Understanding typedef in C/C++

GOAL

To understand “typedef” in C++

What is typedef?

typedef is a reserved keyword that gives a new name to existing data type used in C/C++ programming. That new name like this is called alias or synonym.

The following is an example of typedef. typedef is used just for user convenience.

typedef int USER_Id;
USER_Id john_id; //this is same as int john_id
john_id = 24;

When to use typedef

These are some of many use cases of type def.

To clarify the meaning of variable

For example, when you handle data of date many times, you may want to identify month and day.

typedef  int Month;  //define alias "Month"
typedef int Day;  // define alias "Day"

This is an example of use.

#include <iostream>
using namespace std;

typedef  int Month;
typedef int Day;

void print_birthday(Month m,  Day d){
    printf("your birthday is %d/%d\n",m, d);
}
Day last_day(Month m){
    if(m == 4 && m==6 && m==9 && m==11)
        return 30;
    if(m == 2)
        return 28; //ignore leap year 
    else
        return 31;
}
int main(void){
    print_birthday(12,24);  /* output is "your birthday is 12/24" */
    cout << last_day(10);  /* output is "31"*/
}

To omit long type

typedef long long unsigned int LLUInt;

int main(void){
    LLUInt test1 = 10;
    LLUInt test2 = 20;
    cout << test1+test2 << endl;  /* output is "30"*/
}

To omit the “struct” keyword in C language

When you declare a defined structure, you should use the “struct” keyword in C language.

struct person{
    char *nickname;
    int age;
};
//declare
struct person john;

If you use typedef and rename the structure, you don’t need the “struct” keyword.

typedef struct person{
    char *nickname;
    int age;
} person_type;

//declare
person_type john;

The definition of the structure in above source code is the same as this.

struct person{
    char *nickname;
    int age;
};
typedef person person_type;

self-referencing structure

The following is the Node structure used in linked list.

struct Node {
    int data;
    struct Node *next;
}

This self-referencing structure is also defined by using typedef.

typedef struct _node{
int data;
struct _node * next;
} Node;
Node* top;

To create a pointer type

Usually you should use * to declare a pointer.

int *p1, *p2, *p3, *p4;

You can define pointer type by using typedef.

typedef int* IntP
IntP p1, p2, p3, p4;

string

#include <iostream>
using namespace std;

typedef char* my_str;

int main(void){
    my_str name = "Nako";
    printf("My name is %s\n", name); /* output is "My name is Nako"*/
}

Variations

Structure and its pointer

typedef struct _node{
    int data;
    struct _node * next;
} Node, *pNode;

Function

In this example, variable f can store a pointer to function int FUNC(int n).

#include <iostream>
using namespace std;

int next_int(int x) { return x + 1; } //example of int FUNC(int n)
int half_int(int x) { return x/2;}  //example of int FUNC(int n)

int main(void){
    int (*f1)(int n);
    int (*f2)(int n);
    f1 = next_int;
    f2 = half_int;
    cout << f1(4) << " " << f2(4) << endl;  /*output is "5 2"*/
}

Typedef can add the name to the pointer of function.

typedef int (*FUNCT)(int n);

The above example of function pointer can be changed as below.

#include <iostream>
using namespace std;

int next_int(int x) { return x + 1; }
int half_int(int x) { return x/2;}

typedef int (*FUNCT)(int n);

int main(void){
    FUNCT f1;
    FUNCT f2;
    f1 = next_int;
    f2 = half_int;
    cout << f1(4) << " " << f2(4) << endl;    /*output is "5 2"*/
}

list

typedef can define list as type.

#include <iostream>
using namespace std;

typedef int Members[5];

int main(void){
    Members team_a = {13, 25, 66, 89, 100};
    Members team_b = {21, 22, 50, 78, 81};
    for(int i=0; i<5; i++){
        cout << team_a[i] << " ";  /*output is "13 25 66 89 100 "*/
    }
    cout << endl;
    for(int i=0; i<5; i++){
        cout << team_b[i] << " ";  /*output is "21 22 50 78 81 "*/
    }
}

using in C++

The “using” declaration introduces a name into the declarative region in C++. The “using” declaration is usually used for making a declaration of namespace such as “using namespace std;”, but it can be also used for alias.

using Month = int;
using Day = int;
using FUNCT = int (*)(int);

I’ll write about the difference between typedef and using and its features someday. Thank you!

[C++]How to separate header file and source file.

GOAL

To understand the rules and general method to separate header files and source files in C++.

What is header files?

Header files are files where class definitions and function declarations are summarized. You can use the class written in the header file by writig #include or #include “myheader.h” in source file as follows.

#include <iostream>
#include "myClass.h"
using namespace std;
int main(){
   myClass instance1;
   cout << instance1.getName() << endl;
}

Why are header files necessary?

In C++, variables, classes, functions and so on should be declared before used, like sayHello() or myClass in the sample code below.

