Compile CPP files using G++ in Ubuntu

When I used g++ to compile the cpp file for the first time, an undefined reference error was reported. All the functions in the custom class could not be found. After searching for information, I found that the called class needed to be linked. This article uses a small example to describe the compilation process. The class2 class calls the member function of the class1 class and depends on the class1 class, while the test file calls the member function of the class2 class and depends on the class1 class.

//class1.h
#ifndef _CLASS1_H
#define _CLASS1_H
class class1
{
public:
  int f(int i);
};
#endif

//class1.cpp
#include<iostream>
#include "class1.h"
using namespace std;
int class1::f(int i)
{
  if (i==0 || i==1)
   return 1;
  else
   return f(i-1)+f(i-1);
}

//class2.h
#ifndef _CLASS2_H
#define _CLASS2_H
#include "class1.h"
class class2
{
public:
  int double_f(int i);
};
#endif

//class2.cpp
#include<iostream>
#include "class2.h"
using namespace std;
int class2::double_f(int i)
{
  class1 c;
  return 2*cf(i);
}

//test.cpp
#include<iostream>
#include "class2.h"
using namespace std;
int main()
{
  class2 c2;
  cout << c2.double_f(4) <<endl;
  return 0;
}

First compile class1, class2 and test files into .o format files

g++ -c class1.cpp
g++ -c class2.cpp
g++ -c test.cpp

Since class1 and class2 are dependent, they need to be packaged into static library files (.a format) for linking

ar -rc class1.a class1.o
ar -rc class2.a class2.o

The last step is to link test.o into an executable file

g++ -o test test.o class2.a class1.a

After execution, the executable file test is obtained, which can be executed using the ./test command. Pay attention to the order of static link libraries when linking. It is necessary to ensure the order of the dependencies in front and the back. For example, here test depends on class2, and class2 depends on class1. Then when linking, test needs to be written before calss2, and class2 needs to be written before class1, otherwise there will be an undefined reference error.

How to compile multiple files at the same time with g++

For this example, the files are placed in the /CPPPFrtory directory. Use the CD command to change into the folder containing the source files.
To compile a program by listing the files on the command line, type:

Method 1: g++ Hello.cpp Welcome2.cpp -o Welcome2.out -ansi //Two cpp source files, only one .out file is generated after compilation Method 2: Use "wildcard" to compile the program, type: g++ *.cpp -o Welcome2.out -ansi //Use * to replace all the files to be compiled in the directory

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