An old C++ library for the Raspberry Pi Revision 2 (please note: Raspberry Pi 1, revision 2, *not* Raspberry Pi 2) GPIO pins, using the standard /sys/class/gpio driver. It's probably outdated for newer Pi models, so beware. Also I don't think I ever actually tested input.

gpio.cpp

#include <fstream>
#include <string>
#include <iostream>
#include <sstream>
#include "gpio.hpp"
 
using namespace std;

namespace GPIO {
 
    Connection::Connection() : Connection{Pin::GPIO4} {} //because why not default to 4
    Connection::Connection(Pin pin) : Connection{pin, Direction::UnInit} {}
    Connection::Connection(Pin pin, Direction direct) : pin(pin), direct(direct)
    {
        this->exportPin();
        if (this->direct != Direction::UnInit)
        {
            this->setDirection(this->direct);
        }
    }
    
    Connection::~Connection()
    {
        this->unexportPin();
    }
     
    int Connection::exportPin()
    {
        string export_str = "/sys/class/gpio/export";
        ofstream exportgpio(export_str.c_str()); // Open "export" file. Convert C++ string to C string. Required for all Linux pathnames
        if (exportgpio < 0){
            cerr << " OPERATION FAILED: Unable to export GPIO"<< static_cast<int>(this->pin) << endl;
            return -1;
        }
     
        exportgpio << static_cast<int>(this->pin) ; //write GPIO number to export
        exportgpio.close(); //close export file
        return 0;
    }
     
    int Connection::unexportPin()
    {
        string unexport_str = "/sys/class/gpio/unexport";
        ofstream unexportgpio(unexport_str.c_str()); //Open unexport file
        if (unexportgpio < 0){
            cerr << " OPERATION FAILED: Unable to unexport GPIO"<< static_cast<int>(this->pin) << endl;
            return -1;
        }
     
        unexportgpio << static_cast<int>(this->pin); //write GPIO number to unexport
        unexportgpio.close(); //close unexport file
        return 0;
    }
     
    int Connection::setDirection(Direction direct)
    {
        if (direct == Direction::UnInit) {
            cerr << " OPERATION FAILED: Can't set the direction of a GPIO connection to Direction::UnInit" << endl;
            return -1;
        }
     
        stringstream sstr;
        sstr << "/sys/class/gpio/gpio" << static_cast<int>(this->pin) << "/direction";
        ofstream setdirgpio(sstr.str().c_str()); // open direction file for gpio
        if (setdirgpio < 0){
            cerr << " OPERATION FAILED: Unable to set direction of GPIO"<< static_cast<int>(this->pin) << endl;
            return -1;
        }
 
        setdirgpio << (direct == Direction::In ? "in" : "out"); //write direction to direction file
        setdirgpio.close(); // close direction file
        
        this->direct = direct;
        
        return 0;
    }
     
    int Connection::setValue(bool val)
    {
        if (this->direct != Direction::Out) {
            cerr << " OPERATION FAILED: Can't set the value of an input GPIO connection" << endl;
            return -1;
        }
     
        stringstream sstr;
        sstr << "/sys/class/gpio/gpio" << static_cast<int>(this->pin) << "/value";
        ofstream setvalgpio(sstr.str().c_str()); // open value file for gpio
        if (setvalgpio < 0){
            cerr << " OPERATION FAILED: Unable to set the value of GPIO"<< static_cast<int>(this->pin) << endl;
            return -1;
        }
 
        setvalgpio << val ;//write value to value file
        setvalgpio.close();// close value file 
        return 0;
    }
     
    int Connection::getValue(bool& val)
    {
        if (this->direct != Direction::In) {
            cerr << " OPERATION FAILED: Can't get the value of an output GPIO connection" << endl;
            return -1;
        }
     
        stringstream sstr;
        sstr << "/sys/class/gpio/gpio" << static_cast<int>(this->pin) << "/value";
        ifstream getvalgpio(sstr.str().c_str()); // open value file for gpio
        if (getvalgpio < 0){
            cerr << " OPERATION FAILED: Unable to get value of GPIO"<< static_cast<int>(this->pin) << endl;
            return -1;
        }
        
        string tmp;
        getvalgpio >> tmp;  //read gpio value
     
        // nonzero is on, zero is off
        val = (tmp != "0");
     
        getvalgpio.close(); //close the value file
        return 0;
    }
} // namespace GPIO

gpio.hpp

#ifndef GPIO_H

#define GPIO_H

namespace GPIO
{
    
    // Directions in which a pin can be set
    enum class Direction {
        UnInit,
        In,
        Out
    };
    
    // Valid GPIO pin numbers on Revision 2 Raspberry Pi
    enum class Pin : int {
        GPIO2  = 2,
        GPIO3  = 3,
        GPIO4  = 4,
        GPIO7  = 7,
        GPIO8  = 8,
        GPIO9  = 9,
        GPIO10 = 10,
        GPIO11 = 11,
        GPIO14 = 14,
        GPIO15 = 15,
        GPIO17 = 17,
        GPIO18 = 18,
        GPIO22 = 22,
        GPIO23 = 23,
        GPIO24 = 24,
        GPIO25 = 25,
        GPIO27 = 27
    };

    class Connection
    {
        public:
            Connection();
            Connection(Pin pin);
            Connection(Pin pin, Direction direction);
            ~Connection();
            
            int setDirection(Direction direction);
            
            int setValue(bool value);
            int getValue(bool& value);

        private:
            int exportPin();
            int unexportPin();
            
            Pin pin;
            Direction direct;
    };

} // namespace GPIO

#endif // !def GPIO_H