C++模板调用成员函数如果匹配类型否则抛出异常？

Question

考虑以下类：

class Example {
    const void * pValue;
    (Example& (*SIndex(const void *,char *)));
    (Example& (*NIndex(const void *,unsigned long long *)));

    template<class T,typename I> static  Example & Index(const T* value,I index)
    { return Example(value->operator[](index)); }

public:
    template<class T> Example(const T& value) {
        pValue = &value;
        SIndex = (Example& (*(const void *,char *)))Index<T,char *>;
        NIndex = (Example& (*(const void *,unsigned long long)))Index<T,unsigned long long>;
    };
    Example& operator[](char * index) { return SIndex(pValue,index); };
    Example& operator[](unsigned long long) { return NIndex(pValue,index); };
}

我希望使用 SFINAE（替换失败不是错误）（参见 http://en.wikipedia.org/wiki/Substitution_failure_is_not_an_error）来更改 _Index，这样如果类没有匹配的运算符，它将抛出异常而不是返回新的 Example 对象。即_Index<T,char *> 将匹配operator[](char *) 或operator[](std::string&)，因为std::string 可以从char * 构造，而_Index<T,unsigned long long> 将匹配任何数字类型，因为unsigned long long 可以转换。

我只是在寻找这个问题的解决方案时遇到了 SFINAE，很明显解决方案就在这个问题上，但是我不确定如何实现它。

示例（根据要求）： 这将编译，当前输出是指针值没有内容，但这是可修复的，另外我禁用了字符串索引选项，因为这会导致编译器错误，而 SFINAE 无法正常工作。请注意，现实世界的应用程序 obj1 和 obj2 可能是不同的类型，即 然而，vector 和 map 将产生错误，直到将 SFINAE 添加到代码中以产生异常。此外，打开的“索引路径”将在运行时而不是编译时受到损害。 注意：我正在扩展此代码 https://github.com/vitaut/format 以允许 {0[4][first]} 或 {1[error]} 等类型的 id。由于并非所有对象都支持这些方法，因此不支持它的对象应该当格式字符串要求时产生运行时异常，也不得产生编译器错误，因为用作参数的任何类型都将传递给 Arg 类，例如，只显示了我需要的更复杂的 Arg 类的一部分修改以使其正常工作。

#include <iostream>
#include <string>
#include <vector>
#include <map>

using namespace std;

class Example {
    void * pValue;
    /*Example& (*SIndex)(void*,const char *);*/
    Example& (*NIndex)(void*,unsigned int);
    template<class T,typename I> static Example& Index(T* obj,I index) {
        return *(new Example((*obj)[index])); }
public:
    Example(char value) { pValue = &value; }
    template<class T> Example(T& value) {
        pValue = &value;
        /*SIndex = (Example& (*)(void*,const char *))Index<T,const char *>;*/
        NIndex = (Example& (*)(void*,unsigned int))Index<T,unsigned int>;
    }
    /*Example& operator[](const char * index) { return SIndex(pValue,index); }*/
    Example& operator[](unsigned int index) { return NIndex(pValue,index); }
    void * Get() { return pValue; }
};

int main()
{
    vector<Example> Objs;
    vector<string> obj1;
    vector<string> obj2;
    obj1.push_back("Hello ");
    obj1.push_back("World");
    obj2.push_back("Olleh ");
    obj2.push_back("Dlorw");
    Objs.push_back(Example(obj1));
    Objs.push_back(Example(obj2));
    cout << (string *)(Objs[0][0].Get()) << endl;
    cout << (string *)(Objs[1][1].Get()) << endl;
    cin.ignore();
    return 0;
}

