A static value is associated with the class rather than individual objects. In C++, the static data members are declared inside the class definition but defined outside the class definition.
When you declare a static variable in the header file, it creates a declaration of the variable in every file that includes the header. And when you define the static variable in the implementation file, it creates a definition of the variable in only one file. This is why you need to initialize the static variable in the implementation file to ensure that there is only one definition of the variable.
If you call the set function to set the static value as true, the value of the static variable will be changed to true for all instances of the class. So if you initialize the static value as false in the implementation file but set it to true using the set function, the value of the static variable will be true for all instances of the class.
To clarify, the static value is not stored as a member of each object, but as a single value that is shared by all instances of the class. So when you create a new object, it does not create a new copy of the static variable. Instead, it simply uses the existing static variable that is associated with the class.
Initializing the static value in the implementation file won’t affect the value of the static variable that is associated with the class. The purpose of initializing the static variable in the implementation file is to ensure that there is only one definition of the static variable across all files that include the header file.
If you initialize the static variable in the header file, then the static variable will be defined in every file that includes the header file. This can lead to multiple definitions of the static variable, which can cause errors at compile-time or link-time.
By initializing the static variable in the implementation file, you ensure that there is only one definition of the static variable, which is what you want. Once the static variable is defined in the implementation file, you can set its value using the set function and it will be shared by all instances of the class.
When you include a header file in your C++ program using the #include directive, the contents of the header file are copied and pasted into the file that includes it. This means that any definitions in the header file, including static variable definitions, will be duplicated in every file that includes the header file.
For example, suppose you have a header file student.h that contains the following code:
class Student {
public:
static bool show;
};If you include this header file in two different source files main.cpp and helper.cpp using #include "student.h", the contents of student.h will be copied and pasted into both files. This means that the Student class and its show static variable will be defined twice, once in main.cpp and once in helper.cpp.
This can lead to multiple definitions of the show variable, which can cause errors when you try to compile or link your program. To avoid this, it’s best to define the static variable in only one source file, typically the implementation file (student.cpp) for the Student class.
When the compiler processes each source file separately, it generates an object file for each source file. The object file contains information about the functions, variables, and other symbols defined in that source file. When it comes time to link the object files together to create the final executable, the linker will combine the object files and resolve any references to symbols (such as functions and variables) that are defined in other object files.
If the linker encounters multiple definitions of a global or static variable, it doesn’t know which definition to use. This can cause linker errors, such as “multiple definition of symbol”, and prevent the program from being linked successfully.
To avoid these errors, you should define static variables in only one source file to ensure that there is only one definition of the variable in the entire program.