C++ “使用命名空间 std”有什么用？
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What is the use of "using namespace std"?
提问by Jarvis
What is the use of using namespace std?
有什么用using namespace std？
I'd like to see explanation in Layman terms.
我想看看外行人的解释。
回答by Daniel Daranas
- using: You are going to use it.
- namespace: To use what? A namespace.
- std: The
stdnamespace (where features of the C++ Standard Library, such asstringorvector, are declared).
- using：您将要使用它。
- 命名空间：使用什么？一个命名空间。
- std：
std命名空间（声明C++ 标准库的特性，例如stringorvector）。
After you write this instruction, if the compiler sees stringit will know that you may be referring to std::string, and if it sees vector, it will know that you may be referring to std::vector. (Provided that you have included in your compilation unit the header files where they are defined, of course.)
写完这条指令后，如果编译器看到string它就会知道你可能指的是std::string，如果它看到vector，它就会知道你可能指的是std::vector。（当然，前提是您在编译单元中包含了定义它们的头文件。）
If you don'twrite it, when the compiler sees stringor vectorit will not know what you are refering to. You will need to explicitly tell it std::stringor std::vector, and if you don't, you will get a compile error.
如果你不写它，当编译器看到string或者vector它不会知道你指的是什么。你需要明确地告诉它std::stringor std::vector，如果你不这样做，你会得到一个编译错误。
回答by Ivaylo Strandjev
When you make a call to using namespace <some_namespace>;all symbols in that namespace will become visible without adding the namespace prefix. A symbol may be for instance a function, class or a variable.
当您调用using namespace <some_namespace>;该名称空间中的所有符号时，无需添加名称空间前缀即可可见。例如，符号可以是函数、类或变量。
E.g. if you add using namespace std;you can write just coutinstead of std::coutwhen calling the operator coutdefined in the namespace std.
例如，如果您添加using namespace std;，则可以在调用命名空间中定义的运算符时编写cout而不是编写。std::coutcoutstd
This is somewhat dangerous because namespaces are meant to be used to avoid name collisions and by writing using namespaceyou spare some code, but loose this advantage. A better alternative is to use just specific symbols thus making them visible without the namespace prefix. Eg:
这有点危险，因为名称空间旨在用于避免名称冲突并通过编写using namespace您节省一些代码，但失去了这一优势。更好的选择是仅使用特定符号，从而使它们在没有名称空间前缀的情况下可见。例如：
#include <iostream>
using std::cout;
int main() {
cout << "Hello world!";
return 0;
}