示例 2（使用 Simple 的 SFINALE 代码）v2：

#include <iostream>
#include <string>
#include <vector>
#include <map>
#include <utility>
#include <type_traits>

using namespace std;

template<typename T, typename I>
decltype(void((*declval<T>())[declval<I>()]), true_type())
has_subscript_test(int);

template<typename, typename>
false_type
has_subscript_test(...);

template<typename T, typename I>
struct has_subscript : decltype(has_subscript_test<T, I>(0))
{
};

class Example {
    void * pValue;
    Example& (*SIndex)(void*,const char *);
    Example& (*NIndex)(void*,unsigned int);
    template<class T,typename I> static
    typename enable_if<has_subscript<T, I>::value, Example&>::type
    Index(T* obj,I index) { return *(new Example((*obj)[index])); }
    template<class T,typename I> static
    typename enable_if<!has_subscript<T, I>::value, Example&>::type
    Index(T* obj,I index) { throw "Invalid Index Type"; }
public:
    //Example(char value) { pValue = &value; }
    template<class T> Example(T& value) {
        pValue = &value;
        SIndex = (Example& (*)(void*,const char *))Index<T,const char *>;
        NIndex = (Example& (*)(void*,unsigned int))Index<T,unsigned int>;
    }
    Example& operator[](const char* index) { return SIndex(pVale,index); }
    Example& operator[](unsigned int index) { return NIndex(pValue,index); }
    void * Get() { return pValue; }
};

int main()
{
    vector<Example> Objs;
    vector<string> obj1;
    map<string,string> obj2;
    obj1.push_back("Hello ");
    obj1.push_back("World");
    obj2["A"] = "Olleh ";
    obj2["B"] = "Dlorw";
    Objs.push_back(Example(obj1));
    Objs.push_back(Example(obj2));
    cout << (string *)(Objs[0][(unsigned int)0].Get()) << endl;
    cout << (string *)(Objs[1][(const char *)"B"].Get()) << endl;
    cin.ignore();
    return 0;
}

