how to use getline() properly and how to use strcpy with a string a const char*

[Ardaos]

I get an error when I try to use cin.getline(line). Also, I can get strcpy(line, tempString) to work. It says no conversion exists between sting and const char*. Not sure why I get this when both are declared as string.

Any help is appreciated. thanks for your time.

  // Purpose:   prompts the user for a 5 line limerick and writes the data to the root directory in binary format
// Date:       9/8/2014
// Filename:   SaveLimerick.cpp
// Input:       5 line limerick poem
// Output:     5 line limerick in binary format

#include<iostream>
#include<string>
#include<fstream>
#include<iomanip>
using namespace std;

int main()
{
	string line;
	const char QUIT = 'Q';
	const char* FILENAME = "C:\\Limerick.dat";
	ofstream writingFile(FILENAME);

	if (!writingFile.good())
		cout << "File could not be opened for writing" <<
		endl;
	else
	{
		cout << "Enter a line or Q to quit: ";
		cin.getline(line);
		while (line[0] != QUIT)
		{
			string tempString = " ";
			writingFile.write(reinterpret_cast <const char*>(&line), sizeof(line));
			cout << "Enter a line or Q to quit ";
			cin.getline (tempString);
			strcpy(line, tempString);
		}

	}

	//closes file
	writingFile.close();

	system("Pause");
	return 0;
}

[Chervil]

There are two differing syntaxes for getline, depending on whether you use a char array, or std::string.
change

    cin.getline(line);

to

    getline(cin, line);

strcpy() is not needed when assigning to a std::string. Change this

    strcpy(line, tempString);

to this

    line = tempString;

http://www.cplusplus.com/reference/string/string/getline/

http://www.cplusplus.com/reference/string/string/operator=/

Also this line looks both overly complex, as well as incorrect:

    writingFile.write(reinterpret_cast <const char*>(&line), sizeof(line));

you could perhaps put

    writingFile.write(line.c_str(), line.size());

but it would be simpler to put

    writingFile << line;

[Ardaos]

Hey, Thanks. That getline(cin, line); seemed to work. I revised my code as shown. I changed it a bit because I am required to store it in binary output.

// Purpose:   prompts the user for a 5 line limerick and writes the data to the root directory in binary format
// Date:       9/8/2014
// Filename:   SaveLimerick.cpp
// Input:       5 line limerick poem
// Output:     5 line limerick in binary format

#include<iostream>
#include<string>
#include<fstream>
#include<iomanip>
using namespace std;

int main()
{
	string line;
	int lineNum = 1;

	const char* FILENAME = "C:\\Users\\Admin\\Limerick.dat";
	ofstream writingFile(FILENAME, ios::out | ios::binary);



	if (!writingFile.good())
		cout << "File could not be opened for writing" <<
		endl;
	else
	{
		cout << "You will be prompted for 5 lines of a Limerick poem." << endl << endl;

		while (lineNum != 6)
		{
			string tempString;
			cout << "Enter line " << lineNum << ": ";
			getline(cin, tempString);
			writingFile.write(reinterpret_cast <const char*>(&tempString), sizeof(tempString));
			++lineNum;
		}

	}

	//closes file
	writingFile.close();

	system("Pause");
	return 0;
}

however when I try to retrieve the Limerickdat file using a seperate program i get gibberish for some of the lines. I'm not sure where the problem lies. Any ideas.

// Purpose:   retrieves a limerick from file Limerick.dat and displays it
// Date:       9/8/2014
// Filename:   DisplayLimerick.cpp
// Input:       5 line limerick poem from Limerick.dat file
// Output:     displays the limerick

#include<iostream>
#include<string>
#include<fstream>
#include<iomanip>
using namespace std;

int main()
{
	string line;
	int lineNum = 1;
	const char* FILENAME = "C:\\Users\\Admin\\Limerick.dat";
	ifstream readingFile(FILENAME, ios::in | ios::binary);

	if (!readingFile.good())
		cout << "File could not be opened for reading" <<
		endl;
	else
	{
		while (lineNum != 6)
		{

			readingFile.read(reinterpret_cast <char*>(&line), sizeof(line));
			cout << lineNum << ":  " << line << endl;
			++lineNum;
		}

	}

	//closes file
	readingFile.close();

	system("Pause");
	return 0;
}

Thanks for looking.

