编译时constexpr错误，但运行时无开销 [英] constexpr error at compile-time, but no overhead at run-time

问题描述

有一个众所周知的技巧，通过执行类似以下操作，会导致在 constexpr 函数的评估中导致编译时错误：

There is a well-known trick to cause a compile-time error in the evaluation of a constexpr function by doing something like this:

constexpr int f(int x) {
    return (x != 0) ? x : throw std::logic_error("Oh no!");
}

如果该函数用于 constexpr 上下文，如果 x == 0 ，您将收到编译时错误。但是，如果 f 的参数不是 constexpr ，则在运行时如果 x == 0 ，出于性能原因可能并不总是希望如此。

And if the function is used in a constexpr context you will get a compile-time error if x == 0. If the argument to f is not constexpr, however, then it will throw an exception at run time if x == 0, which may not always be desired for performance reasons.

类似于 assert 受 NDEBUG 的保护，有没有一种方法导致 constexpr 函数，但在运行时什么都不做？

Similar to the theory of assert being guarded by NDEBUG, is there a way to cause a compile-time error with a constexpr function, but not do anything at run time?

最后，在C语言中使用宽松的 constexpr 规则++ 1y（C ++ 14）更改任何内容吗？

Finally, do relaxed constexpr rules in C++1y (C++14) change anything?

推荐答案

是否有导致编译时的方法？ constexpr 函数出错，但在运行时什么都不做？

Is there a way to cause a compile-time error with a constexpr function, but not do anything at run time?

您可以使用完全相同的技巧，但不是使用 throw-expression ，而是使用不是常量表达式而是在运行时执行所需操作的表达式。例如：

You can use the exact same trick, but instead of using a throw-expression, use an expression that is not a constant expression but does what you want at runtime. For instance:

int runtime_fallback(int x) { return x; } // note, not constexpr
constexpr int f(int x) {
  return (x != 0) ? x : runtime_fallback(0);
}

constexpr int k1 = f(1); // ok
constexpr int k2 = f(0); // error, can't call 'runtime_fallback' in constant expression
int k3 = f(0);           // ok

放松 constexpr C ++ 1y（C ++ 14）中的规则会发生变化吗？

Do relaxed constexpr rules in C++1y (C++14) change anything?

不在此区域，否。有些形式的表达式在C ++ 14的常量表达式中有效，但在C ++ 11的常量表达式中无效，但 throw-expressions 或对non- constexpr <的调用/ code>函数在该列表上。

Not in this area, no. There are some forms of expression that are valid in constant expressions in C++14 but not in C++11, but neither throw-expressions nor calls to non-constexpr functions are on that list.

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