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'.
Jul 03, 2019 1. What is a Header File in C and C? The C/C Standard Library offers its users a variety of functions, one of which is header files. In C, all the header files may or may not end with the.h extension but in C, all the header files must necessarily begin with the.h extension. I have been searching to get the source code of the header file and its associated library in order to integrate it with my C program. At the same time, I am interested in those.
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
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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:
-->Header files for the C++ standard library and extensions, by category.
Headers by category
| Category | Headers |
|---|---|
| Algorithms | <algorithm>, <cstdlib>, <numeric> |
| Atomic operations | <atomic>11 |
| C library wrappers | <cassert>, <ccomplex>11 a b, <cctype>, <cerrno>, <cfenv>11, <cfloat>, <cinttypes>11, <ciso646>b, <climits>, <clocale>, <cmath>, <csetjmp>, <csignal>, <cstdalign>11 a b, <cstdarg>, <cstdbool>11 a b, <cstddef>, <cstdint>11, <cstdio>, <cstdlib>, <cstring>, <ctgmath>11 a b, <ctime>, <cuchar>11, <cwchar>, <cwctype> |
| Concepts | <concepts>20 |
| Containers | |
| Sequence containers | <array>11, <deque>, <forward_list>11, <list>, <vector> |
| Ordered associative containers | <map>, <set> |
| Unordered associative containers | <unordered_map>11, <unordered_set>11 |
| Container adaptors | <queue>, <stack> |
| Container views | <span>20 |
| Errors and exception handling | <cassert>, <exception>, <stdexcept>, <system_error>11 |
| General utilities | <any>17, <bitset>, <charconv>17, <cstdlib>, <execution>17, <functional>, <memory>, <memory_resource>17, <optional>17, <ratio>11, <scoped_allocator>11, <tuple>11, <type_traits>11, <typeindex>11, <utility>, <variant>17 |
| I/O and formatting | <cinttypes>11, <cstdio>, <filesystem>17, <fstream>, <iomanip>, <ios>, <iosfwd>, <iostream>, <istream>, <ostream>, <sstream>, <streambuf>, <strstream>c, <syncstream>20 |
| Iterators | <iterator> |
| Language support | <cfloat>, <climits>, <codecvt>11 a, <compare>20, <contract>20, <coroutine>20, <csetjmp>, <csignal>, <cstdarg>, <cstddef>, <cstdint>11, <cstdlib>, <exception>, <initializer_list>11, <limits>, <new>, <typeinfo>, <version>20 |
| Localization | <clocale>, <codecvt>11 a, <cvt/wbuffer>, <cvt/wstring>, <locale> |
| Math and numerics | <bit>20, <cfenv>11, <cmath>, <complex>, <cstdlib>, <limits>, <numeric>, <random>11, <ratio>11, <valarray> |
| Memory management | <allocators>, <memory>, <memory_resource>17, <new>, <scoped_allocator>11 |
| Multithreading | <atomic>11, <condition_variable>11, <future>11, <mutex>11, <shared_mutex>14, <thread>11 |
| Ranges | <ranges>20 |
| Regular expressions | <regex>11 |
| Strings and character data | <cctype>, <cstdlib>, <cstring>, <cuchar>11, <cwchar>, <cwctype>, <regex>11, <string>, <string_view>17 |
| Time | <chrono>11, <ctime> |
11 Added in the C++11 standard.
14 Added in the C++14 standard.
17 Added in the C++17 standard.
20 Added in the draft C++20 standard.
a Deprecated in the C++17 standard.
b Removed in the draft C++20 standard.
c Deprecated in the C++98 standard.
C++ Header File Tutorial
| Category | Headers |
|---|---|
| Algorithms | <algorithm> |
| C library wrappers | <cassert>, <cctype>, <cerrno>, <cfenv>, <cfloat>, <cinttypes>, <ciso646>, <climits>, <clocale>, <cmath>, <csetjmp>, <csignal>, <cstdarg>, <cstdbool>, <cstddef>, <cstdint>, <cstdio>, <cstdlib>, <cstring>, <ctgmath>, <ctime>, <cwchar>, <cwctype> |
| Containers | |
| Sequence containers | <array>, <deque>, <forward_list>, <list>, <vector> |
| Ordered associative containers | <map>, <set> |
| Unordered associative containers | <unordered_map>, <unordered_set> |
| Adaptor containers | <queue>, <stack> |
| Errors and exception handling | <exception>, <stdexcept>, <system_error> |
| I/O and formatting | <filesystem>, <fstream>, <iomanip>, <ios>, <iosfwd>, <iostream>, <istream>, <ostream>, <sstream>, <streambuf>, <strstream> |
| Iterators | <iterator> |
| Localization | <codecvt>, <cvt/wbuffer>, <cvt/wstring>, <locale> |
| Math and numerics | <complex>, <limits>, <numeric>, <random>, <ratio>, <valarray> |
| Memory Management | <allocators>, <memory>, <new>, <scoped_allocator> |
| Multithreading | <atomic>, <condition_variable>, <future>, <mutex>, <shared_mutex>, <thread> |
| Other utilities | <bitset>, <chrono>, <functional>, <initializer_list>, <tuple>, <type_traits>, <typeinfo>, <typeindex>, <utility> |
| Strings and character data | <regex>, <string>, <string_view> |
See also
Examples Of Header Files
Using C++ library headers
C++ standard library
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