foo.cc: In function 'int main()':
foo.cc:90:32: warning: 'char* gets(char*)' is deprecated [-Wdeprecated-declarations]
  for(int i=0;i<4;i++) gets(a[i]);
                                ^
In file included from /usr/include/stdio.h:862:0,
                 from /usr/include/c++/7/cstdio:42,
                 from /usr/include/c++/7/ext/string_conversions.h:43,
                 from /usr/include/c++/7/bits/basic_string.h:6361,
                 from /usr/include/c++/7/string:52,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:232:1: note: declared here
 gets (char *__str)
 ^~~~
foo.cc:90:27: warning: ignoring return value of 'char* gets(char*)', declared with attribute warn_unused_result [-Wunused-result]
  for(int i=0;i<4;i++) gets(a[i]);
                       ~~~~^~~~~~
In file included from /usr/include/stdio.h:862:0,
                 from /usr/include/c++/7/cstdio:42,
                 from /usr/include/c++/7/ext/string_conversions.h:43,
                 from /usr/include/c++/7/bits/basic_string.h:6361,
                 from /usr/include/c++/7/string:52,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
In function 'char* gets(char*)',
    inlined from 'int main()' at foo.cc:90:27:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:236:28: warning: call to '__gets_warn' declared with attribute warning: please use fgets or getline instead, gets can't specify buffer size
   return __gets_warn (__str);
                            ^
/usr/include/x86_64-linux-gnu/bits/stdio2.h:236:28: warning: call to '__gets_warn' declared with attribute warning: please use fgets or getline instead, gets can't specify buffer size
/usr/include/x86_64-linux-gnu/bits/stdio2.h:236:28: warning: call to '__gets_warn' declared with attribute warning: please use fgets or getline instead, gets can't specify buffer size
/usr/include/x86_64-linux-gnu/bits/stdio2.h:236:28: warning: call to '__gets_warn' declared with attribute warning: please use fgets or getline instead, gets can't specify buffer size
/tmp/ccmsbgsk.o: In function `main':
foo.cc:(.text.startup+0xc): warning: the `gets' function is dangerous and should not be used.