//mysource.cpp

#include <string>
#include <iostream>
using namespace std;
void sayHello(){
    cout << "Hello!" << endl;
}
class myClass {
    private:
        string name;
    public:
        myClass();
        myClass(string name_str);
        string getName();
};
myClass::myClass(){
    name = "nothing";
}
myClass::myClass(string name_str){
    name = name_str;
}
string myClass::getName(){
    return name;
}

int main() {
    sayHello();
    myClass cat1("Joe");
    cout << cat1.getName() << endl;
}

//console result
Hello!
Joe

If this declaration is missed or called by wrong name, an error will occur. And when you use the same function or class in multiple files, you have to declare it many times in each file. It is difficult to understand the structured of code if the names of functions that have same function are different. And it is also difficult to make corrections collectively. For this reasons, the declaration part are split into the header files separately from the main source files.

//myClass.h
#include <string>
#include <iostream>

class myClass {
    private:
        std::string name;
    public:
        myClass();
        myClass(std::string name_str);
        std::string getName();
};
myClass::myClass(){
    name = "nothing";
}
myClass::myClass(std::string name_str){
    name = name_str;
}
std::string myClass::getName(){
    return name;
}

//mysource.cpp
#include <iostream>
#include "myClass.h"
using namespace std;
int main() {
    myClass cat1("Joe");
    cout << cat1.getName() << endl;
}

//console result
Joe

About #include

Header files can be loaded with #include or #include “filename”. The difference between the two styles is that when #include is used, the system searches the directory of the library specified by the compiler preferentially, but on the other hand, when #include “filename” is used, the system searches the current directory.

See below for C and C++ standard libraries.

• C standard library header files
• C++ standard library header

What to put in a header file

Reference: What to put in a header file from Microsoft C++ language reference

In header files, write classes, methods and variable declarations. You can also make class-related definitions.

• Definitions
  • Class definition
  • Inline function definition
  • Macro definition
• Declarations
  • extern variable declaration
  • typedef declaration
  • Global function declaration

The declared functions in the header file are defined in the source file. Be aware of items not to be included in the header file. For example, the next chapter explains why variables are only declarated and are’nt defined in the header file.

Items that should not be written in the header file

Reference : What to put in a header file from Microsoft C++ language reference

Items such as duplicate definitions, that may cause confusion.

• Definition of built-in types (int, float, string, etc.)
• Anonymous namespace
  • That is definition a class in a namespace that is not named.
• Using directive
  • If “using” is used, the namespace is also applied indirectly to the source file that read the header file, and it is easy to get confused.

Items that allocate memory area. (When multiple source files are read, each item reserves another area with the same name, which results in duplicate definition.)

• Non-inline function definition
  • Inline function will be expanded to the call destination, so they have no problem.
• Defining non-const variables (variables that are not constants)
  • Even if it is static, the variables are not shared and the entity is created for each source file.
  • Use const variables to define constants.

How to define a function?

Write only the declaration in the header file myClass.h.

//myClass.h
void myFunction();

Define the function in one of the .cpp files that are compiled together (either the main source file or myClass.cpp created for each header file).

//myClass.cpp
#include "myClass.h"
void myFunction(){
    printf("This is my function!");
}

In other .cpp files, you can use and use myFuction() defined in myClass.cpp by including only myClass.h.

How to define a variable?

Only “extern” is used in the header file myClass.h.

//myClass.h
extern int myVar;

Define the global variables in one of the .cpp files that are compiled together (either the main source file or myClass.cpp created for each header file).

//myClass.cpp
#include "myClass.h"
int myVar;

In other .cpp files, you can use variables by including only myClass.h. However, it is not recommended because it rarely uses variables in header file and it’s good to use class variables in C++.

Use include guards

Reference: Include guards from Microsoft C++ language reference

Include guard is a mechanism to prevent duplicate inclusion of header files. Duplicate inclusion can occur, for example, if you include one header file A in another header file B, then include A and B from a C++ source file.

//myClass.h
#ifndef MYCLASS_H
#define MYCLASS_H
/*myClass.h本体*/
#endif

Installing g++ in Windows 10

GOAL

To install g++ in Windows10 and compile a c++ file. g++ is included in

What is g++?

g++ is the command to compile c++ and the name of C++ compiler included in GCC, the GNU Compiler Collection. You can use g++ on GNU such as Cygwin and MinGW.

Method

I’ll use MinGW

Get mingw-get-setup.exe

Access the MinGW web site and go to the Downloads page. Click and downloads mingw-get-setup.exe.

Run the downloaded file.

Click “Install” an “Continue” with your preference.

Install Basic MinGW and GCC(g++).

Mark “mingw32-base-bin” and “mingw32-gcc-g++-bin” on MinGW Installation Manager.

Click Installtion > Apply Changes

Export path

Add C:\MinGW\bin into environment variables Path.

Operation check

Open Command Prompt and input the command “g++ -v”.

Command to compile C++ file

g++ helloWorld.cpp

You can see command line options at the die.net g++(1) – Linux man page.

Appendix

Additionally install msys if you’d like to use shell with MinGW.