Should the trailing return type syntax style become the default for new C++11 programs?

Question

C++11 supports a new function syntax:

auto func_name(int x, int y) -> int;

Currently this function would be declared as:

int func_name(int x, int y);

The new style does not seem to be widely adopted yet (say in the gcc stl)

However, should this new style be preferred everywhere in new C++11-programs, or will it only be used when needed?

Personally, I prefer the old style when possible, but a code-base with mixed styles looks pretty ugly.

Answer 1

There are certain cases where you must use a trailing return type. Most notably, a lambda return type, if specified, must be specified via a trailing return type. Also, if your return type utilizes a decltype that requires that the argument names are in scope, a trailing return type must be used (however, one can usually use declval<T> to work around this latter issue).

The trailing return type does have some other minor advantages. For example, consider a non-inline member function definition using the traditional function syntax:

struct my_awesome_type
{
    typedef std::vector<int> integer_sequence;

    integer_sequence get_integers() const;
}; 

my_awesome_type::integer_sequence my_awesome_type::get_integers() const
{
    // ...
}

Member typedefs are not in scope until after the name of the class appears before ::get_integers, so we have to repeat the class qualification twice. If we use a trailing return type, we don't need to repeat the name of the type:

auto my_awesome_type::get_integers() const -> integer_sequence
{
    // ...
}

In this example, it's not such a big deal, but if you have long class names or member functions of class templates that are not defined inline, then it can make a big difference in readability.

In his "Fresh Paint" session at C++Now 2012, Alisdair Meredith pointed out that if you use trailing return types consistently, the names of all of your functions line up neatly:

auto foo() -> int;
auto bar() -> really_long_typedef_name;

I've used trailing return types everywhere in CxxReflect, so if you're looking for an example of how code looks using them consistently, you can take a look there (e.g, the type class).