constexpr的等效三元运算符是否? [英] Equivalent ternary operator for constexpr if?

问题描述

也许我错过了一些东西，但是我找不到任何提示:C ++ 17中是否有一个相当于constexpr-if的constexpr三元运算符?

Maybe I missed something, but I can't find any hints: is there a constexpr ternary operator in C++17 equivalent to constexpr-if?

template<typename Mode>
class BusAddress {
public:
    explicit constexpr BusAddress(Address device) : 
        mAddress(Mode::write ? (device.mDevice << 1) : (device.mDevice << 1) | 0x01) {}
private:
    uint8_t mAddress = 0;    
};

推荐答案

否，没有constexepr条件运算符.但是您可以将整个内容包装在一个lambda中，然后立即对其进行评估( IIFE ):

No, there is no constexepr conditional operator. But you could wrap the whole thing in a lambda and immediately evaluate it (an IIFE):

template<typename Mode>
class BusAddress {
public:
    explicit constexpr BusAddress(Address device)
     : mAddress([&]{
          if constexpr (Mode::write) {
            return device.mDevice << 1;
          }
          else {
            return (device.mDevice << 1) | 0x01;
          }         
        }())
     { }
private:
    uint8_t mAddress = 0;    
};

这可能不是有史以来最性感的代码，但是它可以完成工作.请注意，自 N4487 和 P0170 .

It may not be the sexiest code ever, but it gets the job done. Note that lambdas are constexpr by default where possible as of N4487 and P0170.

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