I am having some trouble with vector declarations in the header file of a C class I am making. My entire header file looks like this: #ifndef PERSONH #define PERSONH #include 'Message.h' #inc. Make Your Own Header File? Step1: Type this Code crayon-5e929ed7cf43/ In this Code write only function definition as you write in General C Program Step 2: Save Code Save Above Code with.h Extension. Let name of our header file be myhead myhead.h Compile Code if required.

1. Create C++ Header
2. C++ Header File Tutorial

Create C++ Header

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:

Many C++ newbies want to create the projects they find in books by typing in the code they come across to better see how the process works. However, sometimes just getting the project started can be a serious problem. You may find step-by-step instructions online that provide you with techniques you can use to create the projects, but often, these procedures build on what you’ve done before. The following steps help you get a project started that includes a header, even if you jump to this point directly without getting all the prep work done. Even though the screenshots show a Windows system, the same procedure works with both the Mac and Linux platforms.

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1. Open your copy of Code::Blocks using a technique appropriate for your operating system.

   You see the Code::Blocks IDE open with the Start Here tab opened.

2. Choose File→New→Project or click Create a New Project on the Start Here page that appears when you start the application.

   The New from Template dialog box appears.

3. In the New from Template dialog box, click the Console Application icon found in the Projects tab, then click Go.

   You may see the Welcome page of the Console Application wizard. If so, click Next to get past it. The first usable page asks which language you want to use.

4. Highlight C++ and click Next.

   You see a list of project-related questions. These questions define project basics, such as the project name.

5. Type a name for your project in the Project Title field and a location for your project in the Folder to Create Project In field.

   The example uses SampleProject as the project title. However, you need to give your project a name that matches the application you want to create. Notice that the wizard automatically starts creating an entry for you in the Project Filename field. It’s best to simply use the default name in the Project Filename field.

6. Click Next.

   You see the compiler settings. Most of the projects in this book use the default compiler settings, which means using the GNU GCC Compiler option in the Compiler drop down list box. However, if you look at the Compiler drop-down list, you see that Code::Blocks supports a number of compilers and you can add more to the list. The other settings control the creation and location of a Debug version (the version you use for finding problems in your code) and a Release version (the version that you send to a customer) of the application.

7. Change any required compiler settings and click Finish.

   The wizard creates the application for you. It then displays the Code::Blocks IDE with the project loaded. However, the source code file isn’t loaded yet. To load the source code file, you simply double click its entry in the Workspace hierarchy (such as main.cpp).

8. Highlight the project entry (such as Sample Project) and choose File→New→File.

   The New from Template dialog box shows the kinds of files you can add to your project.

9. Highlight one of the file types, such as C/C++ Header or C/C++ Source, and click Next.

   You see the appropriate wizard for the kind of file you’re adding. This article assumes you’re adding a header file. If you see a welcome page, simply click Next to get past it.

10. Configure the header file as needed for your project.

    1. Type a filename in the Filename with Full Path field.

    2. Check individual build options or click All to add the header to all of the builds for this project. (All is the best selection for the book).

    You must provide a full path to the filename. Notice that the IDE automatically creates a Header Guard Word field entry for you — that prevents the compiler from adding the header to a project more than once. Using this default normally works just fine.

11. Click Finish.

    The IDE creates the new file for you and automatically opens it for editing.

C++ Header File Tutorial

You can repeat steps 10 through 12 as often as needed to create the entire project hierarchy. The process for adding a C++ source file is about the same as adding a header file. The only difference is that you don’t need to add a header guard word. Each time you add a new file, the IDE will add it to the project hierarchy and automatically open the file for you.