std::stringstream returning back values

[TBass]

Hi.

I wrote a "tag" class to store values. The user gets to store most any
type he would need. Instead of getting too complicatated, I decided I
would store the value as a stringstream, then overload the SetValue
method.

....

protected:
std::stringstre am m_szValue;

....


void
CFusionTag::Set Value( char *szValue )
{
m_szValue << szValue;

MessageBox::Sho w( m_szValue.str() .c_str() );

} /* ::SetValue */


void
CFusionTag::Set Value( unsigned int uiValue )
{
m_szValue << uiValue;
}

void
CFusionTag::Set Value( int iValue )
{
m_szValue << iValue;
}

void
CFusionTag::Set Value( float fValue )
{
m_szValue << fValue;
}

void
CFusionTag::Set Value( double dValue )
{
m_szValue << dValue;
}

void
CFusionTag::Set Value( long lValue )
{
m_szValue << lValue;
}


That works fine (I'm aware of the obvious flaw in this code, but I use
taking advantage of that for debugging purposes). The value gets
stored.

My problem (which is probably obvious to everyone but me) is returning
the value when someone calls the GetValue method.

void
CFusionTag::Get Value( char *szValue )
{

MessageBox::Sho w( "Sending back a char value!" );
szValue = (char *)m_szValue.str ().c_str();
}

The char string returned is always blank. I thought this would be
pretty simple, but STL keeps getting the better of me. Can anyone
point me in the right direction?

Thanks in advance,
T

[TBass]

Re: std::stringstre am returning back values

I don't know if this would be the "C++" way, but doing it the way I
would do it in C worked.

void
CFusionTag::Get Value( char *szValue )
{
memcpy( szValue, m_szValue.str() .c_str(), m_szValue.str() .length() );
}

[Daniel T.]

Re: std::stringstre am returning back values

In article
<32f67c60-fcdf-4fb6-8974-dd0d59fc2103@e4 g2000hsg.google groups.com>,
TBass <tbj@automatedd esign.comwrote:

Hi.
>
I wrote a "tag" class to store values. The user gets to store most any
type he would need. Instead of getting too complicatated, I decided I
would store the value as a stringstream, then overload the SetValue
method.
>
...
>
protected:
std::stringstre am m_szValue;
>
...
>
>
void
CFusionTag::Set Value( char *szValue )
{
m_szValue << szValue;
>
MessageBox::Sho w( m_szValue.str() .c_str() );
>
} /* ::SetValue */
>
>
void
CFusionTag::Set Value( unsigned int uiValue )
{
m_szValue << uiValue;
}
>
void
CFusionTag::Set Value( int iValue )
{
m_szValue << iValue;
}
>
void
CFusionTag::Set Value( float fValue )
{
m_szValue << fValue;
}
>
void
CFusionTag::Set Value( double dValue )
{
m_szValue << dValue;
}
>
void
CFusionTag::Set Value( long lValue )
{
m_szValue << lValue;
}
>
>
That works fine (I'm aware of the obvious flaw in this code, but I use
taking advantage of that for debugging purposes). The value gets
stored.
>
My problem (which is probably obvious to everyone but me) is returning
the value when someone calls the GetValue method.
>
void
CFusionTag::Get Value( char *szValue )
{
>
MessageBox::Sho w( "Sending back a char value!" );
szValue = (char *)m_szValue.str ().c_str();
}
>
The char string returned is always blank.

Your not returning anything. Try this:

std::string
CFusionTag::Get Value() const
{
return m_szValue.str() ;
}

I thought this would be pretty simple, but STL keeps getting the
better of me. Can anyone point me in the right direction?

Your having problems because you are trying to use C idioms (char*)
instead of C++ idioms (string.)

