I have a C++ program (developed in Visual C++ 2017) that need to output a lot of lines to files. Below are simplified version just to illustrate the problem.
#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <filesystem>
#include <algorithm>
#include <numeric>
#include <fstream>
using namespace std;
class Test{
public:
void output(int a){
static ofstream out("a.txt", fstream::in | fstream::out | fstream::app);
out<<a<<"\n";
}
};
int main()
{
Test t;
for(int i = 0; i < 10; i++){
t.output(i);
}
t.~Test();
remove("a.txt");
}
The remove("a.txt") never works. I understand that the text file is never properly closed. I explicitly call the t.~Test() in hope it will close the static ofstream but it seems id does not work.
The reason to make the ofstream static is it significantly improve the performance of my production code since I assume it just open the file one time and use it instead of open and close the file for each call
I know the design is bad, but this is from legacy code and I am reluctant to change the function signature.
Is there a simple way to make this work without changing the "out" function signature? Thanks
The life-time of a local static variable is the life-time of the program itself. It will not be destructed until the program exits.
One possible way to go around your problem could be to use a member variable in the class instead. Open the stream in the constructor, and close it in the destructor. If multiple objects of the class needs to share the stream them make it a static member variable which is opened by its definition (remember that static member variables might need separate declaration and definition).
Related
I have two similar methods that open a file identically, but process them and return values a bit differently, yet while the first method does that successfully, the second method, which is called after the first one, fails.
I have tried changing the path to this file, its extension, but I think I miss some important knowledge about ifstream.
vector<User> Database::createUserDatabase()
{
vector<User> users;
ifstream inputFile;
inputFile.open(pathToFile, ios::in);
//Some file processing
inputFile.close();
return users;
}
And that works perfectly, while
vector<User> Database::createBookDatabase()
{
vector<Book> books;
ifstream inputFile;
inputFile.open(pathToFile, ios::in);
//Some file processing
inputFile.close();
return books;
}
fails to end whenever I check if the file has been opened or not using
inputFile.is_open()
These functions are defined in class files Database.cpp, User.cpp, Book.cpp, which are correctly linked to the main.cpp with the following content:
#include <iostream>
#include <cstdlib>
#include <string>
#include <cstring>
#include <sstream>
#include <vector>
#include <fstream>
#include "../lib/Book.h"
#include "../lib/User.h"
#include "../lib/Database.h"
using namespace std;
int main()
{
Database userDatabase("../database/users.txt", "users");
Database bookDatabase("../database/lmsdb.txt", "books");
vector<User> users = userDatabase.createUserDatabase();
vector<Book> books = bookDatabase.createBookDatabase();
return 0;
}
Here are my Project directories
Using gdb debugger, I have confirmed that the file is not being opened at all. I assume that I did not close the files properly, but I have a little knowledge of C++ yet (been learning it for only a week or so).
Looking forward to see what you can suggest reading/researching, yet I really would like to see a straightforward solution to this problem.
I assume that I did not close the files properly, [..]
Yes, but that probably isn't the cause of the issue. The C++ way is to not close them explicitly. Due to RAII, the ifstream will close itself once it goes out of scope (i.e. when the enclosing function terminates).
There are many reasons why a file could fail to open, including:
It doesn't exist.
Trying to open a read-only file in write mode.
The file is in use by another process. (Maybe you have it opened in an editor?)
Insufficient privileges (e.g. due to the file being protected).
So I was trying to access a method that is defined in another class and has the prototype in the header. I'm pretty positive I defined it but it keeps popping up undefined reference to SafeCracker.
Main.cpp
#include <iostream>
#include "mystuff.h"
using namespace std;
void BigDog(int KibblesCount);
int main()
{
cout << SafeCracker(1);
return 0;
}
mystuff.cpp
#include <iostream>
using namespace std;
string SafeCracker(int SafeID)
{
return "123456";
}
mystuff.h
using namespace std;
#ifndef MYSTUFF_H_INCLUDED
#define MYSTUFF_H_INCLUDED
string SafeCracker(int SafeID);
#endif // MYSTUFF_H_INCLUDED
Here it tells you that you have an undefined reference, so you don't really have a problem with the prototype.
