

What do I do?

[Thomas Jakob]

Re: Creating a stringstream in a parameter

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Hash: SHA1

anongroupaccoun t@googlemail.co m wrote:[color=blue]
> I have a base class that takes a string to its constructor. I want to
> construct a concrete class that does this:
>
> class concrete: public base
> {
> public:
> concrete() : base( /* create a stringstream here and convert to
> string */ ){}
> };
>
> However, I can't get it to work. I can pass this:
>
> ostringstream(" String").str()
>
> but not this:
>
> (ostringstream( "String") << 3).str()
>
> or this:
>
> ((ostringstream )(ostringstream ("String") << 3)).str()
>
> which is what I want to do (as I need to build the string on the fly.
>
> What do I do?
>[/color]


I would prefer to use a template function to convert any type to a
string using a stringstream:

template <class StringType, class SourceType>
inline StringType string_cast(con st SourceType& value)
{
std::basic_ostr ingstream<typen ame StringType::val ue_type> stream;
stream << value;
return stream.str();
}

Using it:
"String" + string_cast<std ::string>(123)

Or using boost:
"String" + boost::lexical_ cast<std::strin g>(123)

Hope this helps.

- --
Greetings, Thomas Jakob
quicix (at) gmail (dot) com
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[John Carson]

Re: Creating a stringstream in a parameter

<anongroupaccou nt@googlemail.c om> wrote in message
news:1136290385 .549960.296830@ z14g2000cwz.goo glegroups.com[color=blue]
> I have a base class that takes a string to its constructor. I want to
> construct a concrete class that does this:
>
> class concrete: public base
> {
> public:
> concrete() : base( /* create a stringstream here and convert to
> string */ ){}
> };
>
> However, I can't get it to work. I can pass this:
>
> ostringstream(" String").str()
>
> but not this:
>
> (ostringstream( "String") << 3).str()
>
> or this:
>
> ((ostringstream )(ostringstream ("String") << 3)).str()
>
> which is what I want to do (as I need to build the string on the fly.
>
> What do I do?[/color]

You need to cast to a reference to ostringstream, not to an ostringstream
object:

((ostringstream &)(ostringstrea m("String")<<3) ).str()

It would also be nicer if you used a static_cast:

static_cast<ost ringstream&>(os tringstream("St ring")<<3).str( )


--
John Carson

[Dervish]

Re: Creating a stringstream in a parameter

>static_cast<os tringstream&>(o stringstream("S tring")<<3).str ()

To make this work I have added seekp:
(static_cast<os tringstream&>(( ostringstream(" String").seekp( 0,ios::end)
<< 3))).str()

[John Carson]

Re: Creating a stringstream in a parameter

"Dervish" <DAkhlynin@yand ex.ru> wrote in message
news:1136293978 .774962.193450@ g44g2000cwa.goo glegroups.com[color=blue][color=green]
>> static_cast<ost ringstream&>(os tringstream("St ring")<<3).str( )[/color]
>
> To make this work I have added seekp:
> (static_cast<os tringstream&>(( ostringstream(" String").seekp( 0,ios::end)
> << 3))).str()[/color]

What "works" depends on what the objective is, on which the OP was not
explicit. If the objective is to add a 3 to the end of "String", then you
are right. You can, incidentally, make do with less brackets:

static_cast<ost ringstream&>(os tringstream("St ring").seekp(0, ios::end)<<
3).str()


--
John Carson

[Earl Purple]

Re: Creating a stringstream in a parameter


John Carson wrote:[color=blue]
> "Dervish" <DAkhlynin@yand ex.ru> wrote in message
> news:1136293978 .774962.193450@ g44g2000cwa.goo glegroups.com[color=green][color=darkred]
> >> static_cast<ost ringstream&>(os tringstream("St ring")<<3).str( )[/color]
> >
> > To make this work I have added seekp:
> > (static_cast<os tringstream&>(( ostringstream(" String").seekp( 0,ios::end)
> > << 3))).str()[/color]
>
> What "works" depends on what the objective is, on which the OP was not
> explicit. If the objective is to add a 3 to the end of "String", then you
> are right. You can, incidentally, make do with less brackets:
>
> static_cast<ost ringstream&>(os tringstream("St ring").seekp(0, ios::end)<<
> 3).str()
>[/color]

Do not cast away what the compiler tells you is not allowed. The
compiler is telling you that for a reason.

ostringstream(" String") creates a temporary. Now you can call str() on
it because that is a const function. But you cannot call << 3 because
that is a non-const function - it modifies the temporary. And that is
undefined behaviour.

No need for a one-liner. Use an inline function. One was supplied a few
posts above.

[John Carson]

Re: Creating a stringstream in a parameter

"Earl Purple" <earlpurple@gma il.com> wrote in message
news:1136309414 .300233.221280@ f14g2000cwb.goo glegroups.com[color=blue]
> John Carson wrote:[color=green]
>> "Dervish" <DAkhlynin@yand ex.ru> wrote in message
>> news:1136293978 .774962.193450@ g44g2000cwa.goo glegroups.com[color=darkred]
>>>> static_cast<ost ringstream&>(os tringstream("St ring")<<3).str( )
>>>
>>> To make this work I have added seekp:
>>> (static_cast<os tringstream&>(( ostringstream(" String").seekp( 0,ios::end)
>>> << 3))).str()[/color]
>>
>> What "works" depends on what the objective is, on which the OP was
>> not explicit. If the objective is to add a 3 to the end of "String",
>> then you are right. You can, incidentally, make do with less
>> brackets:
>>
>> static_cast<ost ringstream&>(os tringstream("St ring").seekp(0, ios::end)<<
>> 3).str()
>>[/color]
>
> Do not cast away what the compiler tells you is not allowed. The
> compiler is telling you that for a reason.
>
> ostringstream(" String") creates a temporary. Now you can call str() on
> it because that is a const function. But you cannot call << 3 because
> that is a non-const function - it modifies the temporary. And that is
> undefined behaviour.[/color]

The reason for the non-compilation is not because a temporary has been
modified. Modifying a temporary is perfectly legal. You may note that the
following argument compiles without warnings:

string("String" ).append(" extra bit")

The compilation fails because operator<< is inherited from ostringstream's
base class of basic_ostream<c har> and thus operator<< returns a reference to
this base class of basic_ostream<c har> rather than a reference to
ostringstream. basic_ostream<c har> has no str() member function and that is
the error message given --- nothing about modifying a temporary. The cast is
a downcast, made with knowledge that the object referenced is indeed an
ostringstream and not merely a basic_ostream<c har>.

--
John Carson