Function Templates vs. Auto Keyword

Question

Can the auto keyword in C++11 replace function templates and specializations? If yes, what are the advantages of using template functions and specializations over simply typing a function parameter as auto?

template <typename T>
void myFunction(T &arg)
{
    // ~
}

vs.

void myFunction(auto &arg)
{
    // ~
}

There is a way in C++14: `auto myFunction = [](auto &arg) {...};` — chris, Aug 08 '13 at 20:44
C++20 will have this feature: `void myFunction(auto &arg) {...}` — Sadeq, Nov 23 '18 at 05:59

Michael Goldshteyn · Accepted Answer · 2013-08-08T20:58:23.943

15

In a nutshell, auto cannot be used in an effort to omit the actual types of function arguments, so stick with function templates and/or overloads. auto is legally used to automatically deduce the types of variables:

auto i=5;

Be very careful to understand the difference between the following, however:

auto x=...
auto &x=...
const auto &x=...
auto *px=...; // vs auto px=... (They are equivalent assuming what is being 
              //                 assigned can be deduced to an actual pointer.)
// etc...

It is also used for suffix return types:

template <typename T, typename U>
auto sum(const T &t, const U &u) -> decltype(t+u)
{
  return t+u;
}

edited Aug 08 '13 at 20:58

answered Aug 08 '13 at 20:36

Michael Goldshteyn

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it's important to outline that auto is fine and good 99% of the time, sometimes you have to stop for a second and think about what auto is really doing http://stackoverflow.com/questions/12773257/does-auto-type-assignments-of-a-pointer-in-c11-require pointers can be a corner case for auto. – user2485710 Aug 08 '13 at 20:53
1

@user2485710, added more info to my answer based on your comment. – Michael Goldshteyn Aug 08 '13 at 20:58

score 7 · Answer 2 · answered Aug 08 '13 at 20:47

Can the auto keyword in C++11 replace function templates and specializations?

No. There are proposals to use the keyword for this purpose, but it's not in C++11, and I think C++14 will only allow it for polymorphic lambdas, not function templates.

If yes, What are the advantages of using template functions and specializations over simply typing a function parameter as auto.

You might still want a named template parameter if you want to refer to the type; that would be more convenient than std::remove_reference<decltype(arg)>::type or whatever.

score 5 · Answer 3 · edited May 15 '19 at 22:36

The only thing which makes it the auto keyword different from template that is you cannot make a generic class using the auto keyword.

class B { auto a; auto b; }

When you create an object of the above class it will give you an error.

B b; // Give you an error because compiler cannot decide type so it can not be assigned default value to properties

Whereas using template you can make a generic class like this:

template <class T> 

class B {
    T a;
};

void main() {
    B<int> b; //No Error
}

Function Templates vs. Auto Keyword

3 Answers3

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