Had you forgotten to include the header file that contains the prototype you would have gotten something like
main.cpp: In function ‘int main()’:
main.cpp:8:13: error: ‘SafeCracker’ was not declared in this scope
cout << SafeCracker(1);
Your undefined reference is a linker error. The most likely cause would be that you did not use mystuff.cpp when compiling
If you're compiling from the command line, you should give both files as parameters.
If you're using an IDE that calls the compiler, make sure that the file is part of the project.
For example in Code::Blocks right-click on the file name and go "add to project" (If I remember correctly)
It is also possible that you made a typo in the function declaration in mystuff.cpp (that doesn't seem to be the case here though)
Now there is one important thing about your code you should take note of:
It is very bad practice to put a using namespace in a header file.
using namespace std; in a .cpp source file is mostly up to you, and that using statement will only apply to that particular file.
But if you put it in a header file that is meant to be included through #include , the using there will be forced upon any code that includes it.
Here is an example:
main.cpp
#include <iostream>
// including mystuff.h to use that awesome SafeCracker()
#include "mystuff.h"
// I need to use an std::map (basically an associative array)
#include <map>
// the map of my game
class map
{
int tiles[10][10];
};
int main()
{
// The std map I need to use
std::map<int, int> mymappedcontainer;
// The map of my game I need to use
map mytiles;
// The reason why I need to include mystuff.h
cout << SafeCracker(1);
return 0;
}
Normally, my class map should not be a problem since the map I included from the standard library is inside the namespace std, so to use it you would need to go std::map.
The problem here is that, since mystuff.h has using namespace std; in it, the symbol map is already used, and that creates a conflict.
You do not now who will use your header files, or if you will use them again a long time from now, and maybe then you will want to use name that is already used in the std namespace.
I advise you to use std:: before things taken from the standard libraries instead (std::string instead of just string for example)
PS: In C++, "class" refers to a class data structure, and the functions you made here are not part of any class. You should say "defined in another file" or "defined in another translation unit" instead
I'm working on making a game in C++. I have declared a Constant namespace used for global values that I need access to throughout the program. In there I have an ofstream for debugging purposes (yeah, I know it's not "constant" but it fits best there), which outputs only when it feels like it. I was able to make a small program demonstrating the problem. I apologize for it being spread across 4 files, but it is important, I promise.
main.cpp:
// Include necessary files
#include "test.h"
#include "constants.h"
#include <fstream>
using namespace std;
int main(int argc, char* argv[])
{
// Start of program
Constant::outstream.open("test.txt");
// ...
// Do stuff
// Output debugging info
Test test;
test.print("Test", Constant::outstream);
// ...
// Do other stuff
// End of program
Constant::outstream.close();
return 0;
}
constants.h:
#ifndef _CONSTANTS_H
#define _CONSTANTS_H
#include <fstream>
namespace Constant
{
static ofstream outstream;
}
#endif
test.h:
#ifndef _TEST_H
#define _TEST_H
#include <string>
#include <fstream>
#include "constants.h"
class Test
{
public:
void print(string str, ofstream& out);
};
#endif
test.cpp:
#include "test.h"
using namespace std;
void Test::print(string str, ofstream& out)
{
out << "out: " << str << endl << flush; // Works
Constant::outstream << "Constant::outstream: " << str << endl << flush; // Doesn't
}
In the test.cpp file, the out << ... line works as it should, while the Constant::outsream << ... line doesn't do anything even though I'm passing Constant::outstream as the out parameter! I don't see any reason why these two lines should be in any way different.
Before posting this, I tried putting test.cpp's code in test.h, just to have less files for the question, and was amazed to see it work. If I copy-paste the Test::print() function into test.h (whether inside or out of the class Test { ... }), then both output commands work correctly. the problem only occurs if Test::print()'s implementation is in a separate file.
