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 构造函数可以采用 char* 或 std::string 引用,这就是赋值不明确的原因。但为什么我的编译器 (VC++10) 提示第二个赋值而不是第一个?
我正在寻找一种方法让模板转换运算符优先于重载的构造函数。
我收到以下错误:
1>------ Build started: Project: Plasma4Test, Configuration: Debug Win32 ------
1> Plasma4Test.cpp
1>d:\bitbucket\vx\projects\plasma4test\plasma4test.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:\bitbucket\vx\projects\plasma4test\plasma4test.cpp(36): error C2593: 'operator =' is ambiguous
1> c:\program files\microsoft visual studio 10.0\vc\include\xstring(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 files\microsoft visual studio 10.0\vc\include\xstring(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 files\microsoft visual studio 10.0\vc\include\xstring(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:\bitbucket\vx\projects\plasma4test\plasma4test.cpp(37): error C2593: 'operator =' is ambiguous
1> c:\program files\microsoft visual studio 10.0\vc\include\xstring(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 files\microsoft visual studio 10.0\vc\include\xstring(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 files\microsoft visual studio 10.0\vc\include\xstring(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 files\microsoft visual studio 10.0\vc\include\xstring(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 ==========
最佳答案
对我来说这似乎是一个 VC10 错误,并且它与 std::string
不相关。
错误隔离:
我把它归结为以下示例:
#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采用const char*
的构造函数。当尝试从 rvalue
初始化 b2
时,VC10 似乎无法选择到 B
的转换运算符。
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++ 标准第 8.5/16 段要求:
[对于这种复制初始化的情况,] “用户定义的转换序列,可以从源类型转换为目标类型或(当使用转换函数时)到其派生类如 13.3.1.4 中所述进行枚举,并通过重载决议(13.3)选择最佳的“
如果我们考虑示例中所有可用的转换序列,从源类型 (A
) 到目标类型 (B
),那么涉及 A
到 char const*
的用户定义转换函数需要进一步转换(通过 B
的构造函数完成接受 char const*
) 以到达目标类型 B
。因此,它比使用A
的用户自定义转换函数到B
(根据13.3.3.2)多了一步,这使得后者更可取。
这似乎证实了这是一个 VC10 错误。
关于c++ - 返回值和局部变量的区别,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/14379295/