IF Statement Always False

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I have an if statement that compares two IP addresses. It's always returning NOT EQUAL. Here are the values.

char* addr2
char ipSrc[20]

The statement...

C++

if (addr2 == ipSrc)
	{
	printf("\n   EQUAL");
	}
else
	{
	printf("\n   NOT EQUAL");
	}

Is it the types or the statement? And how can this be corrected? Thank you.

Posted 9-Nov-11 14:13pm

Member 7766180

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LanFanNinja 9-Nov-11 20:34pm

What is the value of ipSrc and what does addr2 point to??

Member 7766180 9-Nov-11 20:55pm

They are both IP addresses as strings.

3 solutions

Solution 2

Sorry, typo error.

I meant:

Don't forget to include string at the top. ie:

#include <string.h>

Posted 9-Nov-11 14:48pm

The_Real_Chubaka

Updated 9-Nov-11 14:51pm

v3

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Albert Holguin 9-Nov-11 21:03pm

You can improve your initial solution instead of posting an additional solution. See the links in your first solution.

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Sergey Alexandrovich Kryukov · Accepted Answer · 2011-11-09T15:12:00

Solution 3

Those two variables cannot be equal, even when the strings are identical in content. More exactly, you can only make them equal by assignment addr2 = ipSrc in local context. Here is why: both variables are pointers. First pointer can be initialized or assigned to the address of some object or to the address of memory allocated in heap; second pointer points to the fixed location in memory. If these two pointer point to different locations in memory, comparison "==" will always return false, even if you put exact same characters in these areas of memory. The comparison operator "==" compares pointers, not contents of memory.

As hervebags advised, you can use strcmp, but remembers that both strings should be null-terminated. I would advise to use std::string instead, see http://www.cplusplus.com/reference/string/string/[^].

—SA

Posted 9-Nov-11 15:12pm

Sergey Alexandrovich Kryukov

Updated 9-Nov-11 15:27pm

v2

Comments

Member 7766180 9-Nov-11 21:17pm

Are you saying use this instead?

if (addr2.compare(ipSrc) != 0)
cout << addr2 << " is not " << ipSrc << "\n";

Sergey Alexandrovich Kryukov 9-Nov-11 21:25pm

No, I did not say that. Use comparison of std::string. Still "==", but not for pointers.
--SA

LanFanNinja 9-Nov-11 21:19pm

+5 good explanation

Sergey Alexandrovich Kryukov 9-Nov-11 21:26pm

Do you think so? Thank you very much, but OP did not get it. My bad :-(
--SA

Member 7766180 9-Nov-11 21:28pm

You mean change the pointers into strings, and then use == ?

LanFanNinja 9-Nov-11 21:31pm

Yes he does. Like this
std::string addr2 = "192.168.254.1";
std::string ipSrc = "192.168.254.2";
if (addr2 == ipSrc)
{
printf("TRUE");
}

Member 7766180 9-Nov-11 21:36pm

Thats great except I don't know the IP addresses. They are variables..addr2 and ipSrc.

Something like this?
pHostEnt2 = gethostbyname(NewLogURL.c_str());
ppaddr2 = (int**)pHostEnt2->h_addr_list;
sockAddr2.sin_addr.s_addr = **ppaddr2;
addr2 = inet_ntoa(sockAddr2.sin_addr);
char currentaddress[100] = { 0 };
strcpy( currentaddress, inet_ntoa(sockAddr2.sin_addr ) );

Getting even more lost!

LanFanNinja 9-Nov-11 21:38pm

Then use strcmp() if you must. the string class is just much nicer to work with IMO.

Member 7766180 9-Nov-11 21:42pm

I'm sure it is. Am
i going to have problems with strcmp()? Is there a way with string class even though I don't know thw IP Addresses? Thank you.

LanFanNinja 9-Nov-11 21:47pm

No you should not have problems comparing two character strings using strcmp().

LanFanNinja 9-Nov-11 22:34pm

Also yes you should be able to convert your C style character strings to C++ style strings using something like this:
std::string ip2 = std::string(addr2);

The_Real_Chubaka · Accepted Answer · 2011-11-09T14:45:00

When comparing strings in C++, it is better to use "strcmp" instead of "==".

Your if statement should look like this:

if(strcmp(addr2,ipSrc) == 0 )
        {
                cout << "Equal\n";
        }
        else
        {
                cout << "Not equal\n";
        }

Don't forget to include string at the top. ie:

#include>

"strcmp" returns an integral value indicating the relationship between the strings:
A zero value indicates that both strings are equal.
A value greater than zero indicates that the first character that does not match has a greater value in str1 than in str2; And a value less than zero indicates the opposite. Check out this link[]

I also suggest that you declare 'addr2' like this: char addr2[20]
Because, in your version, you are trying to convert a string constant to a character.

IF Statement Always False

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