Creating a stringstream in a parameter

[anongroupaccount]

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]

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an**************@googlemail.com wrote:

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?

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(const SourceType& value)
{
std::basic_ostringstream<typename StringType::value_type> stream;
stream << value;
return stream.str();
}

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

Or using boost:
"String" + boost::lexical_cast<std::string>(123)

Hope this helps.

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

<an**************@googlemail.com> wrote in message
news:11**********************@z14g2000cwz.googlegr oups.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?

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

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

It would also be nicer if you used a static_cast:

static_cast<ostringstream&>(ostringstream("String" )<<3).str()
--
John Carson

[Dervish]

>static_cast<ostringstream&>(ostringstream("String ")<<3).str()

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

[John Carson]

"Dervish" <DA*******@yandex.ru> wrote in message
news:11**********************@g44g2000cwa.googlegr oups.com

static_cast<ostringstream&>(ostringstream("String" )<<3).str()

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

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<ostringstream&>(ostringstream("String" ).seekp(0,ios::end)<<
3).str()
--
John Carson

[Earl Purple]

John Carson wrote:

"Dervish" <DA*******@yandex.ru> wrote in message
news:11**********************@g44g2000cwa.googlegr oups.com

static_cast<ostringstream&>(ostringstream("String" )<<3).str()

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

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<ostringstream&>(ostringstream("String" ).seekp(0,ios::end)<<
3).str()

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]

"Earl Purple" <ea********@gmail.com> wrote in message
news:11**********************@f14g2000cwb.googlegr oups.com

John Carson wrote:

"Dervish" <DA*******@yandex.ru> wrote in message
news:11**********************@g44g2000cwa.googlegr oups.com

static_cast<ostringstream&>(ostringstream("String" )<<3).str()

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

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<ostringstream&>(ostringstream("String" ).seekp(0,ios::end)<<
3).str()

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.

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<char> and thus operator<< returns a reference to
this base class of basic_ostream<char> rather than a reference to
ostringstream. basic_ostream<char> 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<char>.

--
John Carson