std::vector :无法将“std::ostream {aka std::basic_ostream<char>}"左值绑定到“std::basic_ostream<char>&amp;&" [英] std::vector : cannot bind &#39;std::ostream {aka std::basic_ostream&lt;char&gt;}&#39; lvalue to &#39;std::basic_ostream&lt;char&gt;&amp;&amp;&#39;

问题描述

我在尝试做一些像

std::cout << std::vector<int>{1,2,3};

说的是

 cannot bind 'std::ostream {aka std::basic_ostream<char>}' lvalue to 'std::basic_ostream<char>&&'
 int main() {  std::cout << std::vector<int>{1,2,3}; }

(使用 gcc-4.8.1 和 -std=c++11 测试)

(tested using gcc-4.8.1 with -std=c++11)

SO 有类似的问题，例如 重载运算符<<:无法绑定左值到'std::basic_ostream<char>&&'，它是关于一些用户定义的带有嵌套类的类.还有一个解决该问题的公认答案的工作.

SO has similar questions like Overloading operator<<: cannot bind lvalue to ‘std::basic_ostream<char>&&’, which is about some user defined class with nested classes. There is also a work around the accepted answer to that question.

但我不知道这是否适用于 std::vector.有人可以解释为什么 std::vector 会发生这个错误，以及如何解释它?

But I don't know whether this applies to std::vector. Can someone explain why this error happens to std::vector, and how to interpret it?

谢谢

推荐答案

与模板相关的错误消息有时会令人困惑.问题是标准库没有定义 operator << 的重载以将 std::vector(或任何其他容器，就此而言)插入std::ostream.因此，编译器无法为 operator << 找到合适的重载，并尽其所能报告此失败(不幸的是，这在您的情况下不太好/不太可读).

Template-related error messages can be confusing at times. The problem is that the standard library does not define an overload of operator << for inserting std::vector (or any other container, for that matter) into a std::ostream. So the compiler fails to find a suitable overload for operator <<, and reports this failure as best as it's able (which is unfortunately not too good/readable in your case).

如果要流式传输整个容器，可以使用 std::ostream_iterator 为此:

If you want to stream an entire container, you can use std::ostream_iterator for that:

auto v = std::vector<int>{1, 2, 3};
std::copy(begin(v), end(v), std::ostream_iterator<int>(std::cout, " "));

<小时>

至于为什么你会得到这个神秘的错误，它有助于分析完整的错误消息:

---

As for why you're getting precisely this cryptic error, it helps to analyse the full error message:

prog.cpp: In function ‘int main()’:
prog.cpp:13:37: error: cannot bind ‘std::ostream {aka std::basic_ostream<char>}’ lvalue to ‘std::basic_ostream<char>&&’
  std::cout << std::vector<int>{1,2,3};
                                     ^
In file included from /usr/include/c++/4.8/iostream:39:0,
                 from prog.cpp:3:
/usr/include/c++/4.8/ostream:602:5: error:   initializing argument 1 of ‘std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&&, const _Tp&) [with _CharT = char; _Traits = std::char_traits<char>; _Tp = std::vector<int>]’
     operator<<(basic_ostream<_CharT, _Traits>&& __os, const _Tp& __x)
     ^

显然有一个 operator<< 的模板重载，它采用 std::ostream&& 类型的 lhs 参数和模板化类型的 rhs 参数;它的存在是为了允许插入临时流.由于它是一个模板，它成为您代码中表达式的最佳匹配.然而，std::cout 是一个左值，所以它不能绑定到 std::ostream&&.因此出现错误.

There is apparently a template overload of operator<< which takes a lhs argument of type std::ostream&& and a rhs argument of the templated type; it exists to allow insertion into temporary streams. Since it's a template, it becomes the best match for the expression in your code. However, std::cout is an lvalue, so it cannot bind to std::ostream&&. Hence the error.

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