Namespaces and libraries

Why propose namespaces

stay C/C++ in , Variable 、 There are lots of functions and classes to learn later , These variables 、 The names of functions and classes will exist in the global scope , It could lead to a lot of conflict . The purpose of using a namespace is to localize the name of the identifier , To avoid naming conflicts or names Pollution ,namespace The emergence of keywords is aimed at this problem .

We hope to solve the problem of naming conflict and pollution through namespaces

Before the namespace is proposed , These names will all exist in the global scope .

The problem also arises here

Through the division of naming scope , Make the names originally named in the global scope , Separated by namespaces

All names have their own namespace

Definition of namespace

Define the namespace , Need to be used namespace keyword , Followed by the name of the namespace , And then a couple {} that will do ,{} It is named in Members of the space .

namespace name {
	 Statement and definition 
}

Three characteristics of namespaces

1. Variables can be defined in the namespace / function / type

2. Namespaces can be nested

3. Multiple namespace with the same name are allowed in the same project , The compiler will eventually synthesize in the same namespace .

Use of namespaces

Add a namespace name and scope qualifier

Through the scope qualifier , Tell the compiler which scope the name is in

int main()
{
 printf("%d\n", N::a);
 return 0; 
}

Use using Bring a member from a namespace into

using N::b;
int main()
{
 printf("%d\n", N::a);
 printf("%d\n", b);
 return 0; 
}

  Use using namespace Namespace name introduce

using namespce N;
int main()
{
 printf("%d\n", N::a);
 printf("%d\n", b);
 Add(10, 20);
 return 0; 
}

Namespace vs std（ Standard namespace ）

c++98 when , Namespace is introduced to solve the problem of naming conflict and pollution .

But in c++98 Before , already

There are some library functions , for example ：<iostream.h>、<fstream.h> etc.

C++ The first version of the standard , Most compilers support , International Organization for Standardization (ISO) And American standardization The association recognizes , Rewrite... As a template C++ Standard library , Introduced STL( Standard template library )

Talk about all the Libraries （ Include c The library of languages ） Has been rewritten , All libraries with standard namespaces , It's not with .h Is the suffix of the file

In the rewritten Library , All names are named after std In this namespace , When calling the name in the standard library , Indicate the namespace where the name is located