c++ - 访问静态 constexpr float 成员时 undefined reference

Question

此代码有效:

struct Blob {
    static constexpr int a = 10;
};

int main() {
    Blob b;
    auto c = b.a;
}

但是如果我将 int 更改为 float 我会得到一个错误:

struct Blob {
    static constexpr float a = 10.0f;
};

/tmp/main-272d80.o: In function main': main.cpp:(.text+0xe):
undefined reference toBlob::a'

为什么我不能以这种方式使用 constexpr float？

编译器:Ubuntu clang 版本 3.5.0-4ubuntu2 (tags/RELEASE_350/final)

在gcc version 4.9.1 (Ubuntu 4.9.1-16ubuntu6)上测试，没有报错。

编辑:

如果我使用 -O1、-O2、-O3 或 -Os 但使用 -O0 会失败

Answer 1

C++11 阅读

A variable whose name appears as a potentially-evaluated expression is odr-used unless it is an object that satisfies the requirements for appearing in a constant expression (5.19) and the lvalue-to-rvalue conversion (4.1) is immediately applied.

很明显，l-t-r 转换会立即应用，浮点类型的 constexpr 变量可以按照 [expr.const]/(2.7.1) 出现在常量表达式中:

A conditional-expression is a core constant expression unless it involves one of the following as a potentially evaluated subexpression [..]

• an lvalue-to-rvalue conversion (4.1) unless it is applied to
  • a glvalue of literal type that refers to a non-volatile object defined with constexpr, or that refers to a sub-object of such an object, or

似乎是一个 Clang 错误。