std::conditional<表達(dá)式, 類型1, 類型2>

typedef typename std::conditional<true, int, double>::type Type1;// => int Type1

#include <iostream>
#include <type_traits>
#include <typeinfo>
 
int main() 
{
    typedef typename std::conditional<true, int, double>::type Type1;
    typedef typename std::conditional<false, int, double>::type Type2;
     typedef typename std::conditional<sizeof(int) == sizeof(double), int, double>::type Type3;
 
    std::cout << typeid(Type1).name() << std::endl; // 輸出：i (代表int類型)
    std::cout << typeid(Type2).name() << std::endl; // 輸出：d （代表double類型）
    std::cout << typeid(Type3).name() << std::endl; // 輸出：d (代表double類型)
    
    Type1 a = 3.1; // 由于Type1的類型為int所以3.1被強(qiáng)制轉(zhuǎn)換為3
    Type2 b = 4.2; // Type2的類型為double，4.2保存在變量b中
    std::cout << a +  b << std::endl; // 3+4.2，最終輸出為7.2
}

//std::conditional<bool, A, B>::type 
const bool kEnableOffsetRender = false; 
class LoginDialog : public std::conditional<kEnableOffsetRender, ui::WindowImplBase, nim_comp::ShadowWndBase>::type
{
public:
    //todo
};
 
/*
    if(kEnableOffsetRender)
    {
        type = ui::WindowImpBase;
    }
    else
    {
        type = nim_comp::ShadowWndBase;
    }
*/