在 C++ 中防止名称空间中毒的优雅方法

我们假设，Bob已将他的库包装到名称空间中"bob", and Alice将使整个命名空间在她自己的函数中可见“使用命名空间鲍勃”，代替“使用鲍勃::XYZ”对于每一个项目：

// This file is written by Alice:
#include <iostream>

// She uses Bobs library:
#include "bob.hpp"

int main(void) {
    // Import Bobs library and use it:
    using namespace bob;

    unsigned short value = 50000;
    bob::dump_as_signed(value);

    // Should not be possible without std:: prefix:
    cout << "foobar" << endl;
}

另一方面，鲍勃试图通过以下方式防止这种情况发生：将实现包装在虚拟命名空间内，并制作只有这些符号可用，供其他用户使用：

// This file is written by Bob
#include <iostream>
#include <type_traits>

// Namespace for public use:
namespace bob {

    // Implementation details:
    namespace impl_ {

        // Visible ONLY within sub-namespace:
        using std::cout;
        using std::endl;

        using std::is_integral;
        using std::make_signed;

        // No repeated std:: prefixes at all:
        template <typename T,
            typename S = typename make_signed<T>::type>
        void dump_as_signed(const T i) {
            static_assert(is_integral<T>::value, "no integer");

            // Do something very very useful:
            cout << "signed:" << static_cast<S>(i) << endl;
        }
    }

    // Make available without poisoning with std::*:
    using impl_::dump_as_signed;

}

由于所有 using 指令都被包装到 Bob 主命名空间内的虚拟“impl_”命名空间中，因此 Alice 不存在以下风险：意外地从 std:: 命名空间导入符号， too.

所以，我的问题是：

1. 我不喜欢有一个用于实现细节的虚拟命名空间，“理论上”每个人都可以看到它。能够使用 for e 中的许多符号的正确方法是什么？ G。 std:: 不会泄漏这些并且不会在每个符号显式添加 std::? 前缀（我也在考虑生成的 API 文档，它显示“bob::impl_::XYZ”而不是“bob::XYZ”。）
2. 我认为，重复 std:: a 并不是很 DRY。 s。 o。到处一次又一次。我也明白，在更大的范围（例如类）内相对全局的“使用命名空间 std”并不是那么漂亮，但在我看来，数百个 std:: 前缀要丑陋得多。这里哪种方法更好？

我们假设Alice正在使用两个库，由Bob and Charlie.

// This file is written by Alice:
#include <bob.hpp>
#include <charlie.hpp>

int main(void) {
  using namespace bob;
  using namespace charlie;

  // Do a bunch of stuff
}

现在，查理发明了一项新功能，称为foobar他将其添加到他的图书馆中。foobar很棒，他的用户喜欢它。爱丽丝也开始使用它。

然后鲍勃说：“我喜欢foobar我也想拥有自己的foobar我可以在我的图书馆使用。但我不想依赖查理。”所以他创建了自己的版本。

呃哦，现在 Alice 的代码无法编译！每次使用foobarAlice 的代码中的内容不明确，她必须重写整个项目。

然后，下个月也会发生同样的事情。在那之后的下个月。

现在，Alice 的所有客户都非常不高兴，因为他们正在构建大型技术并试图保持其依赖项的最新版本，但每次他们尝试升级任何内容时，Alice 的代码都会崩溃。他们在她的错误跟踪器上做了很多错误报告。

爱丽丝向鲍勃和查理发送了一封电子邮件并说

伙计们，你们必须停止用相同的名字制作课程，否则我会失去我所有的生意！

鲍勃和查理给爱丽丝发了一封电子邮件：

不，爱丽丝，你需要停止投入using namespace bob; and using namespace charlie;在你的代码中。鲍勃或查理不支持这一点。

现在，让我们再讲一遍同样的故事，只不过没有查理。只是爱丽丝在她的项目中创建了自己的类，与鲍勃添加的新名称发生了冲突。

简而言之，一个using namespace指令从来都不是一个好主意（在我看来）。尤其当命名空间是外部库时。你不really知道该名称空间将来会如何变化，如果它的变化对您来说完全不利，那么您的手上就会突然陷入一团糟。

Using namespace = to shorten命名空间通常是一个非常好的主意。我喜欢做以下事情：

namespace my_lib {

namespace qi = boost::spirit::qi;

// Do stuff with qi
// ...

