When you open a file, all kinds of things can go wrong. A file lives on a physical device — a fixed disk, for example, or perhaps on a flash drive or SD card — and you can run into problems when working with physical devices.
Creating you header file (and other source code files) You seem to be looking for a create a project that creates a header file option but that just doesn't exist because it doesn't make sense, most projects contain multiple header files, I regularly use 1 header file for every code file plus a few extra header files. The C library function FILE.fopen(const char.filename, const char.mode) opens the filename pointed to, by filename using the given mode. Following is the declaration for fopen function. This function returns a FILE pointer. Otherwise, NULL is returned and the global variable errno is set to. Mar 19, 2010 The compiler log states that it can't find the #include files in C:RobsStuffCSourceFilesCC. My question is why is the compiler appending a C to the end of the directory that I added to the Compiler-Options- Directories. May 02, 2014 C - Classes - Creating Source files and Header files - In this C tutorial, I will show you how to create classes outside of the main.cpp file. Codeblocks makes this easy by autocompleting a. Header files in dev-C. Ask Question Asked 11 years ago. Active 2 years, 3 months ago. Viewed 64k times 3. I'm trying to add an header file to dev-C but when I compile it it doesn't work. Here are my exact steps (for my example, I'm trying to get mysql.h to work). How do I create a Java string from the contents of a file?
For example, part of the disk might be damaged, causing an existing file to become corrupted. Or, less disastrous, you might run out of disk space. Or, even less disastrous, you might try to open a file in a directory that doesn’t exist.
If you try to open a file for writing by specifying a full path and filename but the directory does not exist, the computer responds differently, depending on the operating system you’re using. If you’re unsure how your particular operating system will respond, try writing a simple test application that tries to create and open something like /abc/def/ghi/jkl/abc.txt. (Of course, you’ll want to be sure to use a directory that doesn’t exist.)
Then one of two things will happen: Either the directory and the file will get created, or nothing will happen.
For example, on a Windows system, if we attempt to create a file in a directory that doesn’t exist, the system does not create the directory. That’s because deep down inside, the application ultimately calls an operating system function that does the dirty work of creating the file. And this particular operating system function (it’s called CreateFile(), if you even care) has a rule that it will not create a directory for you.
If you want to determine whether the ostream class was unable to create a file, you can call its fail() member function. This function returns true if the object couldn’t create the file. And that’s what happens when a directory doesn’t exist. The DirectoryCheck01 example shown demonstrates an example of this.
When you run this code, assuming that you don’t have a directory called /abc/def/ghi on your system, you should see the message Couldn’t open the file! Assuming that your particular operating system doesn’t create a directory in this case; if it does, your computer will open the file, write Hi to it, and move on with its happy life after closing things out.
As an alternative to calling the fail() member function, you can use an operator available in various stream classes. This is !, fondly referred to as the “bang” operator, and you would use it in place of calling fail(), as in this code:
Most people prefer to use !outfile instead of outfile.fail(), although !outfile makes confusing code. The reason is that outfile is an object, and the notion of !outfile simply doesn’t make sense.
In fact, !outfile trips up many beginning programmers. They know that outfile is not a pointer in this sample code, and they wonder how you could test it against 0 as you normally can only do with a pointer. (Remember, by saying !x, where x is some pointer, you’re testing x against 0.) And that simply doesn’t make sense! And so, to avoid confusion, just call fail(). It makes more sense.
Here are some reasons your file creation may choke:
- The directory doesn’t exist.
- You’re out of disk space and out of luck.
- Your application doesn’t have the right permissions to create a file.
- The filename was invalid — that is, it contained characters the operating system doesn’t allow in a filename, such as * or ?.
Like any good application, your application should do two things:
Types Of Header File
![File File](/uploads/1/2/6/8/126877916/770799662.png)
- 1.Check whether a file creation succeeded.
- 2.If the file creation failed, handle it appropriately.Don’t just print a horrible message like Oops!Aborting!, leaving your poor users with no choice but to toss the monitor onto the floor. Instead, do something friendlier — such as presenting a message telling them there’s a problem and suggesting that they might free more disk space.
The names of program elements such as variables, functions, classes, and so on must be declared before they can be used. For example, you can't just write
x = 42
without first declaring 'x'.The declaration tells the compiler whether the element is an int, a double, a function, a class or some other thing. Furthermore, each name must be declared (directly or indirectly) in every .cpp file in which it is used. When you compile a program, each .cpp file is compiled independently into a compilation unit. The compiler has no knowledge of what names are declared in other compilation units. That means that if you define a class or function or global variable, you must provide a declaration of that thing in each additional .cpp file that uses it. Each declaration of that thing must be exactly identical in all files. A slight inconsistency will cause errors, or unintended behavior, when the linker attempts to merge all the compilation units into a single program.
To minimize the potential for errors, C++ has adopted the convention of using header files to contain declarations. You make the declarations in a header file, then use the #include directive in every .cpp file or other header file that requires that declaration. The #include directive inserts a copy of the header file directly into the .cpp file prior to compilation.
Note
In Visual Studio 2019, the C++20 modules feature is introduced as an improvement and eventual replacement for header files. For more information, see Overview of modules in C++.
Example
The following example shows a common way to declare a class and then use it in a different source file. We'll start with the header file,
my_class.h
. It contains a class definition, but note that the definition is incomplete; the member function do_something
is not defined:Next, create an implementation file (typically with a .cpp or similar extension). We'll call the file my_class.cpp and provide a definition for the member declaration. We add an
#include
directive for 'my_class.h' file in order to have the my_class declaration inserted at this point in the .cpp file, and we include <iostream>
to pull in the declaration for std::cout
. Note that quotes are used for header files in the same directory as the source file, and angle brackets are used for standard library headers. Also, many standard library headers do not have .h or any other file extension.In the implementation file, we can optionally use a using statement to avoid having to qualify every mention of 'my_class' or 'cout' with 'N::' or 'std::'. Don't put using statements in your header files!
Now we can use
my_class
in another .cpp file. We #include the header file so that the compiler pulls in the declaration. All the compiler needs to know is that my_class is a class that has a public member function called do_something()
.After the compiler finishes compiling each .cpp file into .obj files, it passes the .obj files to the linker. When the linker merges the object files it finds exactly one definition for my_class; it is in the .obj file produced for my_class.cpp, and the build succeeds.
Include guards
Typically, header files have an include guard or a
#pragma once
directive to ensure that they are not inserted multiple times into a single .cpp file.What to put in a header file
Because a header file might potentially be included by multiple files, it cannot contain definitions that might produce multiple definitions of the same name. The following are not allowed, or are considered very bad practice:
- built-in type definitions at namespace or global scope
- non-inline function definitions
- non-const variable definitions
- aggregate definitions
- unnamed namespaces
- using directives
Use of the using directive will not necessarily cause an error, but can potentially cause a problem because it brings the namespace into scope in every .cpp file that directly or indirectly includes that header.
Sample header file
The following example shows the various kinds of declarations and definitions that are allowed in a header file: