Differences between description and definition

Generally speaking:StatementIs to tell the compiler that such an identifier exists.DefinitionIt applies for a piece of memory for the program.

For example:

 
 

  
  
Int x; // This is a definition
  
  
Extern int x; // This is the Declaration
  
  
Int x = 10; // This is also a definition
 
 

Use extern int x = 10 in the program, which is also defined.

Declared operations are generally used in header files, so that the same variable can be referenced in multiple source files. This also shows why definition cannot be used. Because the header file is actually a part of the source file after it is included in the source file.

Therefore, if a variable is defined in the header file, the variable definition will appear in multiple source files. That is, repeated definitions.

As mentioned above, variables cannot be defined in the header file. However, there are three exceptions. The header file can define classes, const objects and inline functions that are known at compilation.

Define these entities in the header file because they are not just declared) because the compiler needs their definitions to generate code.

For example, to generate code that can define and use class objects, the compiler needs to know the data members that make up the class and the operations that can be performed on these objects. The class definition provides this information, so you need to define the class in the header file.

Because the const variable is the local variable that defines its file by default. Therefore, the definition of this variable is valid even in multiple source files.

1. When the const variable is initialized using a constant expression, its initialization process can be implemented in the header file.

2. If the const variable is not initialized using a constant expression, the variable cannot be initialized in the header file, but must be defined and initialized in the source file. Add the extern declaration to the header file so that it can be shared by multiple files.