c++ - 为什么编译时浮点计算的结果可能与运行时计算的结果不同？

Question

在constexpr: Introduction ，演讲者提到“编译时浮点计算可能与运行时计算的结果不同”:

而原因与“交叉编译”有关。

老实说，我不太清楚这个想法。恕我直言，不同的平台也可能有不同的整数实现。

为什么它只影响 float ？或者我错过了什么？

Answer 1

Why does it only affect floating points?

因为标准没有对浮点运算精度施加限制。

根据 expr.const , 强调我的:

[ Note: Since this document imposes no restrictions on the accuracy of floating-point operations, it is unspecified whether the evaluation of a floating-point expression during translation yields the same result as the evaluation of the same expression (or the same operations on the same values) during program execution. [ Example:

bool f() {
    char array[1 + int(1 + 0.2 - 0.1 - 0.1)];  // Must be evaluated during translation
    int size = 1 + int(1 + 0.2 - 0.1 - 0.1);   // May be evaluated at runtime
    return sizeof(array) == size;
}

It is unspecified whether the value of f() will be true or false. — end example ]
— end note ]