c++ - 来自另一个非静态的非静态成员初始值设定项

Question

很简单的问题。这是有效的 C++11 吗？

struct Foo {
    int bar = 1;
    int baz = bar;
};

GCC (4.7.2) 和 Clang (3.1) 都接受了它的迂腐设置:

-std=c++11 -Wall -W -pedantic

Intel C++ (13.0.1.117) does not. It barks at int baz = bar; with:

error: a nonstatic member reference must be relative to a specific object

Who is right?

In case you wonder, I use this for code like this, where it brings initialization code closer together, rather than moving the last line into the constructor:

uint8_t colorR = -1;
uint8_t colorG = -1;
uint8_t colorB = -1;
uint8_t colorA = -1;
GLubyte RGBAVec[4] = {colorR, colorG, colorB, colorA};

Answer 1

5.1p12 An id-expression that denotes a non-static data member or non-static member function of a class can only be used:

• as part of a class member access (5.2.5) in which the object expression refers to the member’s class or a class derived from that class, or
• to form a pointer to member (5.3.1), or
• in a mem-initializer for a constructor for that class or for a class derived from that class (12.6.2), or
• in a brace-or-equal-initializer for a non-static data member of that class or of a class derived from that class (12.6.2), or
• if that id-expression denotes a non-static data member and it appears in an unevaluated operand.

所以是的，这个:

struct Foo {
  int bar = 1;
  int baz = bar;
};

是有效的 C++11。

但要小心顺序，因为:

12.6.2p10 In a non-delegating constructor, initialization proceeds in the following order:

• First, and only for the constructor of the most derived class (1.8), virtual base classes are initialized in the order they appear on a depth-first left-to-right traversal of the directed acyclic graph of base classes, where “left-to-right” is the order of appearance of the base classes in the derived class base-specifier-list.
• Then, direct base classes are initialized in declaration order as they appear in the base-specifier-list (regardless of the order of the mem-initializers).
• Then, non-static data members are initialized in the order they were declared in the class definition (again regardless of the order of the mem-initializers).
• Finally, the compound-statement of the constructor body is executed

因此如 Non-static data member initializers proposal 中所指定(问题 3):

A third issue is that class-scope lookup could turn a compile-time error into a run-time error:

struct S {
    int i = j; // ill-formed without forward lookup, undefined behavior with
    int j = 3;
};

(Unless caught by the compiler, i might be intialized with the undefined value of j.)