string vs dynamic char array

cpjust · 11-03-2009, 07:22 PM

Ah, of course. Using resize() & size() are correct.

Was I at least right about needing to manually add a '\0' to the vector?

Ducky · 11-03-2009, 09:54 PM

Cpjust: vec.push_back( '\0' ); // I can't remember if this is required, but it doesn't hurt.

Apparrently its working without the '\0'.
At least i dont have garbage at the end and nothing missing either.

And by the way why would you need to allocate space in a vector?

Arent vectors like strings?

"Actually you could say that a string is a vector<char> A container of chars, or an advanced array of chars. "

cpjust · 11-04-2009, 08:26 AM

Yes, but only if you use the vector's functions to add things to it. If you pass a pointer to the first element of the vector to a function, it won't be calling the vector's functions, it'll just write to it like it's a raw array.

anon · 11-04-2009, 08:43 AM

Quote:

Apparrently its working without the '\0'.
At least i dont have garbage at the end and nothing missing either.

If things like the following work:

Code:

string s = &v[0];

then this can only be because of pure (un)luck. If you are going to treat it as a C-style string, it has to be null-terminated. If you copy from the string to the vector, then string.end() or string.size() won't include the null-terminator (which doesn't have to be there).

Ducky · 11-04-2009, 10:43 AM

Thank you, anon.
You sure know how to recognize different types of trees from quite a long way away.

I will '\0' terminate it then.

Ducky · 11-04-2009, 10:58 AM

And if i concatenate a vector element to a string like this, do i need to '\0' terminate it too?

Code:

int main()
{
    string  str1 =  "Hello ";
    string  str2 =  "World";
    vector <char> vec( str2.begin(), str2.end());

    str1 += &vec[0];

    cout  << str1 << endl;

    return 0;
}

bithub · 11-04-2009, 11:18 AM

Quote:

Originally Posted by Ducky

And if i concatenate a vector element to a string like this, do i need to '\0' terminate it too?

Code:

int main()
{
    string  str1 =  "Hello ";
    string  str2 =  "World";
    vector <char> vec( str2.begin(), str2.end());

    str1 += &vec[0];

    cout  << str1 << endl;

    return 0;
}

Yes, because with the code you have, there is no guarantee that &vec[0] is null terminated.

Ducky · 11-04-2009, 12:19 PM

Thank you Bithub!

Can it be done simpler than this?

Code:

 
str1 += &vec[0];

str1 += '\0';

bithub · 11-04-2009, 12:26 PM

Quote:

Originally Posted by Ducky

Thank you Bithub!

Is it ok like this too?

Code:

 str1 += '\0';

You need to null terminate the vector before you add it to the string, not after. A push_back('\0') on the vector before adding it to the string should do the trick.

Daved · 11-04-2009, 12:31 PM

That is a contrived example, though. Normally you wouldn't need to add the '\0' to the vector if you do it right.

Earlier there was a discussion about pushing back a null character in cpjust's example (post #6). Pushing it back there is not required and doesn't do anything unless you use the vector as a C-style string later on, but you would normally just use c_str() for that.

When that code is changed to use resize, that actually adds 100 null characters anyway, so filling the vector with data from the old function would only cause a problem if all 100 characters were filled with string data. Then you might get garbage characters at the end. One solution is to resize to 100 (or whatever) and only pass 99 characters to your old function. Another is to assign the vector to the string.

Code:

std::string str = "Hello World";
std::vector<char> vec1( str.begin(), str.end() );
// You would want push_back('\0') here if using the vector as a char*
// but that's not usually necessary, use c_str() on the string instead

Code:

std::vector<char> vec2( 100 );  // Initialize with 100 null characters
old_c_function( &vec[0], vec.size() );

// Now put it back into the string.
str.assign(vec.begin(), vec.end());
// No null needed, we used assign.

Code:

std::vector<char> vec2( 100 );  // Initialize with 100 null characters
old_c_function( &vec[0], vec.size()-1 );

// This is a bit more dangerous, I prefer assign.
str = &vec[0];
// No null needed, only filled size()-1 characters, and the last character was initialized to null.

Ducky · 11-04-2009, 02:35 PM

Thanks Daved!

You are revealing details about vectors that doesnt really explained anywhere. Interesting.

brewbuck · 11-04-2009, 03:45 PM

std::string does not require null termination. The question of how to properly "null terminate" the vector is moot.

Convert vector of char to string:

Code:

std::string foo( vec.begin(), vec.end() );

Convert string to vector of char:

Code:

std::vector< char > foo( str.begin(), str.end() );

And yes, when you call c_str() on the resulting string, it will be null terminated automatically. std::string is nothing like a C-string.

cpjust · 11-04-2009, 08:49 PM

If you just wanted a C string for reading, you'd use string::c_str(). The reason for using a vector is to pass it to a C function for writing into a C string, and for that you'd need to add a '\0' char to the end (assuming it's going to append instead of just overwriting the string).