} // end namespace my_lib

这样我就可以使用简称qi in my_lib，但我不会向我的用户强加任何东西。 （我期望谁会not正在做using namespace my_lib;!)

如果您是用户，您可以执行类似的操作

namespace cha = charlie::name::space::is_way_too_long;

但是，您应该非常乐意键入短名称空间，例如bob:: or std::，无论您是用户还是库实现者，这是否意味着您的代码在库升级时不会中断。

这与 DRY 无关。推杆some名称上的某种限定符可以让您更轻松地阅读代码并理解其含义。

例如，SDL，一个流行的 C 库。据我所知，everySDL 中的宏开始SDL_ and every函数开始sdl_。这是否违反了“DRY”？不。这里没有重复的实现细节——公共前缀是为了避免名称冲突。此外，它还使代码更具可读性和可维护性——每当我看到一个正在谈论 SDL 实体的符号时，我立即就知道了。它对人类和计算机都非常有帮助。

Putting using namespace std; or using namespace my_lib;就像把 C++ 最好的功能之一扔进垃圾箱一样。权衡是，节省自己输入 5 个字符，但代价是对可读性和可维护性造成极大损害。

临别思考：如何using namespace影响您收到的错误消息的质量。

这是一个无法编译的简单程序：

#include <iostream>

struct foo {};

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

当编译器看到这段代码时，它必须尝试它知道的每个流运算符重载并检查是否foo可以转换为任何这些东西。因为std::cout是参数之一，ADL 意味着我们必须搜索整个std命名空间。事实证明，惊喜惊喜，foo不可转换为任何这些东西。在gcc 5.3我收到以下（200 行）错误消息。

