How to default-initialize local variables of built-in types in C++?

Question

How do I default-initialize a local variable of primitive type in C++? For example if a have a typedef:

typedef unsigned char boolean;//that's Microsoft RPC runtime typedef

I'd like to change the following line:

boolean variable = 0; //initialize to some value to ensure reproduceable behavior
retrieveValue( &variable ); // do actual job

into something that would automagically default-initialize the variable - I don't need to assign a specific value to it, but instead I only need it to be intialized to the same value each time the program runs - the same stuff as with a constructor initializer list where I can have:

struct Struct {
   int Value;
   Struct() : Value() {}
};

and the Struct::Value will be default-initialized to the same value every time an instance is cinstructed, but I never write the actual value in the code.

How can I get the same behavior for local variables?

Answer 1

You can emulate that behaviour by the following:

boolean x = boolean();

or, more general,

T x = T();

This will default-initialize x if such a default-initialization exists. However, just writing T x will never do the trick for local variables, no matter what you do.

You can also use placement-new to invoke a “constructor”, even for POD:

T x;
new (&x) T();

Notice that this code produces undefined behaviour for non-POD types (in particular for types that have a non-trivial destructor). To make this code work with user-defined types, we first need to call the object’s destructor:

T x;
x.~T();
new (&x) T();

This syntax can also be used for PODs (guaranteed by §§5.2.4/12.4.15) so the above code can be used indiscriminately for any type.