Value-initializing an automatic object?

Question

I\'m writing a template class and at one point in my code would like to be able to value-initialize an object of the parameterized type on the stack. Right now, I\'m accomp

Answer 1

No, there isn't any other way to reliably value-initialize a template type in C++03.

If you can count on T only being class types with default constructors, you could just write

T valueInitialized;

but if T might as well be a built-in type,

T valueInitialized = T();

is the way to go.

Do you have any reason to not to trust your compiler to optimize away that copy?

Answer 2

You can use curly braces in C++0x:

T valueInitialized{};

Answer 3

The following uses copy-initialization, which is 'probably fine' 95% of the time in C++03:

T var = T();

But for generic (C++03) code, you should always prefer direct-initialization to account for that other 5%:

T var((T())); // extra parentheses avoid the most vexing parse – the extra parentheses
              // force the contents to be evaluated as an expression, thus implicitly
              // *not* as a declaration.

Or better yet, use the Boost.Utility.ValueInit library, which packages up the ideal behavior for you along with workarounds for various compiler deficiencies (sadly, more than one might think):

boost::value_initialized<T> var;

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For C++11, one can use list-initialization syntax to achieve direct value-initialization in a significantly less noisy/ugly manner:

T var{}; // unambiguously value-initialization†

(^†N.b. technically this will invoke std::initializer_list<> constructors instead of performing value-initialization for certain pathological types. Presumably the net result should be the same.)