编译器错误： 看起来只有前 2 个很重要，其余的都是这 2 个的副作用。

g++.exe -Wall -fexceptions  -std=c++11 -g     -c C:/SourceCode/Test/main.cpp -o ../obj/Debug/main.o
cygwin warning:
  MS-DOS style path detected: C:/SourceCode/Test/main.cpp
  Preferred POSIX equivalent is: /cygdrive/c/SourceCode/Test/main.cpp
  CYGWIN environment variable option "nodosfilewarning" turns off this warning.
  Consult the user's guide for more details about POSIX paths:
    http://cygwin.com/cygwin-ug-net/using.html#using-pathnames
C:/SourceCode/Test/main.cpp: In instantiation of ‘static typename std::enable_if<has_subscript<T, I>::value, Example&>::type Example::Index(T*, I) [with T = std::vector<std::basic_string<char> >; I = const char*; typename std::enable_if<has_subscript<T, I>::value, Example&>::type = Example&]’:
C:/SourceCode/Test/main.cpp:37:9:   required from ‘Example::Example(T&) [with T = std::vector<std::basic_string<char> >]’
C:/SourceCode/Test/main.cpp:54:32:   required from here
C:/SourceCode/Test/main.cpp:29:64: error: invalid conversion from ‘const char*’ to ‘std::vector<std::basic_string<char> >::size_type {aka unsigned int}’ [-fpermissive]
In file included from /usr/lib/gcc/i686-pc-cygwin/4.7.3/include/c++/vector:65:0,
                 from C:/SourceCode/Test/main.cpp:3:
/usr/lib/gcc/i686-pc-cygwin/4.7.3/include/c++/bits/stl_vector.h:750:7: error:   initializing argument 1 of ‘std::vector<_Tp, _Alloc>::reference std::vector<_Tp, _Alloc>::operator[](std::vector<_Tp, _Alloc>::size_type) [with _Tp = std::basic_string<char>; _Alloc = std::allocator<std::basic_string<char> >; std::vector<_Tp, _Alloc>::reference = std::basic_string<char>&; std::vector<_Tp, _Alloc>::size_type = unsigned int]’ [-fpermissive]
C:/SourceCode/Test/main.cpp: In instantiation of ‘static typename std::enable_if<has_subscript<T, I>::value, Example&>::type Example::Index(T*, I) [with T = std::map<std::basic_string<char>, std::basic_string<char> >; I = unsigned int; typename std::enable_if<has_subscript<T, I>::value, Example&>::type = Example&]’:
C:/SourceCode/Test/main.cpp:38:9:   required from ‘Example::Example(T&) [with T = std::map<std::basic_string<char>, std::basic_string<char> >]’
C:/SourceCode/Test/main.cpp:55:32:   required from here
C:/SourceCode/Test/main.cpp:29:64: error: invalid user-defined conversion from ‘unsigned int’ to ‘std::map<std::basic_string<char>, std::basic_string<char> >::key_type&& {aka std::basic_string<char>&&}’ [-fpermissive]
In file included from /usr/lib/gcc/i686-pc-cygwin/4.7.3/include/c++/string:54:0,
                 from /usr/lib/gcc/i686-pc-cygwin/4.7.3/include/c++/bits/locale_classes.h:42,
                 from /usr/lib/gcc/i686-pc-cygwin/4.7.3/include/c++/bits/ios_base.h:43,
                 from /usr/lib/gcc/i686-pc-cygwin/4.7.3/include/c++/ios:43,
                 from /usr/lib/gcc/i686-pc-cygwin/4.7.3/include/c++/ostream:40,
                 from /usr/lib/gcc/i686-pc-cygwin/4.7.3/include/c++/iostream:40,
                 from C:/SourceCode/Test/main.cpp:1:
/usr/lib/gcc/i686-pc-cygwin/4.7.3/include/c++/bits/basic_string.h:487:7: note: candidate is: std::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _CharT*, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>] <near match>
/usr/lib/gcc/i686-pc-cygwin/4.7.3/include/c++/bits/basic_string.h:487:7: note:   no known conversion for argument 1 from ‘unsigned int’ to ‘const char*’
C:/SourceCode/Test/main.cpp:29:64: error: invalid conversion from ‘unsigned int’ to ‘const char*’ [-fpermissive]
In file included from /usr/lib/gcc/i686-pc-cygwin/4.7.3/include/c++/string:54:0,
                 from /usr/lib/gcc/i686-pc-cygwin/4.7.3/include/c++/bits/locale_classes.h:42,
                 from /usr/lib/gcc/i686-pc-cygwin/4.7.3/include/c++/bits/ios_base.h:43,
                 from /usr/lib/gcc/i686-pc-cygwin/4.7.3/include/c++/ios:43,
                 from /usr/lib/gcc/i686-pc-cygwin/4.7.3/include/c++/ostream:40,
                 from /usr/lib/gcc/i686-pc-cygwin/4.7.3/include/c++/iostream:40,
                 from C:/SourceCode/Test/main.cpp:1:
/usr/lib/gcc/i686-pc-cygwin/4.7.3/include/c++/bits/basic_string.h:487:7: error:   initializing argument 1 of ‘std::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _CharT*, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’ [-fpermissive]
C:/SourceCode/Test/main.cpp:29:64: error: conversion to non-const reference type ‘std::map<std::basic_string<char>, std::basic_string<char> >::key_type&& {aka class std::basic_string<char>&&}’ from rvalue of type ‘std::basic_string<char>’ [-fpermissive]

Answer 1

这应该可行：

template<typename T, typename I>
decltype(void((*std::declval<T>())[std::declval<I>()]), std::true_type())
has_subscript_test(int);

template<typename, typename>
std::false_type
has_subscript_test(...);

template<typename T, typename I>
struct has_subscript : decltype(has_subscript_test<T, I>(0))
{
};

class Example {
    const void * pValue;
    (Example& (*SIndex(const void *,char *)));
    (Example& (*NIndex(const void *,unsigned long long *)));

    template<class T, typename I> static
    typename std::enable_if<has_subscript<T, I>::value, Example&>::type
    _Index(const T* value,I index) { return Example((*value)[index]); }

    template<class T, typename I> static
    typename std::enable_if<!has_subscript<T, I>::value, Example&>::type
    _Index(const T*,I) { throw "uh-oh!"; }
public:
    template<class T> Example(const T& value) {
        pValue = &value;
        SIndex = (Example& (*(const void *,char *)))_Index<T,char *>;
        NIndex = (Example& (*(const void *,unsigned long long)))_Index<T,unsigned long long>;
    };
    Example& operator[](char * index) { return SIndex(this,index); };
    Example& operator[](unsigned long long) { return NIndex(this,index); };
};

我已经完成了 SFINAE 位，但您的代码还有其他问题。 _Index 不允许作为名称，您正在通过引用返回一个临时值，并且您正在尝试将成员指针强制转换为函数指针（即使使用不同的函数类型！）