What does {0} mean when initializing an object?

Question

When {0} is used to initialize an object, what does it mean? I can\'t find any references to {0} anywhere, and because of the curly braces Google s

Answer 1

{0} is a valid initializer for any (complete object) type, in both C and C++. It's a common idiom used to initialize an object to zero (read on to see what that means).

For scalar types (arithmetic and pointer types), the braces are unnecessary, but they're explicitly allowed. Quoting the N1570 draft of the ISO C standard, section 6.7.9:

The initializer for a scalar shall be a single expression, optionally enclosed in braces.

It initializes the object to zero (0 for integers, 0.0 for floating-point, a null pointer for pointers).

For non-scalar types (structures, arrays, unions), {0} specifies that the first element of the object is initialized to zero. For structures containing structures, arrays of structures, and so on, this is applied recursively, so the first scalar element is set to the zero, as appropriate for the type. As in any initializer, any elements not specified are set to zero.

Intermediate braces ({, }) may be omitted; for example both these are valid and equivalent:

int arr[2][2] = { { 1, 2 }, {3, 4} };

int arr[2][2] = { 1, 2, 3, 4 };

which is why you don't have to write, for example, { { 0 } } for a type whose first element is non-scalar.

So this:

some_type obj = { 0 };

is a shorthand way of initializing obj to zero, meaning that each scalar sub-object of obj is set to 0 if it's an integer, 0.0 if it's floating-point, or a null pointer if it's a pointer.

The rules are similar for C++.

In your particular case, since you're assigning values to sexi.cbSize and so forth, it's clear that SHELLEXECUTEINFO is a struct or class type (or possibly a union, but probably not), so not all of this applies, but as I said { 0 } is a common idiom that can be used in more general situations.

This is not (necessarily) equivalent to using memset to set the object's representation to all-bits-zero. Neither floating-point 0.0 nor a null pointer is necessarily represented as all-bits-zero, and a { 0 } initializer doesn't necessarily set padding bytes to any particular value. On most systems, though, it's likely to have the same effect.