我需要扩展std::basic_string
来处理路径字符串和其他operator+
:
#include <string>
template <class t_elem, class t_traits, class t_alloc>
class path_basic_string : public std::basic_string<t_elem, t_traits, t_alloc>
{
public:
using base_type = std::basic_string<t_elem, t_traits, t_alloc>;
path_basic_string() = default;
path_basic_string(const path_basic_string & ) = default;
path_basic_string & operator =(const path_basic_string &) = default;
path_basic_string(const base_type & r) :
base_type(r)
{
}
path_basic_string(base_type && r) :
base_type(std::move(r))
{
}
};
using path_string = path_basic_string<char, std::char_traits<char>, std::allocator<char> >;
template <class t_elem, class t_traits, class t_alloc>
inline path_basic_string<t_elem, t_traits, t_alloc> &&
operator +(
path_basic_string<t_elem, t_traits, t_alloc> && l,
std::basic_string<t_elem, t_traits, t_alloc> && r)
{
std::basic_string<t_elem, t_traits, t_alloc> && l_str = std::move(l);
std::basic_string<t_elem, t_traits, t_alloc> && r_str = std::move(r);
const bool has_right = !r_str.empty();
return std::move(
path_basic_string<t_elem, t_traits, t_alloc>{
std::move(std::move(l_str) + (has_right ? "/" : "") + (has_right ? std::move(r_str) : std::move(std::basic_string<t_elem, t_traits, t_alloc>{})))
});
}
template <class t_elem, class t_traits, class t_alloc>
inline path_basic_string<t_elem, t_traits, t_alloc>
operator +(
const path_basic_string<t_elem, t_traits, t_alloc> & l,
const std::basic_string<t_elem, t_traits, t_alloc> & r)
{
const std::basic_string<t_elem, t_traits, t_alloc> & l_str = l;
const bool has_right = !r.empty();
return path_basic_string<t_elem, t_traits, t_alloc>{
l_str + (has_right ? "/" : "") + (has_right ? r : std::basic_string<t_elem, t_traits, t_alloc>{})
};
}
int main()
{
path_string a;
std::string b;
std::string c;
const path_string test = a + (b + c);
return 0;
}
在https://godbolt.org/z/jhcWoh,我遇到了以下错误:
x86 MSVC 19 2015 U3 :
/opt/compiler-explorer/windows/19.00.24210/include/xlocale(341): warning C4530: C++ exception handler used, but unwind semantics are not enabled. Specify /EHsc <source>(61): error C2666: 'operator +': 3 overloads have similar conversions <source>(44): note: could be 'path_basic_string<char,std::char_traits<char>,std::allocator<char>> operator +<char,std::char_traits<char>,std::allocator<char>>(const path_basic_string<char,std::char_traits<char>,std::allocator<char>> &,const std::basic_string<char,std::char_traits<char>,std::allocator<char>> &)' <source>(28): note: or 'path_basic_string<char,std::char_traits<char>,std::allocator<char>> &&operator +<char,std::char_traits<char>,std::allocator<char>>(path_basic_string<char,std::char_traits<char>,std::allocator<char>> &&,std::basic_string<char,std::char_traits<char>,std::allocator<char>> &&)' /opt/compiler-explorer/windows/19.00.24210/include/xstring(2310): note: or 'std::basic_string<char,std::char_traits<char>,std::allocator<char>> std::operator +<char,std::char_traits<char>,std::allocator<char>>(const std::basic_string<char,std::char_traits<char>,std::allocator<char>> &,const std::basic_string<char,std::char_traits<char>,std::allocator<char>> &)' /opt/compiler-explorer/windows/19.00.24210/include/xstring(2380): note: or 'std::basic_string<char,std::char_traits<char>,std::allocator<char>> std::operator +<char,std::char_traits<char>,std::allocator<char>>(const std::basic_string<char,std::char_traits<char>,std::allocator<char>> &,std::basic_string<char,std::char_traits<char>,std::allocator<char>> &&)' /opt/compiler-explorer/windows/19.00.24210/include/xstring(2390): note: