C ++ pointer passed as function parameter

 

Today, when I write a function that converts string letters to uppercase and lowercase letters, the modification and modification are completed. The Code is as follows:
# Include "stdafx. H"
# Include <string. h>
# Include <iostream. h>
Char * transform (char * CSTR, char * STR, int length );

Int main (INT argc, char * argv [])
{
Char * STR = "woaini38 ";
Cout <STR <Endl;
Char s [10];
Transform (S, STR, strlen (STR ));
Cout <S <Endl;
Return 0;
}

Char * transform (char * CSTR, char * STR, int length)
{
For (INT I = 0; I <Length + 1; I ++)
{
If (STR [I]> = 'A' & STR [I] <= 'Z ')
{
CSTR [I] = STR [I] + 'a'-'A ';
Continue;
}
If (STR [I]> = 'A' & STR [I] <= 'Z ')
{
CSTR [I] = STR [I]-'A' + 'a ';
}
Else
{
CSTR [I] = STR [I];
}
}
Return CSTR;
}

Ah! I admit that my foundation is not as good as I think. Make a mistake when writing this code.
Originally, we wanted to define transform as (char * CSTR, char * Str ). Then, in the function body, allocating memory dynamically for the CSTR industry is the target pointer, so I wrote:

Char * transform (char * CSTR, char * Str)
{

Int Len = strlen (STR );
CSTR = new char [Len]; // dynamically allocates a len-length memory.

For (INT I = 0; I <Len + 1; I ++) // The following is the same
{
If (STR [I]> = 'A' & STR [I] <= 'Z ')
{
CSTR [I] = STR [I] + 'a'-'A ';
Continue;
}
If (STR [I]> = 'A' & STR [I] <= 'Z ')
{
CSTR [I] = STR [I]-'A' + 'a ';
}
Else
{
CSTR [I] = STR [I];
}
}
Return CSTR;

}

However, a packet memory error occurs during running, indicating that a block of memory cannot be read, or a block of memory cannot be written. After programming for so long, I immediately thought that this error should be caused by a null pointer. I checked it carefully but did not find it. During the single-step operation, it was known that the CSTR of transform had a problem. But I have allocated dynamic memory for him! I am sad. I can't find the reason. I tried another method, so I deleted the dynamically allocated statement: But the variable of the string length is required during the loop. So I passed it a length variable. Then, in the main function, I can define only one array. Then the transform parameter is passed. So you can see the code at the beginning.
Then everything went well.
But I have never figured out why I cannot pass a null pointer to transform and then dynamically allocate memory in the function body?
On the internet I see such an article http://99381837.iteye.com/blog/905237

The analysis by the author is quite good. He told me that if I want to pass a null pointer, I should write char * transform (char ** CSTR, char * Str) {* CSTR = (char *) malloc (strlen (STR) + 1 );}
The analysis later is painful. It cannot be completely viewed.
In fact, writing this article is to prevent yourself from making such a mistake. But this error reminds me of a programming habit. Try not to use a null pointer. However, in programming, we sometimes do not know that we are using a null pointer.
So I wrote the following code in VC:
# Include "stdafx. H"
# Include <iostream. h>
# Include <string. h>

Int main (INT argc, char * argv [])
{
Char * STR = "woaini ";
Char * s;
Strcmp (S, STR );
Cout <S <Endl;
Return 0;
}

At this time, I guess you should know how to write this code.
In fact, an error is ignored when a null pointer is used as a parameter. If you want to dynamically allocate memory in the function body, when will the memory you allocate be released? The dynamically allocated memory needs to be manually released. This is also a problem, right. Therefore, null pointers cannot be passed as parameters.
In most MFC programming, parameters like strings are cstring objects. Sometimes when you need some functions of the SDK, if the parameter is a pointer, you must remember to allocate the memory without errors.