Strange behavior in getline based writing to file.

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01-27-2013
dozerman

Strange behavior in getline based writing to file.

In this simple little practice project I am doing to teach myself a little C++, my program skipping the first word to be output to the file, and then writing the next four characters, followed by nothing. I'm not sure what I'm doing wrong here. Anybody have any clue what's going on here?

Code:

/* Practice project in C++, a command line HTML editor. Project started on Jan. 27, 2013 at 12:18 AM. Yeah, I'm an owl. */ #include <iostream> #include <fstream> #include <cstdlib> #include <string> using namespace std; int main() { int count=0; int sheaderleng; string sheader; /*Here, the program asks the user to enter what the header will be for the title header that is to be used.*/ cout<<"Enter header text.\n"; cin>>sheader; //convert the string into a character array so that getline can accept it. sheaderleng=sheader.length(); char header[sheaderleng]; do { header[count]=sheader[count]; count++; }while(count<=sheaderleng); //Write data to file. cin.getline (header,sheaderleng); ofstream outt("File1.html"); outt<<"<html>"<<"\n"<<"<h1>"<<header<<"<h1/>"<<"\n"<<"<html/>"; outt.close(); //Finish program. return 0; }
01-27-2013
whiteflags

>> //convert the string into a character array so that getline can accept it.

Completely unnecessary. getline works with the std::string type. So you might as well just use it and forget the nonsense about copying it to a character array.

As of now, your code has a warning because you tried to convert a C++ string to a C string:

Code:

mingw32-g++.exe -Wall -g -pedantic -std=c++98 -Wextra -Wall -g -c C:\Users\Josh2\Documents\bar\bar.cpp -o obj\Debug\bar.o C:\Users\Josh2\Documents\bar\bar.cpp: In function 'int main()': C:\Users\Josh2\Documents\bar\bar.cpp:27:28: warning: ISO C++ forbids variable length array 'header' [-Wvla] mingw32-g++.exe -o bin\Debug\bar.exe obj\Debug\bar.o Output size is 960.18 KB Process terminated with status 0 (0 minutes, 1 seconds) 0 errors, 1 warnings
01-28-2013
msh

Why you're calling `getline()` on L36 if you already copied the header text into `header` on L28-L32?

Anyhow. Your strange behaviour is caused by the fact that `>>` operator skips leading whitespace and extracts up to but not including first whitespace character after valid input. `getline()` has a similar behaviour, but it extracts the delimiting whitespace but doesn't store it.

After L22, you have a newline character in your input buffer that was not extracted by `>>`. On L36, `getline()` extracts it but doesn't store it into `header`: you never get a chance to enter anything and `header` ends up empty.

Be careful with `std::string::length()`: it returns the number of characters stored in the string, without the terminating null-character. Any c-style string will need to be `length()` + 1 to properly store contents of `std::string`.
01-28-2013
King Mir

Just use getline(cin, sheader) to read in the header. It's that easy.

Quote:

Originally Posted by whiteflags

Completely unnecessary. getline works with the std::string type. So you might as well just use it and forget the nonsense about copying it to a character array.

The free function getline works with std::string. cin.getline() does not.
01-28-2013
Elkvis

Quote:

Originally Posted by dozerman

Code:

sheaderleng=sheader.length(); char header[sheaderleng];

this is non-standard C++. G++ appears to compile it without warnings, and other compilers might also allow it, but know that variable length arrays are not guaranteed to be supported. according to the standard, the array size must be a constant expression.