Thread: Explicit initialization of class members

Code:

struct some_struct{
    int a;
};
some_struct n = {};

n.a will be 0;

I know this braces form of initialization is inherited from C and is supported for compatibility with C programs, but this only compiles with C++, not with the C compiler. I'm using Visual C++ 2005.

In C this type of initialization

Code:

some_struct n = {0};

is correct and will zero-initialize all members of a structure.

Is the empty pair of braces form of initialization standard? Because I've never seen it before, but seem to be used in winapi.

No, it won't compile in C, at least not in Visual C++ 2005.

In C you can write

Code:

typedef struct {
    int a;
}some_struct;

to be able to use

Code:

some_struct n;

but if I write

Code:

some_struct n = {0};

it compiles.

Originally Posted by grumpy

That is a very contradictory observation if you think about it

Yes that's because the same IDE (Visual C++ 2005 known also as Visual C++ 8.0) alows to compile your code as C or as C++. Also the compiler is invoked indirectly using a command-line interface that is CL.EXE, and I don't know the compiler names that this tool calls.

Originally Posted by grumpy

In any event, the construct is valid in both C and C++. Although I tend to see it discouraged in practice (disallowed in some style guides, compilers often warn about such a construct at higher warning settings, etc).

This is not valid in C.

The standard (C99 - 6.7.8/12 - Initialization) says this:

If there are fewer initializers in a brace-enclosed list than there are elements or members of an aggregate, or fewer characters in a string literal used to initialize an array of known size than there are elements in the array, the remainder of the aggregate shall be initialized implicitly the same as objects that have static storage duration.

In C the grammar requires that there be at least one 'assignment-expression' inside the braces. An 'assignment-expression' can be many things from a constant or identifier up through much more complex expressions. However, an empty string doesn't qualify as an 'assignment-expression', so there has to be something between the braces.

In C++, the grammar specifically allows the empty braces initializer.

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