Trailing return type

In computer programming, a subroutine will often inform calling code about the result of its computation, by returning a value to that calling code.
The data type of that value is called the function's return type.
In the C++ programming language, a function must be declared.
The C++ function's return type is specified as a part of declaring that function.
A trailing return type, a syntax feature available since C++11, is like a traditional return type, except that it is specified in a different location.

Syntax

An ordinary return type is specified before the function's name.
In this example of traditional C++ code, the return type of hasMultipleItems is bool:

using std::vector;
class MyClass ;

A trailing return type is specified after the parameter list, following -> symbols:

using std::vector;
class MyClass ;

Distinction from other language features

In modern C++, the meaning of the auto keyword will depend on its context:

When used in a variable's definition, the auto keyword indicates type inference. The data type for that x will be deduced from its initialization. The return type of a function can also be inferred by using auto without specifying a trailing return type. For example:

auto hasMultipleItems

On the other hand, there is no type inference in the hasMultipleItems example on the previous section. That example only uses the auto keyword as a syntactic element, because a trailing return type is being used.

Rationale

Consider the task of programming a generic version of the following:

int add

A proper expression of this function's return type would use the two formal parameter names with decltype: decltype. However, where a return type is traditionally specified, those two formal parameters are not yet in scope.
Consequently, this code will not compile:

// This will not compile
template L, typename R>
decltype add

However, where a trailing return type is specified, the formal parameters are in scope:

template
auto add -> decltype