Friday, May 2, 2014

Improved Type Inference in C++11: auto, decltype, and the new function declaration syntax

int x = 3;
decltype(x) y = x; // same thing as auto y = x;
You can use decltype for pretty much any expression, including to express the type for a return value from a method. Hmm, that sounds like a familiar problem doesn't it? What if we could write:
decltype( builder.makeObject() )
This would give us the type that is returned from makeObject, allowing us to specify the return value from makeAndProcessObject. We can combine this with the new return value syntax to produce this method:
template <typename Builder>
makeAndProcessObject (const Builder& builder) -> decltype( builder.makeObject() )
    auto val = builder.makeObject();
    // do stuff with val
    return val;

As of this writing, GCC 4.4 and MSVC 10 both support everything I've talked about this article, and I'm used most of these techniques in production code; these aren't theoretical benefits, they're real. So if you're compiling your code with -std=c++0x in GCC or using MSVC 10, you can start using these techniques today. If you're using another compiler, check out this page for details of C++11 compiler support. Since the standard has been ratified, and should be published within weeks, now's the time to start.

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