1、 Header file, output format,::, namespace

1、C++ The header file

C++ Header files do not use extensions , Use the library name directly . In order to distinguish between C Language and C++ Different language libraries , The library name will be prefixed with letters c To distinguish between .
for example ：

• C Language ：math.h
• C++：cmath

2、 Standard output format

cout<<" Output content "<<endl;
This is a C++ Standard output format ,\n And endl The same effect , But it is recommended endl.

3、:: scope resolution

#include<iostream>
using namespace std;

int a = 10;
int main(){
    
    int a = 20;
    // When local variables and global variables conflict , Local variables take precedence over global variables , Global variables are masked 
    cout << " local variable a=" << a << endl;

    //:: Represents the use of global variables , Open access to global variables 
    cout << " Global variables a=" << ::a << endl;
    return 0;
}

 local variable a=20
 Global variables a=10

4、 Namespace

1、 establish

#include<iostream>
using namespace std;
// Create a namespace 
namespace A
{
    
    int a = 10;
} // namespace A

namespace B
{
    
    int a =20;
} // namespace B


int main(){
    
    cout << "namespace A:a=" << A::a << endl;
    cout << "namespace B:a=" << B::a << endl;
}

Be careful , Namespaces can only be defined globally , The following code example is wrong ：

#include<iostream>
using namespace std;

int main(){
    
    namespace A
    {
    
        int a = 10;
    } // namespace A
    
    cout << "namespace A:a=" << A::a << endl;
}

Error message ：

hello.cpp: In function ‘int main()’:
hello.cpp:5:5: error: ‘namespace’ definition is not allowed here
    5 |     namespace A
      |     ^~~~~~~~~
hello.cpp:10:33: error: ‘A’ has not been declared
   10 |     cout << "namespace A:a=" << A::a << endl;
      |                                 ^

2、 Nesting of namespace

#include<iostream>
using namespace std;

namespace A
{
    
    int a = 10;
    namespace B
    {
    
        int a = 20;
    } // namespace B
    
} // namespace A

int main(){
     
    cout << "namespace A:a=" << A::a << endl;
    cout << "namespace A::B::a=" << A::B::a << endl;
}

3、 Namespaces are open , You can join new members at any time , And will retain the original members

#include<iostream>
using namespace std;

namespace A
{
    
    int a = 10;
    namespace B
    {
    
        int a = 20;
    } // namespace B
    
} // namespace A

namespace A
{
    
    int b = 200;
} // namespace A


int main(){
     
    cout << "namespace A:b =" << A::b << endl;
    cout << "namespace A::B::a=" << A::B::a << endl;
}

4、 Declaration and implementation are separate , Declare in namespace , Implement elsewhere

#include<iostream>
using namespace std;

namespace func_space
{
    
    void func();
} // namespace func_space
void func_space::func(){
    
    cout <<"declaration and function are sparate!" << endl;
}

int main(){
    
    func_space::func();
}

5、 Nameless namespace , Namespace alias

#include<iostream>
using namespace std;

namespace func_space
{
    
    void func();
} // namespace func_space

/*  Nameless namespace , It is equivalent to adding static,  Can only be accessed within this file */
namespace 
{
    
    int c = 1000;
} // namespace 

// Alias namespace , It is equivalent to that the same space has different names 
namespace func_space_copy = func_space;
void func_space_copy::func(){
    
    cout <<"declaration and function are sparate!" << endl;
    cout << "c=" << c << endl;
}

int main(){
    
    func_space::func();
}

6、using Declaration and compilation

compile ：using namespace A;
Statement ：using A::a;

#include<iostream>
using namespace std;

namespace A
{
    
    int a = 10;
    int b = 20;
} // namespace A

void test_01(){
    
    // Include relational output directly through namespaces 
    cout << A::a << endl;
    
    // Output through namespace declaration 
    using namespace A;
    //could visit b
    cout << b << endl;
}

void test_02(){
    
    using A::a;
    // You can visit a
    cout << a << endl;

    // No access b, Application error ：error: ‘b’ was not declared in this scope; did you mean ‘A::b’?
    //cout << b << endl;
}

void test_03(){
    
    // Only the namespace is declared here A, Didn't say use a, So the output is 20
    using namespace A;//using Compile instructions , bring A All identifiers in are available 
    cout << a << endl;// out a = 10
    int a = 20;// There is no conflict 
    cout << a << endl;// out a = 20
}

void test_04(){
    
    // Conflict of the same name , Will report a mistake 
    //error: ‘int a’ conflicts with a previous declaration
    using A::a;
    //int a = 20;
}
int main(){
    
    test_03();
}