[keskiverto]

Think about this:

#include<iostream>
#include<string>

int main() {
  std::string line1 = "A";
  std::string line2 = "File could not be opened for reading";
  std::cout << sizeof(line1) << '\n';
  std::cout << sizeof(line2) << '\n';
  return 0;
}

[Ardaos]

hmm...so a string is the same size no matter how many characters it holds. I got 28 for both, line1 and line 2. I'm not sure how to apply this. I think you are telling me to change this somehow: writingFile.write(reinterpret_cast <const char*>(&tempString), sizeof(tempString)); I'm not sure. Can I get another hint?

[keskiverto]

The thing is that string allocates dynamically space for the text that it holds. In other words, the text is not within those 28 bytes (my environment has 8 byte strings).

A websearch tells that you don't simply write a string as binary. You must write the size and then the data. Likewise, you must read the size and then the data. For example:
http://baptiste-wicht.com/posts/2011/06/write-and-read-binary-files-in-c.html

This is usually called serialization. http://en.wikipedia.org/wiki/Serialization
Boost probably has a library for serialization too.

[Ardaos]

Hey, thanks for the feedback. I revised my code to use a char array of size 100. I still get gibberish however. Not sure why. both programs compile but I get a blocks of lines when I try to retrieve the limerick. Any ideas?

// Purpose:   prompts the user for a 5 line limerick 
// Date:       9/8/2014
// Filename:   SaveLimerick.cpp
// Input:       5 line limerick poem
// Output:     5 line limerick in binary format

#include<iostream>
#include<string>
#include<fstream>
#include<iomanip>
using namespace std;

template<typename T>
ostream& binary_write(std ostream& stream, const T& value)
{
	return stream.write(reinterpret_cast<const char*>(&value), sizeof(T));
}

int main()
{
	char tempChar[100];
	const char* FILENAME = "C:\\Users\\Admin\\Limerick.dat";
	ofstream writingFile(FILENAME, ios::out | ios::binary);
	
	//creates fixed length dat file
	for (int x = 0; x < 6; ++x)
		binary_write(cout, tempChar)


	if (!writingFile.good())
		cout << "File could not be opened for writing" <<
		endl;
	else
	{
		cout << "You will be prompted for 5 lines of a Limerick poem." << endl << endl;

		for (int x = 0; x < 6; ++x)
		{
			
			cout << "Enter line " << x << ": ";
			cin.getline(tempChar);
			writingFile.seekp((x - 1) * sizeof(tempChar));
			binary_write(stream, tempChar);
			
		}

	}

	//closes file
	writingFile.close();

	system("Pause");
	return 0;
}

// Purpose:   retrieves a limerick from file Limerick.dat and displays it
// Date:       9/8/2014
// Filename:   DisplayLimerick.cpp
// Input:       5 line limerick poem from Limerick.dat file
// Output:     displays the limerick

#include<iostream>
#include<string>
#include<fstream>
#include<iomanip>
using namespace std;


int main()
{
	
	const char* FILENAME = "C:\\Users\\Admin\\Limerick.dat";
	ifstream readingFile(FILENAME, ios::in | ios::binary);
	

	if (!readingFile.good())
		cout << "File could not be opened for reading" <<
		endl;
	else
	{
		for (int x = 0; x < 6; ++x)
		{
			char tempChar[100];
			readingFile.seekg((x - 1) * sizeof(tempChar));
			readingFile.read(reinterpret_cast <char*>(&tempChar), sizeof(tempChar));
			cout << x << ":  " << tempChar << endl;
			
		}

	}

	//closes file
	readingFile.close();

	system("Pause");
	return 0;
}

[Chervil]

There are several compilation errors in the above code.
Line 14

    ostream& binary_write(std ostream& stream, const T& value)

should be

    ostream& binary_write(std::ostream& stream, const T& value)

Line 27 - missing semicolon

    binary_write(cout, tempChar)

Line 41

    cin.getline(tempChar);

should be

    cin.getline(tempChar, sizeof(tempChar));

Line 43

    binary_write(stream, tempChar);

should be

    binary_write(writingFile, tempChar);

If you still require help, please post the latest version of the actual code which you are running, otherwise there's no way to tell where the problems are.