11-03-2009, 07:22 PM	#16
cpjust and the hat of sweating Join Date: Aug 2007 Location: Toronto, ON Posts: 3,120	Ah, of course. Using resize() & size() are correct. Was I at least right about needing to manually add a '\0' to the vector? __________________ "I am probably the laziest programmer on the planet, a fact with which anyone who has ever seen my code will agree." - esbo, 11/15/2008
cpjust is offline

11-03-2009, 09:54 PM	#17
Ducky Registered User Join Date: Dec 2007 Location: France Posts: 396	Cpjust: vec.push_back( '\0' ); // I can't remember if this is required, but it doesn't hurt. Apparrently its working without the '\0'. At least i dont have garbage at the end and nothing missing either. And by the way why would you need to allocate space in a vector? Arent vectors like strings? "Actually you could say that a string is a vector<char> A container of chars, or an advanced array of chars. " __________________ Using Code::Blocks,MingW with Windows. Last edited by Ducky; 11-03-2009 at 10:38 PM.
Ducky is offline

11-04-2009, 08:26 AM	#18
cpjust and the hat of sweating Join Date: Aug 2007 Location: Toronto, ON Posts: 3,120	Yes, but only if you use the vector's functions to add things to it. If you pass a pointer to the first element of the vector to a function, it won't be calling the vector's functions, it'll just write to it like it's a raw array. __________________ "I am probably the laziest programmer on the planet, a fact with which anyone who has ever seen my code will agree." - esbo, 11/15/2008
cpjust is offline

11-04-2009, 10:43 AM	#20
Ducky Registered User Join Date: Dec 2007 Location: France Posts: 396	Thank you, anon. You sure know how to recognize different types of trees from quite a long way away. I will '\0' terminate it then. __________________ Using Code::Blocks,MingW with Windows.
Ducky is offline

11-04-2009, 10:58 AM	#21
Ducky Registered User Join Date: Dec 2007 Location: France Posts: 396	And if i concatenate a vector element to a string like this, do i need to '\0' terminate it too? Code: int main() { string str1 = "Hello "; string str2 = "World"; vector <char> vec( str2.begin(), str2.end()); str1 += &vec[0]; cout << str1 << endl; return 0; } __________________ Using Code::Blocks,MingW with Windows.
Ducky is offline

11-04-2009, 12:19 PM	#23
Ducky Registered User Join Date: Dec 2007 Location: France Posts: 396	Thank you Bithub! Can it be done simpler than this? Code: str1 += &vec[0]; str1 += '\0'; __________________ Using Code::Blocks,MingW with Windows. Last edited by Ducky; 11-04-2009 at 12:26 PM.
Ducky is offline

11-04-2009, 12:31 PM	#25
Daved Registered User Join Date: Jan 2005 Posts: 7,137	That is a contrived example, though. Normally you wouldn't need to add the '\0' to the vector if you do it right. Earlier there was a discussion about pushing back a null character in cpjust's example (post #6). Pushing it back there is not required and doesn't do anything unless you use the vector as a C-style string later on, but you would normally just use c_str() for that. When that code is changed to use resize, that actually adds 100 null characters anyway, so filling the vector with data from the old function would only cause a problem if all 100 characters were filled with string data. Then you might get garbage characters at the end. One solution is to resize to 100 (or whatever) and only pass 99 characters to your old function. Another is to assign the vector to the string. Code: std::string str = "Hello World"; std::vector<char> vec1( str.begin(), str.end() ); // You would want push_back('\0') here if using the vector as a char* // but that's not usually necessary, use c_str() on the string instead Code: std::vector<char> vec2( 100 ); // Initialize with 100 null characters old_c_function( &vec[0], vec.size() ); // Now put it back into the string. str.assign(vec.begin(), vec.end()); // No null needed, we used assign. Code: std::vector<char> vec2( 100 ); // Initialize with 100 null characters old_c_function( &vec[0], vec.size()-1 ); // This is a bit more dangerous, I prefer assign. str = &vec[0]; // No null needed, only filled size()-1 characters, and the last character was initialized to null.
Daved is offline

11-04-2009, 02:35 PM	#26
Ducky Registered User Join Date: Dec 2007 Location: France Posts: 396	Thanks Daved! You are revealing details about vectors that doesnt really explained anywhere. Interesting. __________________ Using Code::Blocks,MingW with Windows.
Ducky is offline

11-04-2009, 03:45 PM	#27
brewbuck Senior software engineer Join Date: Mar 2007 Location: Portland, OR Posts: 5,381	std::string does not require null termination. The question of how to properly "null terminate" the vector is moot. Convert vector of char to string: Code: std::string foo( vec.begin(), vec.end() ); Convert string to vector of char: Code: std::vector< char > foo( str.begin(), str.end() ); And yes, when you call c_str() on the resulting string, it will be null terminated automatically. std::string is nothing like a C-string. __________________ "Congratulations on your purchase. To begin using your quantum computer, set the power switch to both off and on simultaneously." -- raftpeople@slashdot
brewbuck is offline

11-04-2009, 08:49 PM	#28
cpjust and the hat of sweating Join Date: Aug 2007 Location: Toronto, ON Posts: 3,120	If you just wanted a C string for reading, you'd use string::c_str(). The reason for using a vector is to pass it to a C function for writing into a C string, and for that you'd need to add a '\0' char to the end (assuming it's going to append instead of just overwriting the string). __________________ "I am probably the laziest programmer on the planet, a fact with which anyone who has ever seen my code will agree." - esbo, 11/15/2008
cpjust is offline

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Unable to compare string with 'getter' returned string.	Swerve	C++ Programming	2	10-30-2009 05:56 PM
[Inheritance Hierarchy] User Input on program with constructors. How ?	chandreu	C++ Programming	8	04-25-2008 02:45 PM
Personal Program that is making me go wtf?	Submeg	C Programming	20	06-27-2006 12:13 AM
Character arrays	PsychoBrat	C++ Programming	7	06-21-2002 12:02 PM
How do you search & sort an array?	sketchit	C Programming	30	11-03-2001 05:26 PM