main.cpp: In function ‘int main()’:
main.cpp:6:13: error: no match for ‘operator<<’ (operand types are ‘std::ostream {aka std::basic_ostream<char>}’ and ‘foo’)
   std::cout << foo{} << std::endl;
             ^
In file included from /usr/include/c++/5/iostream:39:0,
                 from main.cpp:1:
/usr/include/c++/5/ostream:628:5: note: candidate: std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&&, const _Tp&) [with _CharT = char; _Traits = std::char_traits<char>; _Tp = foo] <near match>
     operator<<(basic_ostream<_CharT, _Traits>&& __os, const _Tp& __x)
     ^
/usr/include/c++/5/ostream:628:5: note:   conversion of argument 1 would be ill-formed:
main.cpp:6:20: error: cannot bind ‘std::ostream {aka std::basic_ostream<char>}’ lvalue to ‘std::basic_ostream<char>&&’
   std::cout << foo{} << std::endl;
                    ^
In file included from /usr/include/c++/5/iostream:39:0,
                 from main.cpp:1:
/usr/include/c++/5/ostream:108:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__ostream_type& (*)(std::basic_ostream<_CharT, _Traits>::__ostream_type&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
       operator<<(__ostream_type& (*__pf)(__ostream_type&))
       ^
/usr/include/c++/5/ostream:108:7: note:   no known conversion for argument 1 from ‘foo’ to ‘std::basic_ostream<char>::__ostream_type& (*)(std::basic_ostream<char>::__ostream_type&) {aka std::basic_ostream<char>& (*)(std::basic_ostream<char>&)}’
/usr/include/c++/5/ostream:117:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__ios_type& (*)(std::basic_ostream<_CharT, _Traits>::__ios_type&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>; std::basic_ostream<_CharT, _Traits>::__ios_type = std::basic_ios<char>]
       operator<<(__ios_type& (*__pf)(__ios_type&))
       ^
/usr/include/c++/5/ostream:117:7: note:   no known conversion for argument 1 from ‘foo’ to ‘std::basic_ostream<char>::__ios_type& (*)(std::basic_ostream<char>::__ios_type&) {aka std::basic_ios<char>& (*)(std::basic_ios<char>&)}’
/usr/include/c++/5/ostream:127:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::ios_base& (*)(std::ios_base&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
       operator<<(ios_base& (*__pf) (ios_base&))
       ^
/usr/include/c++/5/ostream:127:7: note:   no known conversion for argument 1 from ‘foo’ to ‘std::ios_base& (*)(std::ios_base&)’
/usr/include/c++/5/ostream:166:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
       operator<<(long __n)
       ^
/usr/include/c++/5/ostream:166:7: note:   no known conversion for argument 1 from ‘foo’ to ‘long int’
/usr/include/c++/5/ostream:170:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
       operator<<(unsigned long __n)
       ^
/usr/include/c++/5/ostream:170:7: note:   no known conversion for argument 1 from ‘foo’ to ‘long unsigned int’
/usr/include/c++/5/ostream:174:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(bool) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
       operator<<(bool __n)
       ^
/usr/include/c++/5/ostream:174:7: note:   no known conversion for argument 1 from ‘foo’ to ‘bool’
In file included from /usr/include/c++/5/ostream:638:0,
                 from /usr/include/c++/5/iostream:39,
                 from main.cpp:1:
/usr/include/c++/5/bits/ostream.tcc:91:5: note: candidate: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(short int) [with _CharT = char; _Traits = std::char_traits<char>]
     basic_ostream<_CharT, _Traits>::
     ^
/usr/include/c++/5/bits/ostream.tcc:91:5: note:   no known conversion for argument 1 from ‘foo’ to ‘short int’
In file included from /usr/include/c++/5/iostream:39:0,
                 from main.cpp:1:
/usr/include/c++/5/ostream:181:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(short unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
       operator<<(unsigned short __n)
       ^
/usr/include/c++/5/ostream:181:7: note:   no known conversion for argument 1 from ‘foo’ to ‘short unsigned int’
In file included from /usr/include/c++/5/ostream:638:0,
                 from /usr/include/c++/5/iostream:39,
                 from main.cpp:1:
/usr/include/c++/5/bits/ostream.tcc:105:5: note: candidate: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(int) [with _CharT = char; _Traits = std::char_traits<char>]
     basic_ostream<_CharT, _Traits>::
     ^
/usr/include/c++/5/bits/ostream.tcc:105:5: note:   no known conversion for argument 1 from ‘foo’ to ‘int’
In file included from /usr/include/c++/5/iostream:39:0,
                 from main.cpp:1:
/usr/include/c++/5/ostream:192:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
       operator<<(unsigned int __n)
       ^
