constexpr函数中的GCC constexpr lambda和编译时评估 [英] GCC constexpr lambdas in constexpr functions and evaluation in compile time

问题描述

首先，我们有以下代码用于在编译时累积 constexpr std :: array ：

Code first, we have the following piece of code that is used to accumulate a constexpr std::array in compile time:

template <typename T, std::size_t N, typename O>
constexpr T compile_time_accumulator(const std::array<T, N> const &A, const std::size_t i, const O& op, const T initialValue)
{
  return (i < N)
       ? op(A[i], compile_time_accumulator(A, i + 1, op, initialValue))
       : initialValue;
}

和以下代码示例进行测试/验证（即，它评估

and the following code example to test/varify it (i.e., that it evaluates in compile time):

constexpr std::array<int, 4> v {{4, 5, 6, 7}};
std::cout << std::integral_constant<int, compile_time_accumulator(v, 42, std::plus<int>())>::value 
          << std::endl;

实时演示

现在，如果更改运算符 std :: plus< ; int> 与 constexpr lambda：

Now if change the operator std::plus<int> with a constexpr lambda:

constexpr auto lambda_plus = [] (int x, int y) { return x + y; };

并按以下方式调用：

constexpr std::array<int, 4> v {{4, 5, 6, 7}};
std::cout << std::integral_constant<int, compile_time_accumulator(v, 42, lambda_plus)>::value << std::endl;
                                                                         ^^^^^^^^^^^ 

我得到一个错误， lambda不是 constexpr ：

I get an error, that lambda is not constexpr :

调用非constexpr函数

call to non-constexpr function ''

现在做点小事研究我发现尚不支持 constexpr lambda。

Now doing a litle research I discovered that constexpr lambdas aren't support yet.

为什么不支持 constexpr lambda，我们可以在其中定义 constexpr lambda

Why if constexpr lambdas aren't supported, we are allowed to define a constexpr lambda in the first place?

编辑：

似乎clang不接受代码。那么哪个编译器是正确的呢？

It seems that clang doesn't accep the code. So which compiler is right?

推荐答案

根据[expr.const] /（2.6），代码确实格式错误；尽管相应的提案正在分发中。 GCC接受 lambda_plus 的声明是错误的。

The code is indeed ill-formed as per [expr.const]/(2.6); lambdas aren't yet allowed in constant expressions, though a corresponding proposal is in circulation. GCC is incorrect in accepting lambda_plus's declaration.

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