CODEWITHSUNDEEP
c++templatesc++11variadic-templatesvariadic-functions
melmi
melmi

Reputation: 1008

A function with variable number of arguments with known types, the c++11 way

I already know the stdarg.h way to have a function with variable arguments in c++ as discussed here for example. I also know c++11 standard has variadic templates as explained here.

But in both of aforementioned schemes we don't know (and we can't force) argument types in compile time afaik. What I'm looking for is to pass variable arguments of known types to a function. I think this can be done because I read about it here:

Variadic templates, which can also be used to create functions that take variable number of arguments, are often the better choice because they do not impose restrictions on the types of the arguments, do not perform integral and floating-point promotions, and are type safe.

Is it possible? If yes, how can I do this?

Upvotes: 19

Views: 13907

Answers (2)

nosid
nosid

Reputation: 50074

It is straight forward to write a function with variadic templates, that accept an arbitrary number of arguments. The only difference to the general pattern is, that a concrete type is used as first argument (head) - instead of a template parameter. The following example shows a function foobar, that accepts an arbitrary number of strings.

// used for end of recursion - and for the empty arguments list
void foobar() { }

template <typename ...Tail>
void foobar(const std::string& head, Tail&&... tail)
{
    // do something with head
    std::cout << head << '\n';
    // call foobar recursively with remaining arguments
    foobar(std::forward<Tail>(tail)...);
}

foobar("Hello", "World", "...");

Personally, I prefer using std::initializer_list instead of variadic templates. Because variadic templates are more complex and require additional experience. With std::initializer_list, it might look like this:

void foobar(std::initializer_list<std::string> values)
{
    for (auto& value : values) {
        // do something with value
        std::cout << value << '\n';
    }
}

foobar({ "Hello", "World", "...", });

Unfortunately, the additional curly braces are required when using std::initializer_list with regular functions. They are not required for constructors, if the new initializer syntax is used.

Edit: Rewrote the answer according to the feedback. In particular I have changed the order of the two solutions/examples.

Upvotes: 30

Colin D
Colin D

Reputation: 5661

If variable parameters are all of one type, you can change the function signature to take an array of those types instead of using the '...'.

Upvotes: 2

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