c++ - Size of strcat Destination Array -

take following program:

#include <iostream> #include <cstring>  using namespace std;  int main() {     char a[8] = "hello, ";     char b[7] = "world!";      strcat(a, b);      cout << a;      return 0; } 

notice a , b have same size assigned strings.

the documentation states strcat(a, b) work, a needs large enough contain concatenated resulting string.

nevertheless, cout << a displays "hello, world!". entering undefined behavior?

"am entering undefined behavior?"

yes. region @ end of a[] has been written over. worked, time, might have belonged else.

here use struct control memory layout, , demonstrate it:

#include <iostream> #include <cstring>  using namespace std;  int main() {     struct s {         char a[8];         char b[5];         char c[7];     };      s s;     strcpy( s.a , "hello, " );     strcpy( s.b , "foo!" );     strcpy( s.c , "world!" );       strcat(s.a, s.c);      cout << s.a << endl;     cout << s.b << endl;      cin.get();      return 0; } 

this outputs:

hello, world! orld! 

instead of:

hello, world! foo! 

the strcat() has stomped on b[].

please note in real life example such bugs may far more subtle, , lead wondering why innocent function calls 250 lines later crash , burn horribly. ;-)

edit: may recommend use strcat_s, instead? or, better, std::strings:

#include <string> #include <iostream>  using namespace std;  int main() {     string = "hello, ";     string b = "world!";     = + b;     cout << a; }