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//Функция
	ifstream file(filename);
 
	string str;
	smatch result;
	regex rx ("class\s+([^\s:]*)\s*:\s*(?:virtual)?\s+(?:public\s+|private\s+|protected\s+)?([^\s,]*)(?:\s*,\s*(?:virtual)?\s+(?:public\s+|private\s+|protected\s+)?([^\s,]*))*");
 
	while (!file.eof()) {
		getline(file, str);
		cout << str << endl << endl;
		while (regex_search(str, result, rx)) {
			for (auto x : result) std::cout << x << " ";
			std::cout << std::endl;
			str = result.suffix().str();
		}
	}
 
 
 
//.txt
class Parent: {};
class Child: virtual public Parent {};
class Child2: virtual public Parent {};
class Child3: virtual public Child, virtual public Child2 {};
class Child4: virtual public Child, virtual public Child6 {};
class Child5: virtual public Child {};
class Child6: virtual public Parent, virtual public Child3 {};
class Child7: virtual public Child3, virtual public Child {};

prog.cpp:2:2: error: 'ifstream' does not name a type
  ifstream file(filename);
  ^
prog.cpp:4:2: error: 'string' does not name a type
  string str;
  ^
prog.cpp:5:2: error: 'smatch' does not name a type
  smatch result;
  ^
prog.cpp:6:2: error: 'regex' does not name a type
  regex rx ("class\s+([^\s:]*)\s*:\s*(?:virtual)?\s+(?:public\s+|private\s+|protected\s+)?([^\s,]*)(?:\s*,\s*(?:virtual)?\s+(?:public\s+|private\s+|protected\s+)?([^\s,]*))*");
  ^
prog.cpp:8:2: error: expected unqualified-id before 'while'
  while (!file.eof()) {
  ^

Standard output is empty