返回值和局部变量之间的区别
假设我有
#include <string>
class A
{
public:
template<class T>
operator T();
A child();
};
void f()
{
A a;
std::string s1 = a; // ok
std::string s2 = a.child(); // error (line 34)
s1 = a; // error (line 36)
s2 = a.child(); // error (line 37)
}
std :: string构造函数可以接受char *或std :: string引用，这就是赋值不明确的原因。 但是为什么我的编译器（VC ++ 10）会抱怨第二个任务，但不是第一个？
我正在寻找一种方法来优先模板转换运算符而不是重载的构造函数。
我收到以下错误：
1>------ Build started: Project: Plasma4Test, Configuration: Debug Win32 ------
1> Plasma4Test.cpp
1>d:bitbucketvxprojectsplasma4testplasma4test.cpp(34): error C2440: 'initializing' : cannot convert from 'A' to 'std::basic_string<_Elem,_Traits,_Ax>'
1> with
1> [
1> _Elem=char,
1> _Traits=std::char_traits<char>,
1> _Ax=std::allocator<char>
1> ]
1> No constructor could take the source type, or constructor overload resolution was ambiguous
1>d:bitbucketvxprojectsplasma4testplasma4test.cpp(36): error C2593: 'operator =' is ambiguous
1> c:program filesmicrosoft visual studio 10.0vcincludexstring(772): could be 'std::basic_string<_Elem,_Traits,_Ax> &std::basic_string<_Elem,_Traits,_Ax>::operator =(_Elem)'
1> with
1> [
1> _Elem=char,
1> _Traits=std::char_traits<char>,
1> _Ax=std::allocator<char>
1> ]
1> c:program filesmicrosoft visual studio 10.0vcincludexstring(767): or 'std::basic_string<_Elem,_Traits,_Ax> &std::basic_string<_Elem,_Traits,_Ax>::operator =(const _Elem *)'
1> with
1> [
1> _Elem=char,
1> _Traits=std::char_traits<char>,
1> _Ax=std::allocator<char>
1> ]
1> c:program filesmicrosoft visual studio 10.0vcincludexstring(762): or 'std::basic_string<_Elem,_Traits,_Ax> &std::basic_string<_Elem,_Traits,_Ax>::operator =(const std::basic_string<_Elem,_Traits,_Ax> &)'
1> with
1> [
1> _Elem=char,
1> _Traits=std::char_traits<char>,
1> _Ax=std::allocator<char>
1> ]
1> while trying to match the argument list '(std::string, A)'
1>d:bitbucketvxprojectsplasma4testplasma4test.cpp(37): error C2593: 'operator =' is ambiguous
1> c:program filesmicrosoft visual studio 10.0vcincludexstring(772): could be 'std::basic_string<_Elem,_Traits,_Ax> &std::basic_string<_Elem,_Traits,_Ax>::operator =(_Elem)'
1> with
1> [
1> _Elem=char,
1> _Traits=std::char_traits<char>,
1> _Ax=std::allocator<char>
1> ]
1> c:program filesmicrosoft visual studio 10.0vcincludexstring(767): or 'std::basic_string<_Elem,_Traits,_Ax> &std::basic_string<_Elem,_Traits,_Ax>::operator =(const _Elem *)'
1> with
1> [
1> _Elem=char,
1> _Traits=std::char_traits<char>,
1> _Ax=std::allocator<char>
1> ]
1> c:program filesmicrosoft visual studio 10.0vcincludexstring(762): or 'std::basic_string<_Elem,_Traits,_Ax> &std::basic_string<_Elem,_Traits,_Ax>::operator =(const std::basic_string<_Elem,_Traits,_Ax> &)'
1> with
1> [
1> _Elem=char,
1> _Traits=std::char_traits<char>,
1> _Ax=std::allocator<char>
1> ]
1> c:program filesmicrosoft visual studio 10.0vcincludexstring(707): or 'std::basic_string<_Elem,_Traits,_Ax> &std::basic_string<_Elem,_Traits,_Ax>::operator =(std::basic_string<_Elem,_Traits,_Ax> &&)'
1> with
1> [
1> _Elem=char,
1> _Traits=std::char_traits<char>,
1> _Ax=std::allocator<char>
1> ]
1> while trying to match the argument list '(std::string, A)'
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========
这对我来说似乎是一个VC10的bug，它与 std::string 没有关系 。
错误隔离：
我把它归结为以下例子：
#include <string>
class B
{
public:
B(char const*) { }
B(B&&) { }
};
class A
{
public:
operator char* const () { return 0; }
operator B () { return B(0); }
};
int main()
{
A a;
B b1 = a; // fine
B b2 = A(); // error C2440: 'initializing' : cannot convert from 'A' to 'B'
// No constructor could take the source type, or constructor
// overload resolution was ambiguous.
}
B类有一个移动构造函数和一个构造函数，它们需要一个const char* 。 当尝试从右rvalue初始化b2 ，VC10似乎无法将转换运算符选择为B
Clang 3.2和GCC 4.7.2都将转换运算符选择为B
C ++标准规则：
C ++标准的第8.5 / 16段要求：
[对于这种复制初始化]，“可以从源类型转换为目标类型或者（当使用转换函数时）转换为其派生类的用户定义的转换序列按照13.3.1.4中所述枚举，最好的是通过重载决议（13.3）选择“
如果我们考虑从源类型（ A ）到目标类型（ B ）的所有可用转换序列，那么涉及到A的用户定义转换函数到char const*转换序列需要进一步转换（通过B的构造函数接受一个char const* ）以达到目标类型 B 因此，它比使用A的用户定义的转换函数B （按照13.3.3.2）更长一步，这使得后者更可取。
这似乎证实这是一个VC10的错误。
链接地址: http://www.djcxy.com/p/11731.html