为什么运算符<<不适用于操作员返回的内容-？

Question

这是我编写的一个小测试程序：

#include <iostream>
using namespace std;

class A {

    public:
    int val;

    A(int _val=0):val(_val) { }

    A operator+(A &a) { return A(val + a.val); }
    A operator-(A &a) { return A(val - a.val); }

    friend ostream& operator<<(ostream &, A &);

};

ostream& operator<<(ostream &out, A &a) {
    out<<a.val;
    return out;
}

int main() {
    A a(3), b(4), c = b - a;
    cout<<c<<endl; // this works
    cout<<(b-a)<<endl; // this doesn't
    return 0;
}

我似乎不明白为什么标记为“这有效”的行有效，而标记为“这无效”的行则无效。当我尝试使用 cout<<(ba); 行编译程序时，我得到的结果如下：

[felix@the-machine C]$ g++ test.cpp 

test.cpp: In function ‘int main()’:
test.cpp:26:13: error: no match for ‘operator<<’ in ‘std::cout << b.A::operator-(((A&)(& a)))’
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:108:7: note: candidates are: 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>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:117:7: note:                 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>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:127:7: note:                 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>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:165:7: note:                 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>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:169:7: note:                 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>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:173:7: note:                 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>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/bits/ostream.tcc:91:5: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(short int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:180:7: note:                 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>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/bits/ostream.tcc:105:5: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:191:7: note:                 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>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:200:7: note:                 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>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:204:7: note:                 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>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:209:7: note:                 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>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:213:7: note:                 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>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:221:7: note:                 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>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/ostream:225:7: note:                 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>]
/usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/bits/ostream.tcc:119:5: note:                 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>]
test.cpp:18:11: note:                 std::ostream& operator<<(std::ostream&, A&)
[felix@the-machine C]$

相当令人讨厌。

Answer 1

你的operator+返回一个临时对象——在C++中你不能将临时对象绑定到非常量引用。您想要：

ostream& operator<<(ostream &out, const A &a) {

这是编写 opertaor<< 的规范方式对于流式传输 - 输出的内容应始终作为 const 引用传递。

Answer 2

因为您不允许将临时变量传递给插入运算符。将其更改为：

friend ostream& operator<<(ostream &, const A &);
...
ostream& operator<<(ostream &out, const A &a) {
    out<<a.val;
    return out;
}

Answer 3

在第二行中，您要求编译器从 A 转换为 A&，但 A 是临时的，这是非标准行为。在第一行中， c 不是临时的，因此它可以工作。
将运算符更改为

ostream& operator<<(ostream &out, const A &a)

不仅会更正确，而且还应该被编译器接受。

Answer 4

临时对象基本上被视为 const，因此对于绑定到它的引用，它必须是对 const 的引用：

ostream& operator<<(ostream &out, A const &a) // ...

Answer 5

您将一个临时对象传递给 operator<<，因此它需要采用 const A&。

std::ostream& operator<<(std::ostream&, const A&)

其他一些注意事项：

始终使用“const T&”传递未修改的非原始参数时。这允许您传递临时变量，并指示您不更改参数。

最好将单参数构造函数标记为显式，这样可以防止您到处创建意外的临时对象，从而降低性能。

explicit A(int _val=0):val(_val) { }

Answer 6

std::basic_ostream::operator<< 在您的用例中需要 const 限定的参数。因此，您需要按如下方式更正操作员的签名：

ostream& operator<<(ostream &out, A const& a)

顺便说一句，友谊是多余的，因为 A::val 是公共的，因此无论如何都可以访问。