>>86
念のためにこれも付け加えておくか

After the call to a constructor for class X has completed, if a member of X is neither specified in the
constructor’s mem-initializers, nor default-initialized, nor value-initialized, nor given a value during execution
of the body of the constructor, the member has indeterminate value.

はっきりと「メンバの値は不定」と書いてあるだろう

つまりgcc拡張である事が明らかだ