Namespaces provide a method for preventing name conflicts in large
projects.

Symbols declared inside a namespace block are placed in a named scope
that prevents them from being mistaken for identically-named symbols
in other scopes.

Multiple namespace blocks with the same name are allowed. All
declarations within those blocks are declared in the named scope.

命名空间可以避免名称冲突,例如标准库定义了sort(),但这对于排序函数来说是一个非常好的名称,这要归功于您可以定义自己的sort()的名称空间,因为它不在同一名称空间中作为标准的.

using指令告诉编译器在当前作用域中使用该命名空间,这样就可以了

int f(){
    std::cout << "out!" << std::endl;
}@H_301_3@ 
 
要么：
 
  
 
int f(){
    using namespace std;
    cout << "out!" << endl;
}@H_301_3@ 
 
当你从另一个命名空间中使用很多东西时,它很方便.
 
 
来源：Namespaces – cppreference.com