#include <iostream>
#include <string>
#include <functional>

int main()
{
     // declare the variable func1
     std::function<void(std::string)> func1;

     // set the value (function body) to the variable func1
     func1 = [](std::string text) {
          std::cout << text;
     };

     // func1 This variable indicates a function
     func1("Hello!");
     func1("Thank you");
}

#include <iostream>

#include <string>
#include <functional>
#include <fstream>

// The ReadTextFile function contains 2 parameters:
// 1st peremeter is filename (text)
// 2nd peremeter is callback (function)
void ReadTextFile(const char* filename, std::function<void(std::string)> callback)
{
     std::ifstream file;
     file.open(filename);
     if (!file)
          return;

     std::string line;
     while (file >> line) {
          callback(line);
     }

     file.close();
}

int main()
{
     // The second parameter is to pass the unnamed function
     ReadTextFile("d://1.txt", [](std::string line) {
          std::cout << line;
     });
}