Comparing strings lexicographically

You are comparing "primitive" strings, which are of type char const *.

The following is essentially equivalent to your example:

char const * s1 = "aa";
char const * s2 = "bz";
if ( s1 > s2 ) cout<<"Yes";

This is comparing the pointers (the memory addresses of the strings), not the contents.

@izomorphius has suggested some good solutions.

Comparing std::string -s like that will work. However you are comparing string literals. To do the comparison you want either initialize a std::string with them or use strcmp:

if(std::string("aa") > std::string("bz")) cout<<"Yes";

This is the c++ style solution to that.

Or alternatively:

if(strcmp("aa", "bz") > 0) cout<<"Yes";

EDIT(thanks to Konrad Rudolph's comment): in fact in the first version only one of the operands should be converted explicitly so:

if(std::string("aa") > "bz") cout<<"Yes";

Will again work as expected.

EDIT(thanks to churill's comment): since c++14 you can use string literals:

if("aa"s > "bz") cout<<"Yes";