在比较中使用用户定义的转换和隐式转换 [英] using user-defined conversions with implicit conversions in comparisons
问题描述
我很难理解为什么下面的代码不允许隐式转换发生。
#include< string> ;
using namespace std;
struct HasConversionToString {
HasConversionToString(const string& s_):s {s_} {}
string s;
operator const string&()const {return s; }
};
int main(){
string s {a};
HasConversionToString obj {b};
return s< obj;
}
clang和gcc都找不到一个有效的方法来比较两个对象错误符合以下行:
clang ++ -std = c ++ 14-Wall -Wextra-pedantic conversion.cpp -o test
conversion.cpp:13:12:error:二进制表达式的操作数无效('string'(aka'basic_string< char>')和'HasConversionToString')
return s< obj;
〜^ ~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.0/../../../../include/c++ /5.3.0/bits/stl_pair.h:220:5:注意:候选模板被忽略:无法匹配
'pair'对'basic_string'
运算符<(const pair< _T1,_T2& ; __x,const pair< _T1,_T2>& __y)
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.0/../ .. /../../include/c++/5.3.0/bits/stl_iterator.h:298:5:注意:候选模板忽略:无法匹配
'reverse_iterator'对'basic_string'
运算符;(const reverse_iterator< _Iterator>& __x,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.0/../../ .. /../include/c++/5.3.0/bits/stl_iterator.h:348:5:note:candidate template ignored:could not match
'reverse_iterator'against'basic_string'
operator<(const reverse_iterator< _IteratorL>& __x,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.0/../../../ .. /include/c++/5.3.0/bits/stl_iterator.h:849:5:note:candidate template ignored:could not match
'__normal_iterator'against'basic_string'
operator<(const __normal_iterator< _IteratorL ,_Container>& __lhs,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.0/../../../../include/c++/5.3 .0 / bits / stl_iterator.h:856:5:注意:候选模板忽略:无法匹配
'__normal_iterator'对'basic_string'
运算符<(const __normal_iterator< _Iterator,_Container>& __lhs ,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.0/../../../../include/c++/5.3。 0 / bits / stl_iterator.h:1089:5:注意:候选模板忽略:无法匹配
'move_iterator'对'basic_string'
运算符<(const move_iterator< _IteratorL>& __x,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.0/../../../../include/c++/5.3.0/bits /stl_iterator.h:1095:5:注意:候选模板忽略:无法匹配
'move_iterator'对'basic_string'
运算符<(const move_iterator< _Iterator>& __x,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.0/../../../../include/c++/5.3.0/bits/basic_string。 h:4989:5:注意:候选模板已忽略:无法匹配
'basic_string< type-parameter-0-0,type-parameter-0-1,type-parameter-0-2>'against'HasConversionToString '
operator<(const basic_string< _CharT,_Traits,_Alloc>& __lhs,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.0/../../../../include/c++/5.3 .0 / bits / basic_string.h:5001:5:注意:候选模板忽略:无法匹配
'const _CharT *'对'HasConversionToString'
运算符<(const basic_string< _CharT,_Traits,_Alloc> ;& __lhs,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.0/../../../../include/ c ++ / 5.3.0 / bits / basic_string.h:5013:5:注意:候选模板忽略:无法匹配
'const _CharT *'对'string'(aka'basic_string< char& b运算符<(const _CharT * __lhs,
^
1错误生成。
#include< string>
using namespace std;
struct HasConversionToString {
HasConversionToString(const string& s_):s {s_} {}
string s;
operator const string& ()const {return s; }
};
int main(){
string s {a};
hasConversionToString obj {b};
return s< static_cast< string>(obj);
}
。在cppreference 隐式转换,我看到没有理由这不应该工作。我假设clang和gcc都没有搞错同样的事情,所以我想我有一个概念上的误解。
您要调用的是一个函数模板:
template< class charT,Class Traits,class Alloc>
bool operator<(std :: basic_string< charT,Traits,Alloc> const& lhs,
std :: basic_string< charT,Traits,Alloc> const& rhs);
第二个参数的扣除失败,因为 HasConversionToString
不是 std :: basic_string
- 模板参数扣除不通过隐式转换。因此,该函数模板已从重载解析中移除。
std :: experimental :: basic_string_view
类似的问题,这是由一个足够的额外的重载规则解决(库必须添加足够的重载,使得 basic_string_view
和可转换为一个工作之间的比较) p>
basic_string
虽然 - <
可能默默地导致访问堆是不是一个好主意。
I am struggling to understand why the following code does not allow an implicit conversion to occur.
