What is the Purpose of the Arrow Operator (->) in C Function Headers?

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Understanding Arrow Operators (->) in Function Headers

The code in question introduces the arrow operator (->) in the function heading:

template <typename T, typename T1> auto compose(T a, T1 b) -> decltype(a + b) {
   return a+b;
}

This syntax refers to the alternative function declaration syntax introduced in C 11. It provides an alternative to the traditional method of specifying the return type:

return-type identifier (argument-declarations...)

Function Declaration Syntaxes

The two function declaration syntaxes in C 11 are:

1. Traditional Syntax:

   <return-type> <identifier> (<argument-declarations...>)

2. Alternative Syntax:

   <auto> <identifier> (<argument-declarations...>) -> <return-type>

Determining Return Type with Dectype

The arrow operator (->) allows for deriving the return type based on the argument types using decltype. decltype enables us to specify the type of an expression without explicitly stating it.

In the given example, decltype(a b) determines the return type based on the expression a b. The - > operator indicates that the return type will be the type of the expression.

C 14 Update

C 14 introduces another syntax simplification:

<auto> <identifier> (<argument-declarations...>)

This is permitted if the function is fully defined before use and all return statements deduce to the same type. However, the - > syntax remains useful for hiding function bodies in source files for public functions declared in headers.

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