Default Initialization Versus Zero Initialization

Question

I cannot understand the behavior of gcc 4.8.1 or Visual Studio 2015 with respect to default initialization versus value initialization.

It doesn\'t help that I\'m tr

Answer 1

Default initialisation, of classes like this without user-defined constructors, does nothing, leaving each trivial member with an indeterminate value.

Value initialisation will zero-initialise each member.

In the first case, you're printing:

• the indeterminate value of a default-initialised Foo foo;
• the zero value of a value-initialised Foo()
• the indeterminate value of a default-initialised bar b;

The third one happens to be zero; perhaps because it reuses the storage of the temporary value-initialised Foo.

In the second case, you're printing the indeterminate values of two default-initialised objects. Coincidentally, they have zero values in one case but not the other.

Both programs have undefined behaviour, since they use uninitialised values.