Specific differences between typedef and define

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1) #define It's preprocessing instructions , Make a simple replacement when compiling preprocessing , Do not check for correctness , It doesn't matter whether the meaning is correct or not , Only when compiling the expanded source program will possible errors be found and reported . such as ： #define PI 3.1415926 In program ：area=PI*r*r Will be replaced by 3.1415926*r*r Suppose you put #define The number in the sentence 9 Write in letters g Pretreatment is also brought into .

2）typedef Is processed at compile time . It gives an alias to an existing type in its own scope , But You cannot use the typedef specifier inside a function definition.

3）typedef int * int_ptr; And #define int_ptr int * The function is to use int_ptr representative int * , But the two are different , As mentioned earlier ,#define In pretreatment Make a simple replacement when , and typedef Not a simple replacement , Instead, a type is declared in the same way as a variable . in other words ;

//refer to (xzgyb( Old Dharma )) #define int_ptr int * int_ptr a, b; // amount to int * a, b; Just a simple macro replacement

typedef int* int_ptr; int_ptr a, b; //a, b All point to int The pointer to ,typedef by int* Introduced a new mnemonic

This also explains why the following view holds //QunKangLi( The maintenance cost is proportional to the square of the creativity of the programmer ) typedef int * pint ; #define PINT int *

that ： const pint p ;//p Non modifiable , but p The content pointed to can be changed const PINT p ;//p Changeable , But p The content pointed to cannot be changed .

pint It's a pointer type const pint p Just lock the pointer p Non modifiable and const PINT p yes const int * p The lock is the pointer p Object referred to .

3） Perhaps you have noticed #define It's not a statement Don't put a semicolon at the end of the line , otherwise It will be replaced by a semicolon .

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One 、typedef How to use

stay C/C++ In language ,typedef It is often used to define an identifier and keyword Another name for , It's part of the language compilation process , But it doesn't actually allocate memory space , Examples are like ：

typedef int INT;typedef int ARRAY[10];typedef (int*) pINT;

typedef It can enhance the readability of the program , And the flexibility of identifiers , But it also has “ Non intuition ” Etc .

Two 、#define How to use

#define Define a statement for a macro , It is often used to define constants ( Including no parameter and with parameter ), And to achieve that “ It's kind on the surface 、 There's a long string of ” The macro , It's not in the process of compiling itself , But before that ( Pretreatment process ) It's over , But it is also difficult to find potential errors and other code maintenance problems , Its examples are like ：

#define INT int#define TRUE 1#define Add(a,b) ((a)+(b));#define Loop_10 for (int i=0; i<10; i++)

stay Scott Meyer Of Effective C++ The terms of a Book 1 There are about #define Analysis of the disadvantages of sentences , And good alternatives , You can see .

3、 ... and 、#typedef And #define The difference between

From the above concept, we can basically understand ,typedef Just to add � A new name for the identifier for readability ( Just an alias ), and #define Originally in C To define constants , here we are C++,const、enum、inline The emergence of makes it gradually become an alias tool . Sometimes very easy I don't know the relationship with typedef Which of the two is better , Such as #define INT int This kind of sentence , use typedef It can also be completed , Which one is better ？ I advocate using typedef, Because in the early days of many C This statement in the compiler is illegal , Just today's compilers Expanded again . In order to be as compatible as possible , Generally follow #define Definition “ Can be read ” And the task of some macro statements , and typedef Is often used to define keyword、 Lengthy type of distinction name .

Macro definitions are simply string substitutions ( In situ expansion ), and typedef Instead of expanding in situ , Its new name has a certain encapsulation , So that the newly named identifier has the function of defining variables more easily . Look at the third line of the first big point code above ：

typedef (int*) pINT; And the following line :#define pINT2 int*

The effect is the same ？ It's different ！ There are differences in practice ：pINT a,b; The effect is the same as int *a; int *b; Indicates that two integer pointer variables are defined . and pINT2 a,b; The effect is the same as int *a, b; Indicates that an integer pointer variable is defined a And integer variables b.

Be careful ： The other end of the line ; The difference between numbers ！

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