string vs dynamic char array

Ducky · 11-02-2009, 07:49 AM

We can store data dynamically in a string just like in a dynamic char array, right?

So "string str;" is almost the same as "char * ch = new char[64];"
only much better because we dont have to define the max size?

If the answer is yes i wonder why people continue to complicate their lives by using
dynamic char arrays when they can use strings as well?

Dino · 11-02-2009, 10:07 AM

Yes.

Yes.

Certain functions still require char arrays.

Daved · 11-02-2009, 10:30 AM

>> Certain functions still require char arrays.
Even then the string is usually better with a simple conversion done before working with the other functions.

It might make sense to use a non-dynamic char array instead of the string class in some cases, although those cases are somewhat rare.

gltiich · 11-02-2009, 10:53 AM

Use strings, and if you need an old school string for any reason you can do this:

string lolwat = "hai therreee";
// ... lolwat.c_str();

Of course, don't forget to distinguish between C and C++ when reading random source online, which is probably where you confusion comes from. Most people carelessly mix the two languages, and in C there is no string class because classes don't exist in C. But there still are libs and ways to write functions to manage a 'dynamic' arrays in C.

Ducky · 11-02-2009, 11:45 AM

Thats awesome and crystal clear, strings all the way from now on!

Thanks a lot everyone!

cpjust · 11-02-2009, 10:10 PM

If you need to pass a char* to a function instead of a const char*, you can use a vector<char> like this:

Code:

std::string str = "Hello World";
std::vector<char> vec( str.begin(), str.end() );
vec.push_back( '\0' );  // I can't remember if this is required, but it doesn't hurt.
vec.reserve( 100 );  // Make extra room for the string to be written into.

old_c_function( &vec[0], vec.capacity() );

// Now put it back into the string.
str = &vec[0];

Ducky · 11-03-2009, 03:27 AM

Thank you Cpjust, i think that's just what i was looking for!

Using a vector element as a string

Daved · 11-03-2009, 12:03 PM

Note that this part is not quite correct:

Code:

old_c_function( &vec[0], vec.capacity() );

It should be:

Code:

old_c_function( &vec[0], vec.size() );

The internal data in the vector will not be accurate if you use capacity, because capacity only tells you how much space was allocated, but the size tells you how many actual chars are available to be modified.

bithub · 11-03-2009, 12:25 PM

Quote:

Originally Posted by Daved

Note that this part is not quite correct:

Code:

old_c_function( &vec[0], vec.capacity() );

It should be:

Code:

old_c_function( &vec[0], vec.size() );

The internal data in the vector will not be accurate if you use capacity, because capacity only tells you how much space was allocated, but the size tells you how many actual chars are available to be modified.

No, capacity is correct.

iMalc · 11-03-2009, 12:28 PM

Quote:

Originally Posted by bithub

No, capacity is correct.

Correct in the sense that it might pass the integer value you were expecting, but incorrect in the sense that any characters written to beyond the value of size() are surely undefined behaviour.

brewbuck · 11-03-2009, 12:29 PM

Quote:

Originally Posted by bithub

No, capacity is correct.

I don't think so, because although the characters from vec[0] to vec[capacity-1] may be writable, the vector itself doesn't consider them to be part of the current value. They are just available space.

The vector tracks the size of its contents, so it isn't really correct to go behind vector's back and write into the space beyond end(), even if that space is reserved. It will have no way of knowing that you did that.

**laserlight** · 11-03-2009, 12:30 PM

Quote:

Originally Posted by bithub

No, capacity is correct.

In practice for vectors of objects of built-in types (or maybe POD types in general), it probably works okay. However, semantically and more likely as an issue for non-POD objects, it is not correct.

bithub · 11-03-2009, 12:35 PM

My comment was just for the context of the posted code. In that case, capacity() is correct and size() is incorrect (since size() would return 0, and thus fail for the given example). I realize that after the function call, the vector is in an undefined state (and that size() will probably still return zero after the function call), but in the posted code, this was irrelevant.

**laserlight** · 11-03-2009, 12:39 PM

That does not imply that capacity() is correct. Rather, that implies that reserve() is incorrect, and resize() should be used instead.

Daved · 11-03-2009, 12:51 PM

I misread the original code and thought it used resize instead of reserve.

Using reserve and capacity is wrong in the sense that the vector's internal variables will be out of sync with the data.

You should use resize and size instead as laserlight said. Sorry for the confusion.