/usr/include/c++/5/ostream:192:7: note:   no known conversion for argument 1 from ‘foo’ to ‘unsigned int’
/usr/include/c++/5/ostream:201:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long long int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
       operator<<(long long __n)
       ^
/usr/include/c++/5/ostream:201:7: note:   no known conversion for argument 1 from ‘foo’ to ‘long long int’
/usr/include/c++/5/ostream:205:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long long unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
       operator<<(unsigned long long __n)
       ^
/usr/include/c++/5/ostream:205:7: note:   no known conversion for argument 1 from ‘foo’ to ‘long long unsigned int’
/usr/include/c++/5/ostream:220:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(double) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
       operator<<(double __f)
       ^
/usr/include/c++/5/ostream:220:7: note:   no known conversion for argument 1 from ‘foo’ to ‘double’
/usr/include/c++/5/ostream:224:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(float) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
       operator<<(float __f)
       ^
/usr/include/c++/5/ostream:224:7: note:   no known conversion for argument 1 from ‘foo’ to ‘float’
/usr/include/c++/5/ostream:232:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long double) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
       operator<<(long double __f)
       ^
/usr/include/c++/5/ostream:232:7: note:   no known conversion for argument 1 from ‘foo’ to ‘long double’
/usr/include/c++/5/ostream:245:7: note: candidate: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(const void*) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
       operator<<(const void* __p)
       ^
/usr/include/c++/5/ostream:245:7: note:   no known conversion for argument 1 from ‘foo’ to ‘const void*’
In file included from /usr/include/c++/5/ostream:638:0,
                 from /usr/include/c++/5/iostream:39,
                 from main.cpp:1:
/usr/include/c++/5/bits/ostream.tcc:119:5: note: candidate: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__streambuf_type*) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__streambuf_type = std::basic_streambuf<char>]
     basic_ostream<_CharT, _Traits>::
     ^
/usr/include/c++/5/bits/ostream.tcc:119:5: note:   no known conversion for argument 1 from ‘foo’ to ‘std::basic_ostream<char>::__streambuf_type* {aka std::basic_streambuf<char>*}’
In file included from /usr/include/c++/5/iostream:39:0,
                 from main.cpp:1:
/usr/include/c++/5/ostream:574:5: note: candidate: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const unsigned char*)
     operator<<(basic_ostream<char, _Traits>& __out, const unsigned char* __s)
     ^
/usr/include/c++/5/ostream:574:5: note:   template argument deduction/substitution failed:
main.cpp:6:20: note:   cannot convert ‘foo{}’ (type ‘foo’) to type ‘const unsigned char*’
   std::cout << foo{} << std::endl;
                    ^
In file included from /usr/include/c++/5/iostream:39:0,
                 from main.cpp:1:
/usr/include/c++/5/ostream:569:5: note: candidate: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const signed char*)
     operator<<(basic_ostream<char, _Traits>& __out, const signed char* __s)
     ^
/usr/include/c++/5/ostream:569:5: note:   template argument deduction/substitution failed:
main.cpp:6:20: note:   cannot convert ‘foo{}’ (type ‘foo’) to type ‘const signed char*’
   std::cout << foo{} << std::endl;
                    ^
In file included from /usr/include/c++/5/iostream:39:0,
                 from main.cpp:1:
/usr/include/c++/5/ostream:556:5: note: candidate: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const char*)
     operator<<(basic_ostream<char, _Traits>& __out, const char* __s)
     ^
/usr/include/c++/5/ostream:556:5: note:   template argument deduction/substitution failed:
main.cpp:6:20: note:   cannot convert ‘foo{}’ (type ‘foo’) to type ‘const char*’
   std::cout << foo{} << std::endl;
                    ^
In file included from /usr/include/c++/5/ostream:638:0,
                 from /usr/include/c++/5/iostream:39,
                 from main.cpp:1:
/usr/include/c++/5/bits/ostream.tcc:321:5: note: candidate: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const char*)
     operator<<(basic_ostream<_CharT, _Traits>& __out, const char* __s)
     ^
/usr/include/c++/5/bits/ostream.tcc:321:5: note:   template argument deduction/substitution failed:
main.cpp:6:20: note:   cannot convert ‘foo{}’ (type ‘foo’) to type ‘const char*’
   std::cout << foo{} << std::endl;
                    ^
In file included from /usr/include/c++/5/iostream:39:0,
                 from main.cpp:1:
