这与性能完全无关.但是考虑一下:您正在使用两个名为 Foo 和 Bar 的库:

This is not related to performance at all. But consider this: you are using two libraries called Foo and Bar:

using namespace foo;
using namespace bar;

一切正常，您可以毫无问题地从 Foo 调用 Blah() 和从 Bar 调用 Quux().但是有一天你升级到 Foo 2.0 的新版本，它现在提供了一个名为 Quux() 的函数.现在你遇到了一个冲突:Foo 2.0 和 Bar 都将 Quux() 导入到你的全局命名空间中.这需要一些努力来解决，尤其是在函数参数恰好匹配的情况下.

Everything works fine, and you can call Blah() from Foo and Quux() from Bar without problems. But one day you upgrade to a new version of Foo 2.0, which now offers a function called Quux(). Now you've got a conflict: Both Foo 2.0 and Bar import Quux() into your global namespace. This is going to take some effort to fix, especially if the function parameters happen to match.

如果你曾经使用过foo::Blah()和bar::Quux()，那么引入foo::Quux() 将是一个非事件.

If you had used foo::Blah() and bar::Quux(), then the introduction of foo::Quux() would have been a non-event.