Performance question - char*, vector or similar

[luftwaffe]

Hi,

I need to manipulate unsigned char data. The size of data is unknown by the function which I am about to implement (it comes as an argument).

Is there any speed difference in if it is
1. Allocated as char* using new
2. A std::vector is used
3. Microsoft specific implementation is used?

Which would you recommend and why?

Thanks

[Krishnaa]

Vectors are not suitable for passing strings. Using a NULL terminated strings (char* pointer memory allocated by new or malloc, no probs) are best when you want to pass the argument to function. This way only pointer value is copied to argument variable, and not data.
I assume you mean Win32 API by saying Microsoft specific implementation, I would say that using CRT functions (like new, alloc) and Win32 (HealAlloc) for NULL terminated strings will have almost same performance.

[luftwaffe]

Heya,

I am not about to manipulate string in the sense like you explained. My data can contain many 0s, so a data stream like 0x00 0x81 0x62 0x00 0x05 should be 5 bytes valid data.

Considering that way, what would be the best bet?

[Paul McKenzie]

Vectors are not suitable for passing strings.

Why not?

Using a NULL terminated strings (char* pointer memory allocated by new or malloc, no probs)

First, the OP never mentioned anything about NULL-terminated strings, just char data.

But more to the point, a pointer to the first element in a vector<char> is a char *.

#include <vector>

void foo(char *p)
{
  // do something with p
}

int main()
{
   int n = 10;
   // n has some value
   std::vector<char> p(10);
   foo(&p[0]);  // this works
}

The &p[0] is a char *, so there is no difference whether the data comes from a vector, a char* that you allocated data for using "new", or a straight plain old char array, you end up with the same thing in the foo() function.

Regards,

Paul McKenzie

[Paul McKenzie]

Heya,

I am not about to manipulate string in the sense like you explained. My data can contain many 0s, so a data stream like 0x00 0x81 0x62 0x00 0x05 should be 5 bytes valid data.

Considering that way, what would be the best bet?

The vector<char> is a solution, a std::string is a solution. Asking for "which is best" -- that's for you to decide and test. That's like asking "what is the best car that gets 30 mpg or better".

Regards,

Paul McKenzie

[Krishnaa]

Why not?
First, the OP never mentioned anything about NULL-terminated strings, just char data.

Sure vecror can be used for byte data, I though he might be talking about strings.

But more to the point, what is the difference between a pointer to the first element in a vector<char> is a char *.

#include <vector>

void foo(char *p)
{
  // do something with p
}

int main()
{
   int n = 10;
   // n has some value
   std::vector<char> p(10);
   foo(&p[0]);  // this works
}

The &p[0] is a char *, so there is no difference whether the data comes from a vector, a char* that you allocated data for using "new", or a straight plain old char array, you end up with the same thing in the foo() function.

Regards,

Paul McKenzie

[luftwaffe]

Thank you very much for the detailed explanation!

[AsaF]

I have used vector<CString> to store different strings. I think the stl container has a lot of advantages when you compare to a char array or similar.