为什么 rand() 编译时不包含 cstdlib 或使用命名空间 std?

2021-12-22 00:00:00 visual-studio visual-c++ c++

根据我正在阅读的书，rand() 在 C++ 中需要 #include
但是，我能够编译以下使用 rand() 的代码，而无需 #include <cstdlib> 或 using namespace std; 在 Visual 中工作室 2015.
为什么这两个不需要编译?我应该包括 cstdlib 吗?

According to the book I'm reading, rand() requires #include <cstdlib> in C++
However, I am able to compile the following code that uses rand() without #include <cstdlib> nor using namespace std; in Visual Studio 2015.
Why are these two not needed to compile? Should I include cstdlib?

C++ 代码:

#include <iostream>

int main()
{
    std::cout << rand() << std::endl;
}

推荐答案

有两个问题在起作用:

  1. 标准库头文件可能包含其他标准库头文件.所以 iostream 可以直接或间接包含 cstdlib.
  2. 具有 C 标准库等效项(例如 cstdlib)的头文件可以将 C 标准库名称带入全局命名空间，即在 std 命名空间之外(例如 rand.)这是从 C++11 开始正式允许的，并且在之前被广泛容忍.
  1. Standard library header files may include other standard library header files. So iostream may include cstdlib directly or indirectly.
  2. Header files with C-standard library equivalents (e.g. cstdlib) are allowed to bring C standard library names into the global namespace, that is, outside of the std namespace (e.g. rand.) This is formally allowed since C++11, and was largely tolerated before.

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