C++

So far there have been two definitions of TypeDef, so decided to write them down for later use.

"The const pointer of a typedef"

Case

typedef string * PSTR;

Const PSTR CPSTR;

Q What kind of pointer is cpstr in the above code?

Analysis

A lot of my friends, including myself. When I first came in contact with it, the initial reaction was: CPSTR is defined as a normal pointer to a const string object, for the simple reason that replacing the position of string* and Pstr gets

This is obviously a normal pointer to a const string object

const string * CPSTR;

However, the CPSTR is defined as a const pointer to a string type, rather than a person's wish.

See this answer, compared to a lot of friends slightly surprised, I also so, which internal cause, and listen to my slow way.

The reason for this error is that we "replace it," and the word "permutation" should not be used in a TypeDef, but in #define macros, a typedef defines a type alias, and the result of the definition follows any standard defined by any normal definition. So how exactly does this typedef define a const pointer?

First analyze

Const PSTR CPSTR;

This code defines a PSTR type of const CPSTR, which is actually

pstr Const CPSTR;

For the sake of understanding, take the int definition to say

The two are equivalent, the former is customary, the latter is the standard rule

const int ival = 0;

int const IVAL = 0;

Now that we understand that CPSTR is a const PSTR type of object, what is PSTR? Pstr is string*, so it becomes cpstr a const string* type of object, or CPSTR is a const pointer to a normal string.

　　" Summary"

This misunderstanding stems from the idea that a typedef is equivalent to a #define macro definition, and that a simple substitution character is possible. In fact, otherwise. The easiest way to see this definition in the future is to start by defining the variable

typedef string * PSTR;

Const PSTR CPSTR;

Follow the steps (proficiency is quick) think:

①cpstr is a constant of type pstr

What is ②PSTR?

③pstr is string*.

④ So, Cpstr is a constant of type string*, or string *const cpstr

"function type of typedef"

Case

typedef int FUNC (int);

What type does the typedef define?

Analysis

Don't be surprised, that's it, it's not a function pointer, and I'm not writing it wrong.

It defines a function type Func, the function type requires an int parameter, and returns an int result.

Summary

FUNC TestFunc (FUNC F);

The use of Func is limited, and the above statement causes a compile-time error.

The above code uses two times func, the first error is the Func type of function return type is wrong.

Func is a function type, in another function, a function type variable can be used only as a formal parameter of another function, not as a return type.

When Func is used as a formal parameter, the compiler automatically converts it to the func*//function pointer, and the compiler does not convert Func as a function return type, resulting in a compile-time error that can be amended as follows

func* TestFunc (FUNC F);

Declares a function testfunc, which requires a function of type Func as a formal parameter and returns a function pointer of a func type func*

"TypeDef and #define"

Case

typedef int* TPI;

#define INT* Dpi;

Tpi P1,P2;

Dpi P3,P4;

Ask the four variables defined in the above code, why are they different?

"Analysis"

To differentiate between the types of P1,P2,P3,P4 four variables, first understand the difference between a typedef and a #define macro, a typedef is an alias to an existing type, and a #define is just a simple character replacement. So the code above is equivalent to

The role of int *p1,*p2;//typedef

int *p3,p4;//#define的作用

That is, P3 is defined as an integer pointer, and P4 is just an integral type.

　　Summary

Need to master the difference between typedef and #define, in case these subtle traps, for high-quality code to do well.