Getting started with C++

1. Namespace
Two, input & output
Three, the default parameters
Four, function overloading

We know that the hello world file suffix of C language is'.c', then the file suffix of C++ is'.cpp'.
First write the first C++ program Hello World to understand the language of C++.

#include <iostream>
using namespace std;	//1.命名空间

int main()
{
    
    
  cout<<"Hello World!"<<endl;	//2.输出
  return 0;
}

From the hello world of C++, you can see the intuitive difference with the C language :

The header file in C language ends with'.h', but C++ writes the name of the header file directly
There is one more statement in C++ using namespace std
The output statement in C++ is cout, and the newline statement is endl

Then I will start describing the basic knowledge of C++ from the following aspects:

1. Namespace

In C/C++, there are a large number of variables, functions and classes. The names of these variables, functions and classes will all exist in the global scope, which may cause a lot of conflicts. The purpose of using namespaces is to localize the names of identifiers to avoid naming conflicts or name pollution . The emergence of the namespace keyword addresses this problem.

For example, explain the naming conflict:

Suppose you and your colleague are responsible for different modules of a project. If you define a variable a and your colleague also defines a variable a , who defines the variable when the program uses the variable a ?Use namespace namespace in C++ to solve this problem。

1.1 Definition of Namespace

Define the namespace, you need to usenamespaceKeyword, followed by the name of the namespace, and then a pair of {}, {} is the member of the namespace. There are three ways to define a namespace:

1.1.1 Ordinary namespace

The content in the namespace can be either variable definitions or function definitions .

//1. 普通的命名空间
namespace N1  // N1为命名空间的名称
{
    
    
   // 命名空间中的内容，既可以定义变量，也可以定义函数
   int a;
   int Add(int left, int right)
   {
    
    
       return left + right;
   }
}

1.2.2 Nesting of Namespaces

N3 can be nested in N2

//2. 命名空间可以嵌套
namespace N2
{
    
    
   int a;
   int b;
   int Add(int left, int right)
   {
    
    
       return left + right;
   }
    
   namespace N3	//N3嵌套在N2中
   {
    
    
       int c;
       int d;
       int Sub(int left, int right)
       {
    
    
           return left - right;
       }
   }
}

1.3.1 Multiple namespaces with the same name are allowed in the same project, and the compiler will finally synthesize into the same namespace.

The content of N1 below will be merged with the content of N1 in the previous 1 into one space

namespace N1
{
    
    
   int Mul(int left, int right)
   {
    
    
       return left * right;
   }
}

Note:
A namespace defines a new scope , and all content in the namespace is limited to that namespace, that is, it can only be valid in its own namespace.

1.2 Use of command space

#include<stdio.h>

namespace N2
{
    
    
  int a = 10;
  int b = 20;
  int Add(int x, int y)
  {
    
    
    return x + y;
  }
  namespace N3
  {
    
    
    int c = 30;
    int d = 40;
    int Sub(int x ,int y)
    {
    
    
      return x - y;
    }
  }
}

int main()
{
    
    
  printf("%d\n", a);  // 该语句编译出错，无法识别a
  printf("%d\n", c);  // 该语句编译出错，无法识别c

  return 0;
}

If the above program directly uses a and b variables, an error will be reported. Let's take the second method as an example to explain the three ways of using namespaces:

1.2.1 Add namespace name and scope qualifier

Add the namespace name N1 and the scope qualifier:: before the variable name to access the variables a and c

int main()
{
    
    
  printf("%d\n", N2::a);
  printf("%d\n", N2::N3::c);

  return 0;
}

result:
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