It seems like any references to Constant::outstream simply don't work in class cpp files (no compile error, just nothing happens). It works in main.cpp and in class header files, but any class cpp file it seems not to. Unfortunately, this is a big program I'm writing so pretty much every class has its own cpp implementation file, and that's really the one place I need to use this ofstream. Does anyone know the reason for this?
Thanks in advance,
Doug
Constant::outstream has internal linkage, thus a separate instance is created for each translation unit. In short, Constant::outstream in test.cpp and main.cpp are two different variables.
§3.5.2 A name having namespace scope (3.3.6) has internal linkage if it is the name of
— a variable, function or function template that is explicitly declared static; or,
On the other hand, static class members would be visible throughout the program.
So, if you would write
struct Constant
{
static ofstream outstream;
}
instead of
namespace Constant
{
static ofstream outstream;
}
it would work.
However, note that the class must have external linkage; e.g. you should not put in in anonymous namespace.
I am new in c++, I want to write my program with eclips but it does not know cout and cin however I add include
This is my code:
class READY {
public:
READY();
virtual ~READY();
#include <iostream.h>
int main (){
cout<<"hello";
}
};
#endif /* READY_H_ */
Move the include and main outside the class and qualify cout with std:::
#include <iostream>
class READY {
public:
READY();
virtual ~READY();
};
int main (){
std::cout<<"hello";
}
C++ is not Java, main resides at the global scope, not as a class member.
Also, it's <iostream>, not <iostream.h>.
Whatever tutorial or book you're following... it isn't any good.
You need to put that #include at the top of your file; including headers in the middle of a class will do weird, weird things! At the very least, it will embed all the names in the header into your class; most likely, it will simply fail to compile.
Furthermore, modern C++ puts cout and essentially every other symbol defined in the standard library into a namespace named std, so you need to write std::cout, or put "using namespace std;" before your class definition, but after the #include.
There are a couple of problems with your code:
The #include statement has to be outside the class declaration. It is good practise to put those at the top of the file and not scatter them through the file as it makes it much easier to check for dependencies in your code by eyeballing the top of the file instead of searching the whole file for #includes.
Your main() function also has to be declared and defined outside the class. In contrast to Jave, main() in C and C++ is a standalone function.
As mentioned, cin and cout live in the std namespace. I would recommend referring to them with the fully qualified name (ie, std::cin and std::cout), although you can use using std::cin; and using std::cout; either inside the function or in your implementation file after all includes
You are including iostream.h - that is the "wrong" file as that is for the old iostreams library. The correct include for the standard compliant iostreams is <iostream>
Could any body offer me any reason about that?
If we do it like that, what's the outcome? Compile error?
The problem is that static initialization isnt just initialization, it is also definition. Take for example:
hacks.h :
class Foo
{
public:
static std::string bar_;
};
std::string Foo::bar_ = "Hello";
std::string GimmeFoo();
main.cpp :
#include <string>
#include <sstream>
#include <iostream>
#include "hacks.h"
using std::string;
using std::ostringstream;
using std::cout;
int main()
{
string s = GimmeFoo();
return 0;
}
foo.cpp :
#include <string>
#include <sstream>
#include <iostream>
#include "hacks.h"
using std::string;
using std::ostringstream;
using std::cout;
string GimmeFoo()
{
Foo foo;
foo;
string s = foo.bar_;
return s;
}
In this case, you can't initialize Foo::bar_ in the header because it will be allocated in every file that #includes hacks.h. So there will be 2 instances of Foo.bar_ in memory - one in main.cpp, and one in foo.cpp.
The solution is to allocate & initialize in just one place:
foo.cpp :
...
std::string Foo::bar_ = "Hello";
...
It is just a limitation in the language it self. Hopefully, when C++0x becomes reality, this limitation would go away.
I think this page gives a somehow good reason:
One of the trickiest ramifications of
using a static data member in a class
is that it must be initialized, just
once, outside the class definition, in
the source file. This is due to the
fact a header file is typically seen
multiple times by the compiler. If the
compiler encountered the
initialization of a variable multiple
times it would be very difficult to
ensure that variables were properly
initialized. Hence, exactly one
initialization of a static is allowed
in the entire program.