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 accomplishing this by writing something to this effect:

template <typename T> void MyClass<T>::doSomething() {     T valueInitialized = T();     /* ... */ } 

This code works, but (unless the compiler is smart) it requires an unnecessary creation and destruction of the temporary T object. What I'd like to write is the following, which I know is incorrect:

template <typename T> void MyClass<T>::doSomething() {     T valueInitialized(); // WRONG: This is a prototype!     /* ... */ } 

My question is whether there is a nice way to value-initialize the automatic object without having to explicitly construct a temporary object and assign it over to the automatic object. Can this be done? Or is T var = T(); as good as it gets?

Answer 1

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.)

Answer 2

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

T valueInitialized{};