c++ - using namespace std 有什么用？什么时候用什么时候可以不用？

Question

在 C++ 中 using namespace std; 有什么用，什么时候用什么时候可以不用，

#include<iostream.h>
using namespace std;
class point
{
public:
    int x;
    int y;
     void print()
    {
        cout<<"x="<<x<<endl;
        cout<<"y="<<y<<endl;
    }
};
void main()
{
    point pt;
    pt.x=0;
    pt.y=1;
    pt.print();
}

这段代码如果不注释掉 using namespace std; 运行就会提示出错，为什么？

Answer 1

What’s included in “There’s………………” inside, the rest of it won’t be displayed on my top

Answer 2

Theoretically, it can be unnecessary. But the premise is that you have to write the full name, such as std::cout.

Answer 3

namespaceTo put it simply, it is like this, (take a made-up example)
Once upon a time, someone wrote a file operation module, which contained a open(char*) to open an ordinary file;
later He also wrote a database operation module, which also had a open(char*) used to open a database file;
one day he found that one of his programs used these two modules at the same time. As a result, the compilation failed because the functions had the same name~
The solution is very simple, just change the name~
But it is too troublesome to change so many open() in the two modules to file_open and database_open respectively, so the role of namespace It shows, he just needs to simply modify the two modules to namespace file{文件操作模块的原始内容}; and namespace database{数据库操作模块的原始内容};~ (just add a few lines of code)
so that when he needs to open the file, he writes file::open, and when he needs to open it, If you write database::open in the database, there will be no duplicate name problem
. But in this case, there will be no open function. All functions involving file operations must be prefixed with file::, and database operations require database::Prefix, he suddenly found that the code he wrote before could not be compiled~
Does the previous code need to be significantly changed? No, taking file operations as an example, as long as you add a sentence using namespace file;, the file namespace will be applied globally, and the compiler will try to find the file::open function, so the previous code can be compiled by just adding one more sentence~

Back to the point, because C++ libraries use common function names such as sort and swap, which can easily have the same name as user-defined functions. In order to avoid name conflicts, all library functions are placed by default. In the std namespace, if we also write a function called sort, there will be no naming conflict, because the full name of the thing in the library is std::sort

But we are usually very cautious when writing programs, and the naming is standardized enough. Duplication of names rarely occurs. On the contrary, the prefix of std:: is too redundant, and we have to use std::cout even to write output, so for the sake of convenience, here is One sentence using namespace std; to omit all prefixes. But this is equivalent to blocking the functions provided by namespace. At this time, if you write a swap function and the result is a naming conflict, don’t blame C++ for not providing a solution;

Of course, the previous description is inappropriate. Not only functions can be placed in namespace, variables, classes, structures, etc. are all allowed. cout is an example

Answer 4

means to use the standard namespace. If not, you need to use std::cout to specify the namespace

Answer 5

First of all, we need to understand the role of namespace; namespace is used to avoid naming conflicts;
For example, I defined a cout class myself for output;

//#mycout.h
class cout {
...
};

There is such a section in the program;

#include "mycout.h"
cout << "hello ";

There is no problem with compilation. The compiler finds my cout class and uses it for linking;

If I add the following statement, the system’s cout class is included and visible in the global scope:

#include <iostream>
using namespace std;

At this time, the compiler is confused. Now there are two cout classes. It is unclear which one to use, so it can only report an error;
In order to avoid system errors, we can decide which class to use; there are the following types Solution:

Remove using namespace std

This method will use my custom cout, and can also use other classes in the std space (you need to add the std:: prefix when using it, such as std::string)

#include "mycout.h"
#include <iostream>

cout << "hello";

Add using logo

If you want to use system cout, add std::

#include "mycout.h"
#include <iostream>

std::cout << "hello";

Add a namespace to my self-defined cout (recommended practice)

//#mycout.h
namespace CC{
    class cout {
    ...
    };
}

#include "mycout.h"
#include <iostream>
using namespace std;

cout << "hello"; //使用系统的cout
CC::cout << "colin"; //使用我自定义的cout类