小编yo_*_*gdg的帖子

我正在为Win32的SQL ODBC API开发一个包装器,并且经常有几个函数像GetXXXTextA和GetXXXTextW.我打算选择合适的GetA或GetW取决于用户输入类型.我试过这个:

// test getterA
int _stdcall pruebaA (int, char*, const char*)
{ return 0; }
// test getterW
int _stdcall pruebaW(int, wchar_t*, const wchar_t*)
{ return 0; }
template<typename T>
struct only_char_or_wchar_t
{
    using ct = std::enable_if_t<std::is_same<T, char>::value || std::is_same<T, wchar_t>::value, T>;
};

template<typename char_type> struct char_or_wchart_api: only_char_or_wchar_t<char_type>
{
    constexpr static std::conditional_t<std::is_same<char_type, wchar_t>::value, int (_stdcall*)(int, wchar_t*, const wchar_t*) , int(_stdcall*)(int, char*, const char*)> prueba =
        std::is_same<char_type, wchar_t>::value
        ?
        ::pruebaW : …

我想与你分享一个我无法解决的小问题,这里是代码(仅供测试):

#include <windows.h>
#include <iostream>
#include <vector>
#include <string>
#include <utility>
#include <type_traits>
#include <sstream>

struct Procedure {
    Procedure(HANDLE)
    { std::cout << "ctor w/connection: " << this << std::endl; }

    ~Procedure()
    { std::cout << "dtor: " << this << std::endl; }

    Procedure(Procedure &&rhs) {
        std::cout << "ctor w/move: " << this << std::endl;
        this->m_Params = std::move(rhs.m_Params);
    }

    Procedure& operator= (Procedure &&rhs) {
        std::cout << "operator= w/move: " << this << std::endl;
        if (this != &rhs) this->m_Params = std::move(rhs.m_Params);
        return *this; …

任何人都可以帮我理解为什么这段代码片段无法编译？

#include <iostream>
#include <tuple>
#include <string_view>

constexpr auto Fields()
{
    using namespace std::string_view_literals;

    return std::tuple(
        std::tuple("campo_1"sv, 123),
        std::tuple("campo_2"sv, 456),
        std::tuple("campo_3"sv, 890),
        std::tuple("campo_4"sv, 136));
}

template<typename Tuple>
constexpr auto ProcessTupleElement(Tuple &&tuple)
{
    //constexpr auto ccb = std::get<1>(tuple); // 1 uncomment to get an error
    //char sx[ccb]; // 1 uncomment to get an error
    //(void)sx;
    return std::get<1>(tuple);
}

template<typename Tuple>
constexpr auto IterateTupleImpl(Tuple &&t)
{
    return ProcessTupleElement(std::get<0>(std::forward<Tuple>(t)));
}

template<typename Tuple>
constexpr auto IterateTuple(Tuple &&t)
{
    return IterateTupleImpl(std::forward<Tuple>(t));
}

int main()
{ …