C++ What's the difference between char a[20] = {0} and char *a= new char[20]()

Question

I write a function to return IP address in C++, as I am new to C++ so I want to make my code proficient. I knew to create a new variable needs new, and we need to delete it.

I do not know why char *hostbuffer = new char[1024](); is not as same as char hostbuffer[1024]={0}, both of them are creating a size 1024 int array, right?

std::string ipfunction_Client(){

    char *hostbuffer = new char[1024]();---This cannot work
    //char hostbuffer[1024]={0};---This can work
    char *IPbuffer=new char[1024];
    struct hostent *host_entry;

    gethostname(hostbuffer,sizeof(hostbuffer));

    host_entry=gethostbyname(hostbuffer);

    IPbuffer = inet_ntoa(*((struct in_addr*)host_entry->h_addr_list[0]));-----This is client.cpp 230
    //delete(hostbuffer);
    return std::string(IPbuffer);
}

If I use the above code, the valgrind's feed back is this:

Process terminating with default action of signal 11 (SIGSEGV): dumping core
==19697==  Access not within mapped region at address 0x18
==19697==    at 0x406624: ipfunction_Client() (client.cpp:230)

See e.g. https://stackoverflow.com/questions/408670/stack-static-and-heap-in-c — dedObed, Oct 02 '19 at 17:02
Array is basic topic of the language, SO is not a replacement of a textbook. — Slava, Oct 02 '19 at 17:04

score 4 · Accepted Answer · answered Oct 02 '19 at 17:06

4

When you use

char *hostbuffer = new char[1024]();

sizeof(hostbuffer) evaluates to the size of a pointer, not the size of the array.

When you use

char hostbuffer[1024]={0};

sizeof(hostbuffer) evaluates to the size of the array.

The call

gethostname(hostbuffer,sizeof(hostbuffer));

will work differently based on the declaration you use.

That's the most significant difference in your code.

If you use

const int BUFFER_SIZE = 1024;
char *hostbuffer = new char[BUFFER_SIZE]();

...

gethostname(hostbuffer, BUFFER_SIZE);

you should not see any difference in behavior.

answered Oct 02 '19 at 17:06

R Sahu

204,454
14
159
270

1

Using all uppercase identifiers for constants became really bad habit. – Slava Oct 02 '19 at 17:10
@Slava, guilty as charged. – R Sahu Oct 02 '19 at 17:12
1

It actually bit me once, enum from somebody else code and macro from third party C library, catching it was not pleaseant though it produced compilation error. If it would not - God help he catching the bug. – Slava Oct 02 '19 at 17:16
Thank you, these really help me. – Jambako Oct 02 '19 at 17:19
@Slava, I once ran into the `class Object` from two third party libraries whose headers needed to be `#include`d in one .cpp file. Fun, fun. – R Sahu Oct 02 '19 at 17:19
@Jambako, glad I was able to help. – R Sahu Oct 02 '19 at 17:20
1

Name collision at least does not compile, with those pesky identifiers it is really good chance it would. That would be a disaster. – Slava Oct 02 '19 at 17:22

C++ What's the difference between char a[20] = {0} and char *a= new char[20]()

1 Answers1