Is there a way to pass auto as an argument in C++?

Question

Is there a way to pass auto as an argument to another function?

int function(auto data)
{
    //DOES something
}

Answer 1

Templates are the way you do this with normal functions:

template <typename T>
int function(T data)
{
    //DOES something
}

Alternatively, you could use a lambda:

auto function = [] (auto data) { /*DOES something*/ };

Answer 2

If you want that to mean that you can pass any type to the function, make it a template:

template <typename T> int function(T data);

There's a proposal for C++17 to allow the syntax you used (as C++14 already does for generic lambdas), but it's not standard yet.

Edit: C++ 2020 now supports auto function parameters. See Amir's answer below

Answer 3

C++20 allows auto as function parameter type

This code is valid using C++20:

int function(auto data) {
   // do something, there is no constraint on data
}

As an abbreviated function template.

This is a special case of a non constraining type-constraint (i.e. unconstrained auto parameter). Using concepts, the constraining type-constraint version (i.e. constrained auto parameter) would be for example:

void function(const Sortable auto& data) {
    // do something that requires data to be Sortable
    // assuming there is a concept named Sortable
}

The wording in the spec, with the help of my friend Yehezkel Bernat:

9.2.8.5 Placeholder type specifiers [dcl.spec.auto]

placeholder-type-specifier:

type-constraint_opt auto

type-constraint_opt decltype ( auto )

1. A placeholder-type-specifier designates a placeholder type that will be replaced later by deduction from an initializer.

2. A placeholder-type-specifier of the form type-constraint_opt auto can be used in the decl-specifier-seq of a parameter-declaration of a function declaration or lambda-expression and signifies that the function is an abbreviated function template (9.3.3.5) ...

Answer 4

I dont know when it changed, but currently syntax from Question is possible with c++14:

https://coliru.stacked-crooked.com/a/93ab03e88f745b6c

There is only warning about it:

g++ -std=c++14 -Wall -pedantic -pthread main.cpp && ./a.out main.cpp:5:15: warning: use of 'auto' in parameter declaration only available with -fconcepts void function(auto data)

With c++11 there is an error:

main.cpp:5:15: error: use of 'auto' in parameter declaration only available with -std=c++14 or -std=gnu++14