What Does the Arrow Operator (`->`) Do in a Function Heading?

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Arrow Operator (->) in Function Heading

This article examines the arrow operator (->) employed in function headings, which is a fundamental aspect of C 11's syntax.

Understanding the Arrow Operator

C 11 introduced two equivalent syntaxes for function declaration:

1. return-type identifier (argument-declarations...)
2. auto identifier (argument-declarations...) -> return_type

The arrow operator (->;) in the second syntax serves a crucial purpose. It enables the return type of a function to be deduced from its arguments using the decltype keyword.

Why Use the Auto-Deduced Return Type?

In certain situations, it is advantageous to derive the return type dynamically based on the argument types. For example, consider the following function that calculates the sum of two values:

template <typename T1, typename T2>
decltype(a + b) compose(T1 a, T2 b);

In this case, the decltype argument informs the compiler that the return type should be the same as the type of the expression a b. However, this declaration raises an error since the compiler lacks information about a and b in the decltype argument.

Overcoming the Error

To resolve this issue, you can either use std::declval and the template parameters to manually specify the types:

template <typename T1, typename T2>
decltype(std::declval<T1>() + std::declval<T2>())
compose(T1 a, T2 b);

or utilize the alternate declaration syntax with the arrow operator (->):

template <typename T1, typename T2>
auto compose(T1 a, T2 b) -> decltype(a + b);

The latter option is more concise and streamlines the scoping rules.

C 14 Enhancements

In C 14, the following syntax is also permissible as long as the function is fully defined before use and all return statements deduce to the same type:

auto identifier (argument-declarations...)

However, the arrow operator (->) remains useful for declaring public functions (in headers) while hiding their bodies in source files.

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