Confusing behaviour in concatenate-function

[UlTroX]

Hello everyone,

my question is about an anomaly I encountered while working on an exercise from Stroustrup's book "The C++ Programming Language". The exercise is to write a program that reads file names from cin and prints the file's content with cout.
I actually got the program to work but I encountered some weird "feature" that I don't understand.

First of all, here is my code (I use a Ubuntu 12.04 laptop and g++ as a compiler):

#include<iostream>
#include<fstream>
#include<string>

std::string cat();

int main()
{
	std::string data;
	
	data = cat();
	std::cout << data;
}


//Opens all files in the input line (separated by whitespaces) and gives back the contents.
std::string cat()
{
	
	std::string files;
	std::string data;
	std::string buffer;
	std::ifstream stream;
	
	std::cout << "Enter file names: ";
	
	getline(std::cin, files);
	
	while(files.size() != 0)
	{
		std::string currentfile;
		
		currentfile = files.substr(0, files.find(" "));
		
		std::cout << "Reading: " <<  currentfile << "\n";
		
		char currentf[currentfile.size()];
		
		for(int i=0; i<currentfile.size(); i++)
		{
			currentf[i] = currentfile[i];
		}
		
		stream.open(currentf);
		
		while(stream.good())
		{
			getline(stream, buffer);
			data += buffer;
			data += "\n";
		}
		
		stream.close();
		
		if(files.find(" ") != std::string::npos)
			files = files.substr( files.find(" ")+1, files.size() - files.find(" ") - 1);
		else
			files = "";
	}
	
	return data;
}

Now the weird part happens in the line where I print out the "Reading: [filename]" line. If I remove that line the program does not work anymore (it compiles and reads the filename, but does not print out the files content). If I replace this line by std::cout << "\n"; , it works again.

Can anybody explain to me, why this happens? I just don't see the connection between the problem and the solution (that is, inserting this line).
I feel kind of stupid because I solved my problem but I just don't understand how I did it...

Also I guess my code is not really good, because I don't have very much experience programming. Sorry for that, every advice on how to improve the code is very much appreciated!

Thank you for your help!
Jona

[Peter87]

The string you pass to open has to be null-terminated (like all C-strings). Now weird things will can happen because when the open function reads the string it doesn't find the null character '\0' that marks the end of the string so it will continue reading whatever happens to be stored in memory after the array, until it finds a null character somewhere.

std::string already have a function to get a C-string from it, named c_str() so you can use that instead.
stream.open(currentfile.c_str());
With an up-to-date compiler it is possible to pass an std::string to open directly.
stream.open(currentfile);

[UlTroX]

Thanks, now I understand the problem. I tried passing the std::string at first, but that did not work (so I guess I should look at my compiler version soon). It works perfectly with the currentfile.c_str(), though.