Symbolic constant, const qualifier

#define INT_MAX 32767

         stay cpp During compilation , First pass the source code to the preprocessor . ad locum ,#define and #include equally , It is also a preprocessor compilation instruction . The compile instruction tells the preprocessor ： Find... In the program INT_MAX, And will all INT_MAX Are replaced with 32767. therefore #define Compile instructions work in a similar way to the global search and replace command in a text editor or word processor . The modified program will be compiled after these substitutions . The preprocessor looks for independent tags （ Separate words ）, Skip embedded words . in other words , The preprocessor will not PINT_MAXTM Replace with P32767TM. You can also use #define To define your own symbolic constants , However #define The compile instruction is C The legacy of language ,cpp There is a better way to create symbolic constants , Use keywords const, So it won't be used often #define, However , Some header files , Especially those designed for c and cpp Header file in , You have to use #define.

         The symbolic name indicates what the constant represents , in addition , If the program uses the same constant in multiple places , When you need to modify the constant , Just modify one symbol definition . have access to const Keyword to modify variable declaration and initialization , Such as

const int Months=12;

After the constant is initialized , Its value is fixed , The compiler will not allow you to modify the value of this constant .g++ Will indicate that the program attempted to assign a value to a read-only variable , keyword const It's called a qualifier , Because it defines the meaning of the declaration . A common practice is to capitalize the first letter of the name , Reminded Months It's a constant , Another convention is to capitalize the entire name , Use #define This Convention is often used when creating constants , The general format for creating constants is as follows ：

const type name = value;

If you do not provide a value when declaring a constant , Then the value of the constant will be indeterminate , And cannot be modified .const Than #define good , First , It can clearly indicate the type . secondly , have access to cpp Scope rules restrict definitions to specific functions or files , Third , Can be const For more complex types .