[zhangyw80@yahoo.com.cn]

Re: std::stringstre am returning back values

On 1ÔÂ9ÈÕ, ÉÏÎç10ʱ16·Ö, TBass <t...@automated design..comwrot e:

I don't know if this would be the "C++" way, but doing it the way I
would do it in C worked.
>
void
CFusionTag::Get Value( char *szValue )
{
memcpy( szValue, m_szValue.str() .c_str(), m_szValue.str() .length());
>
>
>
}- Òþ²Ø±»ÒýÓÃÎÄ×Ö -
>
- ÏÔʾÒý

the folowing is another C way through passing argument by pointer:
void
CFusionTag::Get Value(const char** szValue)
{
if(szValue != NULL)
{
*szValue = m_szValue.str() .c_str();
}
}

[acehreli@gmail.com]

Re: std::stringstre am returning back values

On Jan 8, 10:36 pm, zhangy...@yahoo .com.cn wrote:

On 1ÔÂ9ÈÕ, ÉÏÎç10ʱ16·Ö, TBass <t...@automated design.comwrote :I don't know if this would be the "C++" way, but doing it the way I

would do it in C worked.

>

void
CFusionTag::Get Value( char *szValue )
{
memcpy( szValue, m_szValue.str() .c_str(), m_szValue.str() .length() );

Hmmm... Do we have enough space at szValue? Probably not! :(

the folowing is another C way through passing argument by pointer:
void
CFusionTag::Get Value(const char** szValue)
{
if(szValue != NULL)
{
*szValue = m_szValue.str() .c_str();

That doesn't work... The std::string value that str() returns is a
temporary object that dies at the semicolon. So, the C-style string
that is returned by c_str() is not valid beyond that point.

Ali

[TBass]

Re: std::stringstre am returning back values

That doesn't work... The std::string value that str() returns is a
temporary object that dies at the semicolon. So, the C-style string
that is returned by c_str() is not valid beyond that point.
>

Ah ha! That's been my problem. I did not know that it was temporary.
Thanks!


[snip]

I thought this would be pretty simple, but STL keeps getting the
better of me. Can anyone point me in the right direction?

Your having problems because you are trying to use C idioms (char*)
instead of C++ idioms (string.)
[/snip]

The reason for the char * as an argument was because it's actually a
vector that I'm passing into the structure.

std::vector<cha rmyvector(500);
mytag->GetValue( &myvector[0] );

[Daniel T.]

Re: std::stringstre am returning back values

TBass <tbj@automatedd esign.comwrote:

That doesn't work... The std::string value that str() returns is a
temporary object that dies at the semicolon. So, the C-style string
that is returned by c_str() is not valid beyond that point.

>
>
Ah ha! That's been my problem. I did not know that it was temporary.
Thanks!
>
>
[snip]

I thought this would be pretty simple, but STL keeps getting the
better of me. Can anyone point me in the right direction?

>
Your having problems because you are trying to use C idioms (char*)
instead of C++ idioms (string.)
[/snip]
>
The reason for the char * as an argument was because it's actually a
vector that I'm passing into the structure.
>
std::vector<cha rmyvector(500);
mytag->GetValue( &myvector[0] );

Creating a buffer and assuming it is big enough (or making it insanely
huge) is a (rather poor) C idiom. Better would be:


string str = mytag->GetValue();
std::vector<cha rmyvector( str.begin(), str.end() );

Even having the function fill a vector that is provided would be better
than using the raw char*.

std::vector<cha rmyvector;
mytag->GetValue( back_inserter( myvector ) );

[Daniel T.]

Re: std::stringstre am returning back values

In article
<685c8a02-58e0-4ea5-8e64-0907bbcc62b1@j7 8g2000hsd.googl egroups.com>,
TBass <tbj@automatedd esign.comwrote:

I don't know if this would be the "C++" way, but doing it the way I
would do it in C worked.
>
void
CFusionTag::Get Value( char *szValue )
{
memcpy( szValue, m_szValue.str() .c_str(), m_szValue.str() .length() );
}

What if the block passed in isn't big enough? At the very least, even in
C, you should pass in the size of the block handed over...

void
CFusionTag::Get Value( char* value, int value_size ) {
memcpy( value, m_szValue.str() .c_str(),
std::min( value_size, m_szValue.str() .length() ) );
}

The fact that you are having to chain the calls (like
"m_szValue.str( ).c_str()" and "m_szValue.str( ).length()") means there is
probably an easier way.