Automatic Return Type

In this lesson, we'll look at the technique that deduces return type automatically.

Automatic Return Type

A function template is automatically able to deduce their return type.

template <typename T1, typename T2>
auto add(T1 fir, T2 sec) -> decltype( fir + sec ) { 
  return fir + sec;
}

The automatic return type deduction is typically used for function templates but can also be applied to non-template functions.

Rules:

  • auto: introduces the syntax for the delayed return type

  • auto: auto type deduction is based on the function template argument deduction. Function template argument deduction (decays). So it means auto does not return the exact type but a decayed type such as for template argument deduction

  • decltype: declares the return type

  • The alternative function syntax is obligatory

The C++11 syntax for automatically deducing the return type breaks the crucial principle of software development: DRY. DRY stands for Don’t Repeat Yourself.

Automatic Return Type: C++14

A function template is automatically able to deduce their return type.

template <typename T1, typename T2>
auto add(T1 fir, T2 sec){
    return fir + sec;
}

Rules

  • auto: introduces the syntax for the delayed return type
  • decltype: declares the return type
  • The alternative function syntax is obligatory.

With the expression decltype(auto), auto uses the same rules to determine the type as decltype. This means, in particular, no decay takes place.

  • Both declarations are identical.
   decltype(expr) v= expr;
   decltype(auto) v= expr;
  • The syntax also applies for the automatic return type of a function template.
   template <typename T1, typename T2> 
   decltype(auto) add(T1 fir, T2 sec){
       return fir + sec;
   }

When a function template has more than one return statements, all return statements must have the same type.


In the next lesson, we’ll study an example of automatic return type deduction.

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