11-02-2009, 07:49 AM	#1
Ducky Registered User Join Date: Dec 2007 Location: France Posts: 396	string vs dynamic char array We can store data dynamically in a string just like in a dynamic char array, right? So "string str;" is almost the same as "char * ch = new char[64];" only much better because we dont have to define the max size? If the answer is yes i wonder why people continue to complicate their lives by using dynamic char arrays when they can use strings as well? __________________ Using Code::Blocks,MingW with Windows. Last edited by Ducky; 11-02-2009 at 11:54 AM.
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11-02-2009, 10:07 AM	#2
Dino Jack of many languages Join Date: Nov 2007 Location: Katy, Texas Posts: 1,929	Yes. Yes. Certain functions still require char arrays. __________________ Mac and Windows cross platform programmer. Ruby lover. Memorable Quotes From Recent Posts: I can't remember.
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11-02-2009, 10:30 AM	#3
Daved Registered User Join Date: Jan 2005 Posts: 7,137	>> Certain functions still require char arrays. Even then the string is usually better with a simple conversion done before working with the other functions. It might make sense to use a non-dynamic char array instead of the string class in some cases, although those cases are somewhat rare.
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11-02-2009, 10:53 AM	#4
gltiich crazy Join Date: Jun 2008 Posts: 26	Use strings, and if you need an old school string for any reason you can do this: string lolwat = "hai therreee"; // ... lolwat.c_str(); Of course, don't forget to distinguish between C and C++ when reading random source online, which is probably where you confusion comes from. Most people carelessly mix the two languages, and in C there is no string class because classes don't exist in C. But there still are libs and ways to write functions to manage a 'dynamic' arrays in C. Last edited by gltiich; 11-02-2009 at 10:58 AM.
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11-02-2009, 11:45 AM	#5
Ducky Registered User Join Date: Dec 2007 Location: France Posts: 396	Thats awesome and crystal clear, strings all the way from now on! Thanks a lot everyone! __________________ Using Code::Blocks,MingW with Windows.
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11-02-2009, 10:10 PM	#6
cpjust and the hat of sweating Join Date: Aug 2007 Location: Toronto, ON Posts: 3,120	If you need to pass a char* to a function instead of a const char*, you can use a vector<char> like this: Code: std::string str = "Hello World"; std::vector<char> vec( str.begin(), str.end() ); vec.push_back( '\0' ); // I can't remember if this is required, but it doesn't hurt. vec.reserve( 100 ); // Make extra room for the string to be written into. old_c_function( &vec[0], vec.capacity() ); // Now put it back into the string. str = &vec[0]; __________________ "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
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11-03-2009, 03:27 AM	#7
Ducky Registered User Join Date: Dec 2007 Location: France Posts: 396	Thank you Cpjust, i think that's just what i was looking for! Using a vector element as a string __________________ Using Code::Blocks,MingW with Windows.
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11-03-2009, 12:03 PM	#8
Daved Registered User Join Date: Jan 2005 Posts: 7,137	Note that this part is not quite correct: Code: old_c_function( &vec[0], vec.capacity() ); It should be: Code: old_c_function( &vec[0], vec.size() ); The internal data in the vector will not be accurate if you use capacity, because capacity only tells you how much space was allocated, but the size tells you how many actual chars are available to be modified.
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11-03-2009, 12:35 PM	#13
bithub Registered User Join Date: Sep 2004 Location: California Posts: 2,845	My comment was just for the context of the posted code. In that case, capacity() is correct and size() is incorrect (since size() would return 0, and thus fail for the given example). I realize that after the function call, the vector is in an undefined state (and that size() will probably still return zero after the function call), but in the posted code, this was irrelevant. __________________ bit∙hub [bit-huhb] n. A source and destination for information.
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11-03-2009, 12:39 PM	#14
laserlight C++ Witch Join Date: Oct 2003 Location: Singapore Posts: 10,364	That does not imply that capacity() is correct. Rather, that implies that reserve() is incorrect, and resize() should be used instead. __________________ C + C++ Compiler: MinGW port of GCC Build + Version Control System: SCons + Bazaar Look up a C/C++ Reference and learn How To Ask Questions The Smart Way
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11-03-2009, 12:51 PM	#15
Daved Registered User Join Date: Jan 2005 Posts: 7,137	I misread the original code and thought it used resize instead of reserve. Using reserve and capacity is wrong in the sense that the vector's internal variables will be out of sync with the data. You should use resize and size instead as laserlight said. Sorry for the confusion.
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