/usr/include/c++/5/ostream:539:5: note: candidate: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const _CharT*)
     operator<<(basic_ostream<_CharT, _Traits>& __out, const _CharT* __s)
     ^
/usr/include/c++/5/ostream:539:5: note:   template argument deduction/substitution failed:
main.cpp:6:20: note:   mismatched types ‘const _CharT*’ and ‘foo’
   std::cout << foo{} << std::endl;
                    ^
In file included from /usr/include/c++/5/iostream:39:0,
                 from main.cpp:1:
/usr/include/c++/5/ostream:519:5: note: candidate: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, unsigned char)
     operator<<(basic_ostream<char, _Traits>& __out, unsigned char __c)
     ^
/usr/include/c++/5/ostream:519:5: note:   template argument deduction/substitution failed:
main.cpp:6:20: note:   cannot convert ‘foo{}’ (type ‘foo’) to type ‘unsigned char’
   std::cout << foo{} << std::endl;
                    ^
In file included from /usr/include/c++/5/iostream:39:0,
                 from main.cpp:1:
/usr/include/c++/5/ostream:514:5: note: candidate: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, signed char)
     operator<<(basic_ostream<char, _Traits>& __out, signed char __c)
     ^
/usr/include/c++/5/ostream:514:5: note:   template argument deduction/substitution failed:
main.cpp:6:20: note:   cannot convert ‘foo{}’ (type ‘foo’) to type ‘signed char’
   std::cout << foo{} << std::endl;
                    ^
In file included from /usr/include/c++/5/iostream:39:0,
                 from main.cpp:1:
/usr/include/c++/5/ostream:508:5: note: candidate: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, char)
     operator<<(basic_ostream<char, _Traits>& __out, char __c)
     ^
/usr/include/c++/5/ostream:508:5: note:   template argument deduction/substitution failed:
main.cpp:6:20: note:   cannot convert ‘foo{}’ (type ‘foo’) to type ‘char’
   std::cout << foo{} << std::endl;
                    ^
In file included from /usr/include/c++/5/iostream:39:0,
                 from main.cpp:1:
/usr/include/c++/5/ostream:502:5: note: candidate: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, char)
     operator<<(basic_ostream<_CharT, _Traits>& __out, char __c)
     ^
/usr/include/c++/5/ostream:502:5: note:   template argument deduction/substitution failed:
main.cpp:6:20: note:   cannot convert ‘foo{}’ (type ‘foo’) to type ‘char’
   std::cout << foo{} << std::endl;
                    ^
In file included from /usr/include/c++/5/iostream:39:0,
                 from main.cpp:1:
/usr/include/c++/5/ostream:497:5: note: candidate: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, _CharT)
     operator<<(basic_ostream<_CharT, _Traits>& __out, _CharT __c)
     ^
/usr/include/c++/5/ostream:497:5: note:   template argument deduction/substitution failed:
main.cpp:6:20: note:   deduced conflicting types for parameter ‘_CharT’ (‘char’ and ‘foo’)
   std::cout << foo{} << std::endl;
                    ^
In file included from /usr/include/c++/5/bits/ios_base.h:46:0,
                 from /usr/include/c++/5/ios:42,
                 from /usr/include/c++/5/ostream:38,
                 from /usr/include/c++/5/iostream:39,
                 from main.cpp:1:
/usr/include/c++/5/system_error:209:5: note: candidate: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::error_code&)
     operator<<(basic_ostream<_CharT, _Traits>& __os, const error_code& __e)
     ^
/usr/include/c++/5/system_error:209:5: note:   template argument deduction/substitution failed:
main.cpp:6:20: note:   cannot convert ‘foo{}’ (type ‘foo’) to type ‘const std::error_code&’
   std::cout << foo{} << std::endl;
                    ^
In file included from /usr/include/c++/5/string:52:0,
                 from /usr/include/c++/5/bits/locale_classes.h:40,
                 from /usr/include/c++/5/bits/ios_base.h:41,
                 from /usr/include/c++/5/ios:42,
                 from /usr/include/c++/5/ostream:38,
                 from /usr/include/c++/5/iostream:39,
                 from main.cpp:1:
/usr/include/c++/5/bits/basic_string.h:5172:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)
     operator<<(basic_ostream<_CharT, _Traits>& __os,
     ^
/usr/include/c++/5/bits/basic_string.h:5172:5: note:   template argument deduction/substitution failed:
main.cpp:6:20: note:   ‘foo’ is not derived from ‘const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>’
   std::cout << foo{} << std::endl;
                    ^

重点是：如果你这样做using namespace bob;，那么每一个bob可流式传输的类型也将出现在该列表中！如果你这样做using namespace charlie;那么他所有的类型也会在那里！

不仅错误消息会更糟糕，而且您更有可能得到一些您没有预料到的非常奇怪的交互。如果鲍勃的类型偶尔会流式传输到查理的类型之一怎么办？ Charlie 的类型有时可以隐式转换为某种可流式传输的标准类型？

当然，这一切不仅适用于任何运算符重载，还适用于任何模板或函数调用。

因此，最重要的是，如果您避免在一个名称空间中混合大量垃圾，C++ 会更容易推理并且工作得更好。