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#include <map>
#include <string>
using namespace std;
class class1{
	map<string, int> data;
	friend bool func(const class1&);
public:
	class1(){
		data["test"] = 4;
	}
};
 
int main(){
	class1 sample;
	func(sample);
}
 
bool func(const class1& sample){
	if(sample.data["test"] == 4){
		return true;
	}else{
		return false;
	}
}

prog.cpp: In function 'bool func(const class1&)':
prog.cpp:19:23: error: passing 'const std::map<std::basic_string<char>, int>' as 'this' argument discards qualifiers [-fpermissive]
  if(sample.data["test"] == 4){
                       ^
In file included from /usr/include/c++/5/map:61:0,
                 from prog.cpp:1:
/usr/include/c++/5/bits/stl_map.h:494:7: note:   in call to 'std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](std::map<_Key, _Tp, _Compare, _Alloc>::key_type&&) [with _Key = std::basic_string<char>; _Tp = int; _Compare = std::less<std::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::basic_string<char>, int> >; std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = int; std::map<_Key, _Tp, _Compare, _Alloc>::key_type = std::basic_string<char>]'
       operator[](key_type&& __k)
       ^

Standard output is empty