Why does member `float x` get initialized with `0.` for the objects `a` and `b` in main()?
c++, c++11, default-constructor
Solution
`0` is one of the possible arbitrary value an uninitialized variable can acquire: the program has undefined behavior. Given that there is a fair chance that memory starts off zero initialized, `0` is a likely outcome: the IEEE 754 representation for `0` happens to be all zeros. There is, however, no guarantee that the value will be `0`.
Problem
Could somebody indicate which clause in the Standard supports the following behavior obtained in Coliru, for the snippet: ``` #include <iostream> class A { int i; float x; public: A() : i(10) {} A(int i) : i(i) {} int GetI() { return i; } float GetF() { return x; } }; int main() { A a; A b(1); A x{}; A y{1}; std::cout << a.GetI() << '\n'; std::cout << a.GetF() << '\n'; std::cout << b.GetI() << '\n'; std::cout << b.GetF() << '\n'; std::cout << x.GetI() << '\n'; std::cout << x.GetF() << '\n'; std::cout << y.GetI() << '\n'; std::cout << y.GetF() << '\n'; } ``` The code prints: 10 0 <-- Shouldn't be unknown? 1 0 <-- idem 10 0 1 0 Edit: This paragraph was obtained from the TCPL 4th edition, page 490: For this, the rules are not as clean as we might like. For statically allocated objects (§6.4.2), the rules are exactly as if you had used {}, so the value of alpha is {"","",0}. However, for local variables and free-store objects, the default initialization is done only for members of class type, and members of built-in type are left uninitialized, so the value of beta is {"","",unknown}. Mr. Stroustrup doesn't say anything about undefined behavior.