or 'std::basic_string<char,std::char_traits<char>,std::allocator<char>> std::operator +<char,std::char_traits<char>,std::allocator<char>>(std::basic_string<char,std::char_traits<char>,std::allocator<char>> &&,const std::basic_string<char,std::char_traits<char>,std::allocator<char>> &)' /opt/compiler-explorer/windows/19.00.24210/include/xstring(2400): note: or 'std::basic_string<char,std::char_traits<char>,std::allocator<char>> std::operator +<char,std::char_traits<char>,std::allocator<char>>(std::basic_string<char,std::char_traits<char>,std::allocator<char>> &&,std::basic_string<char,std::char_traits<char>,std::allocator<char>> &&)' <source>(61): note: while trying to match the argument list '(path_string, std::basic_string<char,std::char_traits<char>,std::allocator<char>>)' <source>(61): note: note: qualification adjustment (const/volatile) may be causing the ambiguity Compiler returned: 2
x86-64 gcc 5.4 (带有--std=c++11
):
source>: In function 'int main()': <source>:61:40: warning: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second: const path_string test = a + (b + c); ^ <source>:44:5: note: candidate 1: path_basic_string<t_elem, t_traits, t_alloc> operator+(const path_basic_string<t_elem, t_traits, t_alloc>&, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&) [with t_elem = char; t_traits = std::char_traits<char>; t_alloc = std::allocator<char>] operator +( ^ In file included from /opt/compiler-explorer/gcc-5.4.0/include/c++/5.4.0/string:52:0, from <source>:1: /opt/compiler-explorer/gcc-5.4.0/include/c++/5.4.0/bits/basic_string.h:4854:5: note: candidate 2: std::__cxx11::basic_string<_CharT, _Traits, _Alloc> std::operator+(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>] operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs, ^ Compiler returned: 0
我知道至少有一种解决方法。
但是到底发生了什么? 为了避免这种过载冲突的混乱,我又必须再次超载吗?
更新:
通过从所有const
的所有single reference
类参数中删除basic_string
和operator+
来解决。似乎可以解决问题。
答案 0 :(得分:2)
首先,请使用从值移动而不是const&
和&&
重载。
path_basic_string(base_type r) :
base_type(std::move(r))
{
}
并摆脱base_type const&
ctor。
第二,使该ctor明确:
explicit path_basic_string(base_type r) :
base_type(std::move(r))
{
}
因为路径与字符串是不同的东西。
第三,清理template operator+
并将其设置为ADL“ Koenig”运算符,该运算符将其值作为左手。哦,不要通过右值引用返回任何有毒的东西。
friend path_basic_string
operator +(
path_basic_string l,
base_type const& r)
{
base_type& l_str = l;
if (!r.empty())
l = path_basic_string( std::move(l_str) + "/" + r );
return l;
}
并消除所有杂音。
接下来,从base_type
继承ctor。
最后,使用+=
实现附加,并使操作对称:
template <class t_elem, class t_traits, class t_alloc>
class path_basic_string : public std::basic_string<t_elem, t_traits, t_alloc>
{
public:
using base_type = std::basic_string<t_elem, t_traits, t_alloc>;
path_basic_string() = default;
path_basic_string(const path_basic_string & ) = default;
path_basic_string & operator =(const path_basic_string &) = default;
using base_type::base_type;
explicit path_basic_string(base_type r) :
base_type(std::move(r))
{
}
path_basic_string& operator+= ( base_type const& rhs ) & {
if (!rhs.empty())
{
base_type& self = *this;
self += '/';
self += rhs;
}
return *this;
}
friend path_basic_string operator+(
base_type l,
base_type const& r
)
{
path_basic_string l_path(std::move(l));
l+=r;
return l;
}
};
operator+
之所以如此,是因为它只能通过ADL找到,但实际上它是基于该类的 base 类型进行操作。
这意味着至少有一个参数必须是此类型的实例(或具有此类型的实例作为模板参数)才能找到它。
然后根据需要进行基础转换。
我将LHS视为一个值,因为移动字符串很便宜,而且我们需要一个字符串才能输出。通过将LHS按值取值并使用其缓冲区(将其移动后)作为返回值,我们可以获得有效的链式加法:
a+b+c+d+e
成为
(a+b)+c+d+e
现在将a+b
(一个prvalue)的返回值用作(a+b)+c
的lhs参数。
此缓冲区的回收继续进行; (从第一个+
开始,仅创建一个缓冲区,然后将其移动,调整大小(希望有效)并在表达式的其余部分重新使用。