CODEWITHSUNDEEP
c++templatesauto
Werlious
Werlious

Reputation: 594

Identity Function: Difference between templates and auto

I am writing an identity function for some of my classes that keeps a count of its calls (long story -> short: metrics).

At the moment, I'm trying to figure the performance differences/benefits of using a template vs auto.

Here is a short example taken from my code of what I'm doing:

namespace Metrics {
    unsigned long identifications = 0;
    //auto version
    auto identity(auto i) {
        //... other stuffs
        identifications++;
        return i;
    };
    //template version
    template<class I> I identity(I i) {
        //... other stuffs
        identifications++;
        return i;
    };
};

There's a bit more going on, but that is the basics. I know the compiler will just make a function for each, i.e.

identity(5);
identity("5");
//generates the functions
int identity(int i) { ... return i; };
const char* identity(const char* i) { ... return i; };

At run-time, which one is faster? And do they have a compile time difference?

Since this function is meant to be called a lot, I'm more interested in run-time performance, but there also could be a large amount of types to generate the function for, so I'm also interested in which one would be faster at compile-time.

Upvotes: 1

Views: 138

Answers (1)

bolov
bolov

Reputation: 75874

auto identity(auto i)
{
    //...
    return i;
}

is a shorthand for

template <class T>
auto identity(T i)
{
    // ...
    return i;
}

which in turn is a shorthand for

template <class T>
T identity(T i)
{
    // ...
    return i;
}

So no difference whatsoever.

Not applicable to your example, but if you are going to use auto parameters you need to be aware of some gotchas:

auto as return type

auto foo(auto a)

Does not mean

// not this
template <class T>
T foo(T a)

The return type will be deduced from the return expression(s) in foo definition, just like any auto return type. It just happens that in your function the return type is deduced to be the same as the parameter type.

Multiple auto parameters

void foo(auto a, auto b)

Is not equivalent to

// not this
template <class T>
void foo(T a, T b)

but with

template <class T, class U>
void foo(T a, U b)

Upvotes: 5

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