error while inserting into set c++ - c++

Question

/info is a string/
while(getline(cin,info)){
char *a = new char[info.size()+1];
a[info.size()] = 0;
memcpy(a,info.c_str(),info.size());
//cout<<sizeof(a)<<endl;
set<char *>m;
m.insert(a,a+8);
}
//Error:invalid conversion from char to char*

Answer 1

I suspect the thing you're missing is that when you insert a range, it inserts each item in that range. It doesn't try to insert them as a single item but it inserts each item in the range individually.
set<char *>m;
m.insert(a,a+8);
Okay, so m is a set of char*. You've said that you want to insert everything in the range from a to a+8. That's a range of characters. So you're trying to insert characters into a set of pointers to characters. That can't work.
Perhaps you just want m.insert(a);? Since m is a set of char* and a is a char*, I can't imagine what you might want to do other than insert it.

Answer 2

m.insert(a,a+8);
You tried to use a wrong overload of set::insert(). It is mostly useful when you want to do something like this:
int a[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
std::set<int> m;
m.insert(a[0]);
m.insert(a[1]);
...
m.insert(a[6]);
m.insert(a[7]);
This is lengthy and we want to avoid repetitions. Equivalently, you can write
int a[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
std::set<int> m;
m.insert(a, a+8);
Now let's go back to your code...
while(getline(cin,info)){
char *a = new char[info.size()+1];
...
set<char *>m;
m.insert(a,a+8);
Conversely, you can "expand" it into
while(getline(cin,info)){
char *a = new char[info.size()+1];
...
set<char *>m;
m.insert(a[0]);
m.insert(a[1]);
...
m.insert(a[6]);
m.insert(a[7]);
Now you see you ended up in insert-ing a char object to std::set<char *>. This is the reason why you got the error message.
Proposed fix
You defined set<char *> m inside the while loop. That means you are dealing with a freshly constructed m in each iteration of the while loop, which doesn't makes sense. You need to move the definition of m to before while.
It is not recommended to use new char[...] until you spend 5--6 years in C++. C++ already prepared std::string and/or std::vector for you to replace 99% of the use cases of raw new char[...]. So your set should be set<string> m.
std::string has an assignment operator (and a copy constructor) which takes a std::string or NUL-terminated const char * and does memcpy() for you, and you can think set::insert() internally invokes it.
std::string also supports a substring operation returning a fresh std::string such that string("abcde").substr(3) == "abc".
http://en.cppreference.com/w/cpp/string/basic_string/substr
That means you don't need an intermediate variable a at all.
After these corrections, you'll get
set<string> m;
while(getline(cin,info)){
m.insert(info.substr(8));
}