error: no match for `std::string& != long int' operator

[learning c++]

Hi,
I have a short code hope to print the character of a string one by one. but 
there are some errors.

if I declare srting s1, is s1 a pointer? are there some difference between C 
and C++?
#include <iostream>
#include <string>

using namespace std;

int main(){

string s1("0123456789abcdefghijklmnopqrstuvwxyz");
for(s1; s1!=NULL; ++s1)
cout <<*s1<<" " <<int(*s1)<<"  ";

return 0;

}
printc.cpp: In function `int main()':
printc.cpp:9: error: no match for `std::string& != long int' operator
printc.cpp:9: error: no match for `++ std::string&' operator
printc.cpp:10: error: no match for `* std::string&' operator
printc.cpp:10: error: no match for `* std::string&' operator
printc.cpp:9: warning: statement with no effect

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