在 C++14 中，constexpr 成员可以更改数据成员吗？

Question

在 C++14 中，由于 constexpr 不再是隐含的 const，constexpr 成员函数是否可以修改 class 的数据成员：

struct myclass
{
    int member;
    constexpr myclass(int input): member(input) {}
    constexpr void f() {member = 42;} // Is it allowed?
};

Answer 1

据我所知，是的。限制来自 [dcl.constexpr]:

The definition of a constexpr function shall satisfy the following constraints:
— it shall not be virtual (10.3);
— its return type shall be a literal type;
— each of its parameter types shall be a literal type;
— its function-body shall be = delete, = default, or a compound-statement that does not contain

• an asm-definition,
• a goto statement,
• a try-block, or
• a definition of a variable of non-literal type or of static or thread storage duration or for which no initialization is performed.

该功能满足所有这些要求。

Answer 2

是的，我相信这个变化是从 proposal N3598: constexpr member functions and implicit const and eventually became part of N3652: Relaxing constraints on constexpr functions 开始的，它改变了第 7.1.5 段 3 白名单中函数体中允许的内容：

its function-body shall be = delete, = default, or a compound-statement that contains only

• null statements,
• static_assert-declarations
• typedef declarations and alias-declarations that do not define classes or enumerations,
• using-declarations,
• using-directives,
• and exactly one return statement;

加入黑名单：

its function-body shall be = delete, = default, or a compound-statement that does not contain

• an asm-definition,
• a goto statement,
• a try-block, or
• a definition of a variable of non-literal type or of static or thread storage duration or for which no initialization is performed.

并且还在第 C.3.3 节第 7 条中添加了以下注释：声明：

Change: constexpr non-static member functions are not implicitly const member functions.

Rationale: Necessary to allow constexpr member functions to mutate the object.

Effect on original feature: Valid C++ 2011 code may fail to compile in this International Standard. For example, the following code is valid in C++ 2011 but invalid in this International Standard because it declares the same member function twice with different return types:

struct S {
 constexpr const int &f();
 int &f();
};