未初始化的局部变量，默认为c ++ 11

Question

Why does printing bk give a warning when printing ak does not? I Use VS2013

//warning C4700: uninitialized local variable 'b' used
#include<iostream> 

using namespace std;

struct A {
  A() {};
  int k;
};
struct B {
  B() = default;
  int k;
};

int main() {
  A a;
  cout << a.k << endl;
  B b;
  cout << b.k << endl; // this gives a warning, uninitialized local variable

  return 0;
}

Answer 1

Accessing uninitialized variables is undefined behavior and no diagnostic is required. This means that you can get a warning for bk (MSVC++), for ak (g++) or for neither (Clang).

Standard quotes:

12.6.2 Initializing bases and members [class.base.init]

8 In a non-delegating constructor, if a given non-static data member or base class is not designated by a mem-initializer-id (including the case where there is no mem-initializer-list because the constructor has no ctor-initializer) and the entity is not a virtual base class of an abstract class (10.4), then

[bunch of non-applicable clauses]

— otherwise, the entity is default-initialized (8.5).

8.5 Initializers [dcl.init]

7 To default-initialize an object of type T means:

[bunch of non-applicable clauses]

— otherwise, no initialization is performed.

The 12.6.2/8 quote has this example:

struct C {
    C() { }    // initializes members as follows:
    A a;         // OK: calls A::A()
    const B b;   // error: B has no default constructor
    int i;       // OK: i has indeterminate value // <---------- your code
    int j = 5;   // OK: j has the value 5
};

Answer 2

As per § 8.5 Initializers

if T has a non-trivial default constructor, the object is default-initialized;

Which means both ak and bk are not value initialized, access them are UB

To initialize A::k , you could put it in member initialize list

A():k(42) {};

For B::K , you could call it with:

B b = {}; // value initialize  members, k initialized to 0