C++ tips - I

Static members in .h and .cpp

• A static variable in C++ when defined in a global function belongs to the file scope.
• If a C++ static variable is defined as a member variable, regardless of how many objects we instantiate we only have one static variable across all of the instances. The static variable can be accessed via the instance or the class variable.
• If a static variable is defined as member in a class declared in a .h file, the initialization should be done as [variable_type] ClassName::[variable_name] = [value]
• A static function can only access static variables - makes sense huh??
• A static function doesn't have the this pointer
• class_name::static_function() - usage

Friend keyword

	// A freind function
	class A;
	class B
	{
	private:
	int i_Num;
	public:
	B(int iNum) { i_Num = iNum; }
	friend void PrintNumbers(A &a, B &b); // non-member function, cannot use this
	};
	class A
	{
	private:
	int i_Num;
	public:
	A(int iNum) { i_Num = iNum; }
	friend void PrintNumbers(A &a, B &b);
	};
	void PrintNumbers(A &a, B &b)
	{
	std::cout << "A number is " << a.i_Num <<
	" and the B number is " << b.i_Num << std::endl;
	}

view raw freindFunc.cpp hosted with ❤ by GitHub

	// A freind class
	class B
	{
	private:
	int i_Num;
	public:
	B(int iNum) { i_Num = iNum; }
	friend class A;
	};
	class A {
	private:
	int i_Num;
	public:
	void PrintNumbers(B &b) {
	std::cout << "A number is " << i_Num <<
	" and the B number is " << b.i_Num << std::endl; // accessing the private member of B
	}
	}

view raw friendClass.cpp hosted with ❤ by GitHub

	#include <iostream>
	#include <ctime>
	#include <sstream>
	#include <fstream>
	using namespace std;
	// timestamp returns the current time as a string
	std::string timestamp();
	class LogStatement;
	ostream& operator<<(ostream& ost, const LogStatement& ls);
	class LogStatement {
	public:
	LogStatement(std::string s) : data(s), time_string(timestamp()) {
	};
	//This method handles all the outputs.
	friend ostream& operator<<(ostream&, const LogStatement&);
	private:
	std::string data;
	std::string time_string;
	};
	ostream& operator<<(ostream& ost, const LogStatement& ls) {
	ost << "~|" << ls.time_string << '|' << ls.data << "|~";
	return ost;
	}
	std::string timestamp() {
	//Notice the use of a stringstream, yet another useful stream medium!
	ostringstream stream;
	time_t rawtime;
	tm * timeinfo;
	time(&rawtime);
	timeinfo = localtime(&rawtime);
	stream << (timeinfo->tm_year) + 1900 << " " << timeinfo->tm_mon
	<< " " << timeinfo->tm_mday << " " << timeinfo->tm_hour
	<< " " << timeinfo->tm_min << " " << timeinfo->tm_sec;
	// The str() function of output stringstreams return a std::string.
	return stream.str();
	}
	/*
	*
	*/
	int main(int argc, char** argv) {
	char* testArr[3] = {"log 1", "log 2", "log 3"};
	ostringstream log_data;
	// This takes all the char arrays in the argv
	// (except the filename) and produces a stream.
	for (int i = 0; i < 3; i++) {
	log_data << testArr[i] << ' ';
	}
	int iAnyNum=100;
	log_data << iAnyNum;
	LogStatement log_entry(log_data.str());
	clog << log_entry << endl;
	ofstream logfile("logfile", ios::app);
	// check for errors opening the file
	if (!logfile) {
	return -1;
	}
	logfile << log_entry << endl;
	logfile.close();
	return 0;
	}

view raw Logger.cpp hosted with ❤ by GitHub