#include <string>
using namespace std;
struct HasConversionToString {
HasConversionToString(const string& s_) : s{s_} {}
string s;
operator const string&() const { return s; }
};
int main() {
string s{"a"};
HasConversionToString obj{"b"};
return s < obj;
}
Both clang and gcc fail to find a valid way to compare the two objects with errors along the lines of:
clang++ -std=c++14 -Wall -Wextra -pedantic conversion.cpp -o test
conversion.cpp:13:12: error: invalid operands to binary expression ('string' (aka 'basic_string<char>') and 'HasConversionToString')
return s < obj;
~ ^ ~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.0/../../../../include/c++/5.3.0/bits/stl_pair.h:220:5: note: candidate template ignored: could not match
'pair' against 'basic_string'
operator<(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.0/../../../../include/c++/5.3.0/bits/stl_iterator.h:298:5: note: candidate template ignored: could not match
'reverse_iterator' against 'basic_string'
operator<(const reverse_iterator<_Iterator>& __x,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.0/../../../../include/c++/5.3.0/bits/stl_iterator.h:348:5: note: candidate template ignored: could not match
'reverse_iterator' against 'basic_string'
operator<(const reverse_iterator<_IteratorL>& __x,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.0/../../../../include/c++/5.3.0/bits/stl_iterator.h:849:5: note: candidate template ignored: could not match
'__normal_iterator' against 'basic_string'
operator<(const __normal_iterator<_IteratorL, _Container>& __lhs,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.0/../../../../include/c++/5.3.0/bits/stl_iterator.h:856:5: note: candidate template ignored: could not match
'__normal_iterator' against 'basic_string'
operator<(const __normal_iterator<_Iterator, _Container>& __lhs,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.0/../../../../include/c++/5.3.0/bits/stl_iterator.h:1089:5: note: candidate template ignored: could not match
'move_iterator' against 'basic_string'
operator<(const move_iterator<_IteratorL>& __x,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.0/../../../../include/c++/5.3.0/bits/stl_iterator.h:1095:5: note: candidate template ignored: could not match
'move_iterator' against 'basic_string'
operator<(const move_iterator<_Iterator>& __x,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.0/../../../../include/c++/5.3.0/bits/basic_string.h:4989:5: note: candidate template ignored: could not match
'basic_string<type-parameter-0-0, type-parameter-0-1, type-parameter-0-2>' against 'HasConversionToString'
operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.0/../../../../include/c++/5.3.0/bits/basic_string.h:5001:5: note: candidate template ignored: could not match
'const _CharT *' against 'HasConversionToString'
operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.0/../../../../include/c++/5.3.0/bits/basic_string.h:5013:5: note: candidate template ignored: could not match
'const _CharT *' against 'string' (aka 'basic_string<char>')
operator<(const _CharT* __lhs,
^
1 error generated.
whereas the following code works fine, when I explicitly cast the object to a string.
#include <string>
using namespace std;
struct HasConversionToString {
HasConversionToString(const string& s_) : s{s_} {}
string s;
operator const string&() const { return s; }
};
int main() {
string s{"a"};
HasConversionToString obj{"b"};
return s < static_cast<string>(obj);
}
based on the rules and examples listed on cppreference for implicit casts, I see no reason this shouldn't work. I assume that both clang and gcc didn't screw up the same thing, so I imagine that I've got a conceptual misunderstanding.
The one you want to call is a function template:
template<class charT, class Traits, class Alloc>
bool operator<(std::basic_string<charT, Traits, Alloc> const& lhs,
std::basic_string<charT, Traits, Alloc> const& rhs);
Deduction fails for the second argument because a HasConversionToString
is not a std::basic_string
- template argument deduction doesn't look through implicit conversions. As a result, that function template is removed from overload resolution.
std::experimental::basic_string_view
has a similar problem, which was solved by a "sufficient additional overloads" rule (the library must add enough overloads so that comparison between a basic_string_view
and something convertible to one works).
You don't really want such a thing for basic_string
, though - having <
possibly silently causing a trip to the heap is not really a good idea.
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