difference between cout and write in c++

ostream& write ( const char* s , streamsize n );

Is an Unformatted output function and what is written is not necessarily a c-string, therefore any null-character found in the array s is copied to the destination and does not end the writing process.

cout is an object of class ostream that represents the standard output stream.
It can write characters either as formatted data using for example the insertion operator ostream::operator<< or as Unformatted data using the write member function.

You are asking what is the difference between a class member function and an instance of the class? cout is an ostream and has a write() method.

As to the difference between cout << "Some string" and cout.write("Some string", 11): It does the same, << might be a tiny bit slower since write() can be optimized as it knows the length of the string in advance. On the other hand, << looks nice and can be used with many types, such as numbers. You can write cout << 5;, but not cout.write(5).

cout is not a function. Like you said, it is an object of class ostream. And as an object of that class, it possesses the write function, which can be called like this:

cout.write(source,size);